what is the meaning of photosynthesis in life science

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Development of the idea

Overall reaction of photosynthesis.

• Basic products of photosynthesis
• Evolution of the process
• Light intensity and temperature
• Carbon dioxide
• Internal factors
• Energy efficiency of photosynthesis
• Structural features
• Light absorption and energy transfer
• The pathway of electrons
• Evidence of two light reactions
• Photosystems I and II
• Quantum requirements
• The process of photosynthesis: the conversion of light energy to ATP
• Elucidation of the carbon pathway
• Carboxylation
• Isomerization/condensation/dismutation
• Phosphorylation
• Regulation of the cycle
• Products of carbon reduction
• Photorespiration
• Carbon fixation in C 4 plants
• Carbon fixation via crassulacean acid metabolism (CAM)
• Differences in carbon fixation pathways
• The molecular biology of photosynthesis

Why is photosynthesis important?

What is the basic formula for photosynthesis, which organisms can photosynthesize.

photosynthesis

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• Khan Academy - Photosynthesis
• Biology LibreTexts - Photosynthesis
• University of Florida - Institute of Food and Agricultural Sciences - Photosynthesis
• Milne Library - Inanimate Life - Photosynthesis
• National Center for Biotechnology Information - Chloroplasts and Photosynthesis
• Roger Williams University Pressbooks - Introduction to Molecular and Cell Biology - Photosynthesis
• BCcampus Open Publishing - Concepts of Biology – 1st Canadian Edition - Overview of Photosynthesis
• photosynthesis - Children's Encyclopedia (Ages 8-11)
• photosynthesis - Student Encyclopedia (Ages 11 and up)
• Table Of Contents

Photosynthesis is critical for the existence of the vast majority of life on Earth. It is the way in which virtually all energy in the biosphere becomes available to living things. As primary producers, photosynthetic organisms form the base of Earth’s food webs and are consumed directly or indirectly by all higher life-forms. Additionally, almost all the oxygen in the atmosphere is due to the process of photosynthesis. If photosynthesis ceased, there would soon be little food or other organic matter on Earth, most organisms would disappear, and Earth’s atmosphere would eventually become nearly devoid of gaseous oxygen.

The process of photosynthesis is commonly written as: 6CO 2 + 6H 2 O → C 6 H 12 O 6 + 6O 2 . This means that the reactants, six carbon dioxide molecules and six water molecules, are converted by light energy captured by chlorophyll (implied by the arrow) into a sugar molecule and six oxygen molecules, the products. The sugar is used by the organism, and the oxygen is released as a by-product.

The ability to photosynthesize is found in both eukaryotic and prokaryotic organisms. The most well-known examples are plants, as all but a very few parasitic or mycoheterotrophic species contain chlorophyll and produce their own food. Algae are the other dominant group of eukaryotic photosynthetic organisms. All algae, which include massive kelps and microscopic diatoms , are important primary producers.  Cyanobacteria and certain sulfur bacteria are photosynthetic prokaryotes, in whom photosynthesis evolved. No animals are thought to be independently capable of photosynthesis, though the emerald green sea slug can temporarily incorporate algae chloroplasts in its body for food production.

photosynthesis , the process by which green plants and certain other organisms transform light energy into chemical energy . During photosynthesis in green plants, light energy is captured and used to convert water , carbon dioxide , and minerals into oxygen and energy-rich organic compounds .

It would be impossible to overestimate the importance of photosynthesis in the maintenance of life on Earth . If photosynthesis ceased, there would soon be little food or other organic matter on Earth. Most organisms would disappear, and in time Earth’s atmosphere would become nearly devoid of gaseous oxygen. The only organisms able to exist under such conditions would be the chemosynthetic bacteria , which can utilize the chemical energy of certain inorganic compounds and thus are not dependent on the conversion of light energy.

Energy produced by photosynthesis carried out by plants millions of years ago is responsible for the fossil fuels (i.e., coal , oil , and gas ) that power industrial society . In past ages, green plants and small organisms that fed on plants increased faster than they were consumed, and their remains were deposited in Earth’s crust by sedimentation and other geological processes. There, protected from oxidation , these organic remains were slowly converted to fossil fuels. These fuels not only provide much of the energy used in factories, homes, and transportation but also serve as the raw material for plastics and other synthetic products. Unfortunately, modern civilization is using up in a few centuries the excess of photosynthetic production accumulated over millions of years. Consequently, the carbon dioxide that has been removed from the air to make carbohydrates in photosynthesis over millions of years is being returned at an incredibly rapid rate. The carbon dioxide concentration in Earth’s atmosphere is rising the fastest it ever has in Earth’s history, and this phenomenon is expected to have major implications on Earth’s climate .

Requirements for food, materials, and energy in a world where human population is rapidly growing have created a need to increase both the amount of photosynthesis and the efficiency of converting photosynthetic output into products useful to people. One response to those needs—the so-called Green Revolution , begun in the mid-20th century—achieved enormous improvements in agricultural yield through the use of chemical fertilizers , pest and plant- disease control, plant breeding , and mechanized tilling, harvesting, and crop processing. This effort limited severe famines to a few areas of the world despite rapid population growth , but it did not eliminate widespread malnutrition . Moreover, beginning in the early 1990s, the rate at which yields of major crops increased began to decline. This was especially true for rice in Asia. Rising costs associated with sustaining high rates of agricultural production, which required ever-increasing inputs of fertilizers and pesticides and constant development of new plant varieties, also became problematic for farmers in many countries.

A second agricultural revolution , based on plant genetic engineering , was forecast to lead to increases in plant productivity and thereby partially alleviate malnutrition. Since the 1970s, molecular biologists have possessed the means to alter a plant’s genetic material (deoxyribonucleic acid, or DNA ) with the aim of achieving improvements in disease and drought resistance, product yield and quality, frost hardiness, and other desirable properties. However, such traits are inherently complex, and the process of making changes to crop plants through genetic engineering has turned out to be more complicated than anticipated. In the future such genetic engineering may result in improvements in the process of photosynthesis, but by the first decades of the 21st century, it had yet to demonstrate that it could dramatically increase crop yields.

Another intriguing area in the study of photosynthesis has been the discovery that certain animals are able to convert light energy into chemical energy. The emerald green sea slug ( Elysia chlorotica ), for example, acquires genes and chloroplasts from Vaucheria litorea , an alga it consumes, giving it a limited ability to produce chlorophyll . When enough chloroplasts are assimilated , the slug may forgo the ingestion of food. The pea aphid ( Acyrthosiphon pisum ) can harness light to manufacture the energy-rich compound adenosine triphosphate (ATP); this ability has been linked to the aphid’s manufacture of carotenoid pigments.

General characteristics

The study of photosynthesis began in 1771 with observations made by the English clergyman and scientist Joseph Priestley . Priestley had burned a candle in a closed container until the air within the container could no longer support combustion . He then placed a sprig of mint plant in the container and discovered that after several days the mint had produced some substance (later recognized as oxygen) that enabled the confined air to again support combustion. In 1779 the Dutch physician Jan Ingenhousz expanded upon Priestley’s work, showing that the plant had to be exposed to light if the combustible substance (i.e., oxygen) was to be restored. He also demonstrated that this process required the presence of the green tissues of the plant.

In 1782 it was demonstrated that the combustion-supporting gas (oxygen) was formed at the expense of another gas, or “fixed air,” which had been identified the year before as carbon dioxide. Gas-exchange experiments in 1804 showed that the gain in weight of a plant grown in a carefully weighed pot resulted from the uptake of carbon, which came entirely from absorbed carbon dioxide, and water taken up by plant roots; the balance is oxygen, released back to the atmosphere. Almost half a century passed before the concept of chemical energy had developed sufficiently to permit the discovery (in 1845) that light energy from the sun is stored as chemical energy in products formed during photosynthesis.

This equation is merely a summary statement, for the process of photosynthesis actually involves numerous reactions catalyzed by enzymes (organic catalysts ). These reactions occur in two stages: the “light” stage, consisting of photochemical (i.e., light-capturing) reactions; and the “dark” stage, comprising chemical reactions controlled by enzymes . During the first stage, the energy of light is absorbed and used to drive a series of electron transfers, resulting in the synthesis of ATP and the electron-donor-reduced nicotine adenine dinucleotide phosphate (NADPH). During the dark stage, the ATP and NADPH formed in the light-capturing reactions are used to reduce carbon dioxide to organic carbon compounds. This assimilation of inorganic carbon into organic compounds is called carbon fixation.

Van Niel’s proposal was important because the popular (but incorrect) theory had been that oxygen was removed from carbon dioxide (rather than hydrogen from water, releasing oxygen) and that carbon then combined with water to form carbohydrate (rather than the hydrogen from water combining with CO 2 to form CH 2 O).

By 1940 chemists were using heavy isotopes to follow the reactions of photosynthesis. Water marked with an isotope of oxygen ( 18 O) was used in early experiments. Plants that photosynthesized in the presence of water containing H 2 18 O produced oxygen gas containing 18 O; those that photosynthesized in the presence of normal water produced normal oxygen gas. These results provided definitive support for van Niel’s theory that the oxygen gas produced during photosynthesis is derived from water.

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Photosynthesis

Reviewed by: BD Editors

Photosynthesis Definition

Photosynthesis is the biochemical pathway which converts the energy of light into the bonds of glucose molecules. The process of photosynthesis occurs in two steps. In the first step, energy from light is stored in the bonds of adenosine triphosphate (ATP), and nicotinamide adenine dinucleotide phosphate (NADPH). These two energy-storing cofactors are then used in the second step of photosynthesis to produce organic molecules by combining carbon molecules derived from carbon dioxide (CO 2 ). The second step of photosynthesis is known as the Calvin Cycle. These organic molecules can then be used by mitochondria to produce ATP, or they can be combined to form glucose, sucrose, and other carbohydrates. The chemical equation for the entire process can be seen below.

Photosynthesis Equation

Above is the overall reaction for photosynthesis. Using the energy from light and the hydrogens and electrons from water, the plant combines the carbons found in carbon dioxide into more complex molecules. While a 3-carbon molecule is the direct result of photosynthesis, glucose is simply two of these molecules combined and is often represented as the direct result of photosynthesis due to glucose being a foundational molecule in many cellular systems. You will also notice that 6 gaseous oxygen molecules are produced, as a by-produce. The plant can use this oxygen in its mitochondria during oxidative phosphorylation . While some of the oxygen is used for this purpose, a large portion is expelled into the atmosphere and allows us to breathe and undergo our own oxidative phosphorylation, on sugar molecules derived from plants. You will also notice that this equation shows water on both sides. That is because 12 water molecules are split during the light reactions, while 6 new molecules are produced during and after the Calvin cycle. While this is the general equation for the entire process, there are many individual reactions which contribute to this pathway.

Stages of Photosynthesis

The light reactions.

The light reactions happen in the thylakoid membranes of the chloroplasts of plant cells. The thylakoids have densely packed protein and enzyme clusters known as photosystems . There are two of these systems, which work in conjunction with each other to remove electrons and hydrogens from water and transfer them to the cofactors ADP and NADP + . These photosystems were named in the order of which they were discovered, which is opposite of how electrons flow through them. As seen in the image below, electrons excited by light energy flow first through photosystem II (PSII), and then through photosystem I (PSI) as they create NADPH. ATP is created by the protein ATP synthase , which uses the build-up of hydrogen atoms to drive the addition of phosphate groups to ADP.

The entire system works as follows. A photosystem is comprised of various proteins that surround and connect a series of pigment molecules . Pigments are molecules that absorb various photons, allowing their electrons to become excited. Chlorophyll a is the main pigment used in these systems, and collects the final energy transfer before releasing an electron. Photosystem II starts this process of electrons by using the light energy to split a water molecule, which releases the hydrogen while siphoning off the electrons. The electrons are then passed through plastoquinone, an enzyme complex that releases more hydrogens into the thylakoid space . The electrons then flow through a cytochrome complex and plastocyanin to reach photosystem I. These three complexes form an electron transport chain , much like the one seen in mitochondria. Photosystem I then uses these electrons to drive the reduction of NADP + to NADPH. The additional ATP made during the light reactions comes from ATP synthase, which uses the large gradient of hydrogen molecules to drive the formation of ATP.

The Calvin Cycle

With its electron carriers NADPH and ATP all loaded up with electrons, the plant is now ready to create storable energy. This happens during the Calvin Cycle , which is very similar to the citric acid cycle seen in mitochondria. However, the citric acid cycle creates ATP other electron carriers from 3-carbon molecules, while the Calvin cycle produces these products with the use of NADPH and ATP. The cycle has 3 phases, as seen in the graphic below.

During the first phase, a carbon is added to a 5-carbon sugar, creating an unstable 6-carbon sugar. In phase two, this sugar is reduced into two stable 3-carbon sugar molecules. Some of these molecules can be used in other metabolic pathways, and are exported. The rest remain to continue cycling through the Calvin cycle. During the third phase, the five-carbon sugar is regenerated to start the process over again. The Calvin cycle occurs in the stroma of a chloroplast. While not considered part of the Calvin cycle, these products can be used to create a variety of sugars and structural molecules.

Products of Photosynthesis

The direct products of the light reactions and the Calvin cycle are 3-phosphoglycerate and G3P, two different forms of a 3-carbon sugar molecule. Two of these molecules combined equals one glucose molecule, the product seen in the photosynthesis equation. While this is the main food source for plants and animals, these 3-carbon skeletons can be combined into many different forms. A structural form worth note is cellulose , and extremely strong fibrous material made essentially of strings of glucose. Besides sugars and sugar-based molecules, oxygen is the other main product of photosynthesis. Oxygen created from photosynthesis fuels every respiring organism on the planet.

Lodish, H., Berk, A., Kaiser, C. A., Krieger, M., Scott, M. P., Bretscher, A., . . . Matsudaira, P. (2008). Molecular Cell Biology 6th. ed . New York: W.H. Freeman and Company. Nelson, D. L., & Cox, M. M. (2008). Principles of Biochemistry . New York: W.H. Freeman and Company.

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What is photosynthesis?

Photosynthesis is the process plants, algae and some bacteria use to turn sunlight, carbon dioxide and water into sugar and oxygen.

• Photosynthetic processes
• Photosynthesis equation
• The carbon exchange
• How do plants absorb sunlight?

How does photosynthesis start?

• Location of photosynthesis

Light-dependent reactions

• The Calvin cycle

Types of photosynthesis

Additional resources.

Photosynthesis is the process used by plants, algae and some bacteria to turn sunlight into energy. The process chemically converts carbon dioxide (CO2) and water into food (sugars) and oxygen . The chemical reaction often relies on a pigment called chlorophyll, which gives plants their green color.  Photosynthesis is also the reason our planet is blanketed in an oxygen-rich atmosphere.

Types of photosynthetic processes

There are two types of photosynthesis: oxygenic and anoxygenic. They both follow very similar principles, but the former is the most common and is seen in plants, algae and cyanobacteria. 

During oxygenic photosynthesis, light energy transfers electrons from water (H2O) taken up by plant roots to CO2 to produce carbohydrates . In this transfer, the CO2 is "reduced," or receives electrons, and the water is "oxidized," or loses electrons. Oxygen is produced along with carbohydrates.

This process creates a balance on Earth, in which the carbon dioxide produced by breathing organisms as they consume oxygen in respiration is converted back into oxygen by plants, algae and bacteria.

Anoxygenic photosynthesis, meanwhile, uses electron donors that are not water and the process does not generate oxygen, according to "Anoxygenic Photosynthetic Bacteria" by LibreTexts . The process typically occurs in bacteria such as green sulfur bacteria and phototrophic purple bacteria. 

The Photosynthesis equation

Though both types of photosynthesis are complex, multistep affairs, the overall process can be neatly summarized as a chemical equation.

The oxygenic photosynthesis equation is: 

6CO2 + 12H2O + Light Energy → C6H12O6 + 6O2 + 6H2O

Here, six molecules of carbon dioxide (CO2) combine with 12 molecules of water (H2O) using light energy. The end result is the formation of a single carbohydrate molecule (C6H12O6, or glucose) along with six molecules each of oxygen and water.

Similarly, the various anoxygenic photosynthesis reactions can be represented as a single generalized formula:

CO2 + 2H2A + Light Energy → [CH2O] + 2A + H2O

The letter A in the equation is a variable, and H2A represents the potential electron donor. For example, "A" may represent sulfur in the electron donor hydrogen sulfide (H2S), according to medical and life sciences news site News Medical Life Sciences . 

How is carbon dioxide and oxygen exchanged?

Plants absorb CO2 from the surrounding air and release water and oxygen via microscopic pores on their leaves called stomata. 

When stomata open, they let in CO2; however, while open, the stomata release oxygen and let water vapor escape. Stomata close to prevent water loss, but that means the plant can no longer gain CO2 for photosynthesis. This tradeoff between CO2 gain and water loss is a particular problem for plants growing in hot, dry environments. 

How do plants absorb sunlight for photosynthesis?

Plants contain special pigments that absorb the light energy needed for photosynthesis.

Chlorophyll is the primary pigment used for photosynthesis and gives plants their green color, according to science education site Nature Education . Chlorophyll absorbs red and blue light and reflects green light. Chlorophyll is a large molecule and takes a lot of resources to make; as such, it breaks down towards the end of the leaf's life, and most of the pigment's nitrogen (one of the building blocks of chlorophyll) is resorbed back into the plant,  When leaves lose their chlorophyll in the fall, other leaf pigments such as carotenoids and anthocyanins begin to show. While carotenoids primarily absorb blue light and reflect yellow, anthocyanins absorb blue-green light and reflect red light, according to Harvard University's The Harvard Forest .

Related: What if humans had photosynthetic skin?

Pigment molecules are associated with proteins, which allow them the flexibility to move toward light and toward one another. A large collection of 100 to 5,000 pigment molecules constitutes an "antenna," according to an article by Wim Vermaas , a professor at Arizona State University. These structures effectively capture light energy from the sun, in the form of photons.

The situation is a little different for bacteria. While cyanobacteria contain chlorophyll, other bacteria, for example, purple bacteria and green sulfur bacteria, contain bacteriochlorophyll to absorb light for anoxygenic photosynthesis, according to " Microbiology for Dummies " (For Dummies, 2019). 

It was previously hypothesized that just a small number of photons would be needed to kickstart photosynthesis, but researchers never successfully observed this first step. However, in 2023, scientists discovered that photosynthesis appears to begin with a single photon. 

The researchers set up an experiment where a photon source spat out two photons at a time. One was absorbed by a detector, while the other hit a bacteria's chloroplast equivalent. When the second photon hit, photosynthesis began. 

After performing the test over 1.5 million times, the researchers confirmed that just one photon is needed to start photosynthesis.

Where in the plant does photosynthesis take place?

Photosynthesis occurs in chloroplasts, a type of plastid (an organelle with a membrane) that contains chlorophyll and is primarily found in plant leaves. 

Chloroplasts are similar to mitochondria , the energy powerhouses of cells, in that they have their own genome, or collection of genes, contained within circular DNA. These genes encode proteins that are essential to the organelle and to photosynthesis.

Inside chloroplasts are plate-shaped structures called thylakoids that are responsible for harvesting photons of light for photosynthesis, according to the biology terminology website Biology Online . The thylakoids are stacked on top of each other in columns known as grana. In between the grana is the stroma — a fluid containing enzymes, molecules and ions, where sugar formation takes place. 

Ultimately, light energy must be transferred to a pigment-protein complex that can convert it to chemical energy, in the form of electrons. In plants, light energy is transferred to chlorophyll pigments. The conversion to chemical energy is accomplished when a chlorophyll pigment expels an electron, which can then move on to an appropriate recipient. 

The pigments and proteins that convert light energy to chemical energy and begin the process of electron transfer are known as reaction centers.

When a photon of light hits the reaction center, a pigment molecule such as chlorophyll releases an electron.

The released electron escapes  through a series of protein complexes linked together, known as an electron transport chain. As it moves through the chain, it generates the energy to produce ATP (adenosine triphosphate, a source of chemical energy for cells) and NADPH — both of which are required in the next stage of photosynthesis in the Calvin cycle. The "electron hole" in the original chlorophyll pigment is filled by taking an electron from water. This splitting of water molecules releases oxygen into the atmosphere.

Light-independent reactions: The Calvin cycle

The Calvin cycle is the three-step process that generates sugars for the plant, and is named after Melvin Calvin , the Nobel Prize -winning scientist who discovered it decades ago. The Calvin cycle uses the ATP and NADPH produced in chlorophyll to generate carbohydrates. It takes plate in the plant stroma, the inner space in chloroplasts.

In the first step of this cycle, called carbon fixation, an enzyme called RuBP carboxylase/oxygenase, also known as rubiso, helps incorporate CO2 into an organic molecule called 3-phosphoglyceric acid (3-PGA). In the process, it breaks off a phosphate group on six ATP molecules to convert them to ADP, releasing energy in the process, according to LibreTexts.

In the second step, 3-PGA is reduced, meaning it takes electrons from six NADPH molecules and produces two glyceraldehyde 3-phosphate (G3P) molecules.

One of these G3P molecules leaves the Calvin cycle to do other things in the plant. The remaining G3P molecules go into the third step, which is regenerating rubisco. In between these steps, the plant produces glucose, or sugar.

Three CO2 molecules are needed to produce six G3P molecules, and it takes six turns around the Calvin cycle to make one molecule of carbohydrate, according to educational website Khan Academy.

There are three main types of photosynthetic pathways: C3, C4 and CAM. They all produce sugars from CO2 using the Calvin cycle, but each pathway is slightly different.

C3 photosynthesis

Most plants use C3 photosynthesis, according to the photosynthesis research project Realizing Increased Photosynthetic Efficiency (RIPE) . C3 plants include cereals (wheat and rice), cotton, potatoes and soybeans. This process is named for the three-carbon compound 3-PGA that it uses during the Calvin cycle. 

C4 photosynthesis

Plants such as maize and sugarcane use C4 photosynthesis. This process uses a four-carbon compound intermediate (called oxaloacetate) which is converted to malate , according to Biology Online. Malate is then transported into the bundle sheath where it breaks down and releases CO2, which is then fixed by rubisco and made into sugars in the Calvin cycle (just like C3 photosynthesis). C4 plants are better adapted to hot, dry environments and can continue to fix carbon even when their stomata are closed (as they have a clever storage solution), according to Biology Online. 

CAM photosynthesis

Crassulacean acid metabolism (CAM) is found in plants adapted to very hot and dry environments, such as cacti and pineapples, according to the Khan Academy. When stomata open to take in CO2, they risk losing water to the external environment. Because of this, plants in very arid and hot environments have adapted. One adaptation is CAM, whereby plants open stomata at night (when temperatures are lower and water loss is less of a risk). According to the Khan Academy, CO2 enters the plants via the stomata and is fixed into oxaloacetate and converted into malate or another organic acid (like in the C4 pathway). The CO2 is then available for light-dependent reactions in the daytime, and stomata close, reducing the risk of water loss. 

Discover more facts about photosynthesis with the educational science website sciencing.com . Explore how leaf structure affects photosynthesis with The University of Arizona . Learn about the different ways photosynthesis can be measured with the educational science website Science & Plants for Schools .  

This article was updated by Live Science managing editor Tia Ghose on Nov. 3, 2022.

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Daisy Dobrijevic joined  Space.com  in February 2022 as a reference writer having previously worked for our sister publication  All About Space  magazine as a staff writer. Before joining us, Daisy completed an editorial internship with the BBC Sky at Night Magazine and worked at the  National Space Centre  in Leicester, U.K., where she enjoyed communicating space science to the public. In 2021, Daisy completed a PhD in plant physiology and also holds a Master's in Environmental Science, she is currently based in Nottingham, U.K.

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Photosynthesis

Photosynthesis n., plural: photosyntheses [ˌfŏʊ.ɾoʊ.ˈsɪn̪.θə.sɪs] Definition: the conversion of light energy into chemical energy by photolithorophs

Table of Contents

Photosynthesis is a physio-chemical process carried out by photo-auto-lithotrophs by converting light energy into chemical energy . Among the endless diversity of living organisms in the world, producers are a unique breed.

Unlike consumers ( herbivores , carnivores , omnivores , or decomposers ) that rely upon other living organisms for their nutritional requirements and nourishment, producers have been distinguished by their ability to synthesize their own food. This is the reason that we call producers “autotrophic or self-reliable” in nature while consumers of all the different categories are called “heterotrophic or dependent” in nature.

Now among producers, there are different categories of producers, i.e. different mechanisms via which they produce their own food.

• Photo-auto-litho-trophs: Since these organisms tend to derive their nutrition by channeling the sun’s light energy, they are termed phototrophic in nature. Also, since they utilize inorganic carbon and translate it into organic carbon atoms, i.e. their means of deriving food becomes autotrophic. Additionally, since the source of electrons (electron donors) here are inorganic compounds, they are specified as lithotrophic . In totality, they can be called photo-auto-litho-trophic in nature. Example : Green plants are nature’s brilliant entities that come under this category. They carry out a photosynthesis cycle by taking in carbon dioxide and fixing it into carbohydrates (energy storage molecule). Some of them also give out oxygen gas that’s vital for the other life forms to survive in the earth’s atmosphere.
• Chemo-auto-lithotrophs: Many of us might be unaware of the fact that there are some autotrophs that don’t utilize sunlight. Rather they derive their energy stored from a different energy source like oxidation of inorganic compounds.

The scope of today’s discussion is limited to photosynthesis and photoautotrophs. So, let’s get started and get to know the answers to these common questions: what is the photosynthesis process, what are the 3 stages of photosynthesis, what does photosynthesis produce, what is a byproduct of photosynthesis, what is the purpose of photosynthesis, is photosynthesis a chemical change, the various inputs and outputs of photosynthesis, which organisms perform photosynthesis , and many other more questions!!!

What is Photosynthesis?

Photosynthesis definition: Photosynthesis is a physio-chemical process carried out by photo-auto-lithotrophs . In simpler language, photosynthesis is the process by which green plants convert light energy into ‘chemical energy’.

This energy transformation is only possible due to the presence of the miraculous pigment molecule chlorophyll in photosynthesis. The chemical energy as referred to before is the fixed carbon molecules generated during photosynthesis.

Green plants and algae have the ability to utilize carbon dioxide molecules and water and produce food (carbohydrates) for all life forms on Earth. There’s no doubt in the fact that life is impossible and unimaginable without green plants that photosynthesize and sustain the cycles of life.

Let’s give you a brief outline of the topic before we head forward.

• Etymology: The photosynthesis process finds its origin in 2 Greek words, firsts one being “phōs (φῶς)” meaning ‘light’ and the second one being “sunthesis (σύνθεσις)” meaning ‘putting together’ . The process of photosynthesis aids the conversion of light energy to chemical energy in varied forms of carbohydrate molecules like sugar molecules and starches.
• Organisms that perform photosynthesis: The organisms are called photo-auto-litho-trophs or simply photoautotrophs.
• Atmospheric gas consumed: Photosynthesizing organisms utilize carbon dioxide in photosynthesis (CO 2 ).
• Atmospheric gas released by “some” photosynthetic organisms (MIND IT-Not all): Some photosynthesizing organisms convert carbon dioxide and aid the process of producing oxygen gas (O2).
• Examples of photosynthesizing organisms: Green plants, cyanobacteria (earlier termed as blue-green algae), and different types of algae that essentially carry out phytoplankton photosynthesis.
• Why is photosynthesis important? The important function of photosynthesis: Food supply for the organisms on Earth, Oxygen supply for the survival of all organisms.
• Site of photosynthesis: Leaves and green tissues. (So when asked where photosynthesis takes place, we can tell that it is this site.)
• What are the reactants of photosynthesis: Carbon dioxide molecules + Water molecules + Light energy
• Products of photosynthesis: Fixed carbon (carbohydrates) + Oxygen (some cases) + Water

Watch this vid about photosynthesis:

Biology Definition: Photosynthesis is the synthesis of complex organic material using carbon dioxide , water , inorganic salts , and light energy (from sunlight) captured by light-absorbing pigments , such as chlorophyll and other accessory pigments . Photosynthesis may basically be simplified via this equation: 6CO 2 +12H 2 O+energy=C 6 H 12 O 6 +6O 2 +6H 2 O, wherein carbon dioxide (CO 2 ), water (H 2 O), and light energy are utilized to synthesize an energy-rich carbohydrate like glucose (C 6 H 12 O 6 ). Other products are water and oxygen .

• Photosynthesis occurs in plastids (e.g. chloroplasts ), which are membrane-bounded organelles containing photosynthetic pigments (e.g. chlorophyll ), within the cells of plants and algae .
• In photosynthetic bacteria ( cyanobacteria ) that do not have membrane-bounded organelles, photosynthesis occurs in the thylakoid membranes in the cytoplasm .

Etymology: from the Greek photo-, “light”, and synthesis, “putting together” Related forms: photosynthetic (adjective) Compare: chemosynthesis See also: photoautotroph

Types of Photosynthesis

Plant photosynthesis and photosynthetic organisms can be classified under different categories on the basis of some characteristic features. They are:

• Types of organisms that carry out photosynthesis on the basis of “cellular structure” Both prokaryotic and eukaryotic organisms carry out photosynthesis.
• Photosynthetic prokaryotes: for example, cyanobacteria
• Eukaryotic: for example, protists ( diatoms , dinoflagellates , Euglena) and green plants. In particular, algae photosynthesis can be observed in green algae , red algae , brown algae , & land plants, like bryophytes , pteridophytes, gymnosperms , and angiosperms .
• Prokaryotic ONLY (anoxygenic photosynthetic bacteria, green sulfur bacteria and purple bacteria)

Photosynthesis: a two-stage process

Photosynthesis is an example of a metabolic process with 2 stages. Both the stages need light (direct or indirect sunlight). Hence, the long-claimed notion of the 2 processes being ‘absolute LIGHT and DARK reactions’ isn’t apt.

Scientific studies have pointed out that even the 2nd stage of photosynthesis requires indirect sunlight. Therefore, rather than classifying the stages as light and dark photosynthesis reactions, we’ll like to classify the 2 stages as follows:

• Photochemical Reaction Process: Light energy is converted to ATP ; photophosphorylation process (light-dependent reactions)
• Through Calvin cycle: In oxygenic photosynthesis as well as anoxygenic photosynthesis
• Through Non-Calvin cycle: Only is some anoxygenic photosynthesis

Evolution of Photosynthesis Process

It is postulated that the very first photosynthetic beings and photosynthesis evolved quite early down the evolutionary timescale of life.

It is also believed that the first photosynthetic beings would have initially resorted to other available reducing agents like hydrogen ions or hydrogen sulfide in contrast to the modern-day photosynthetic organisms that utilize water as the “prime and only sources of electrons”.

It is believed that cyanobacteria would have appeared on the surface of Earth much later than the first photosynthetic beings. Once appeared they must have saturated the Earth’s atmosphere with oxygen gas and led to its oxygenation. Only after the Earth was oxygenated, the more complex forms of life would have later evolved.

When we compare photosynthesis to other metabolic processes like respiration, we can clearly notice that these two processes are almost opposite to each other. But another point to note is that both the processes in synchrony sustain life on Earth.

You cannot separate respiration from photosynthesis or photosynthesis from respiration and expect life to run normally. It is not possible that way. Let’s try to compare and list some characteristic features of photosynthesis and cellular respiration processes.

Photosynthesis vs. Respiration

• Photosynthesis: Anabolic process
• Cellular respiration: Catabolic process By anabolic, we mean the photosynthesis process “utilizes energy to build biomolecules” like carbohydrates, starch, and sugars. These biomolecules are further utilized by both the plants and the organisms dependent on plants for their nutritional needs. On the other hand, respiration is a catabolic process. This energy is utilized to break down complex molecules to derive nutrition out of them.
• Photosynthesis: In the chloroplasts of the eukaryotic phototrophic cells.
• Respiration: Primarily in the mitochondria of the cell.
• Photosynthesis: Carbon dioxide molecules + Water molecules + Light energy
• Respiration: Glucose + Oxygen
• Photosynthesis: Fixed carbon (carbohydrates) + Oxygen (some cases) + Water
• Respiration: Carbon dioxide + Water +energy (ATP)
• Photosynthesis: Endergonic and endothermic
• Respiration: Exergonic and exothermic Just note that these terms endergonic and endothermic both convey the same meaning of “absorbing heat”. And the terms exergonic and exothermic also convey the same meaning of “releasing heat”. The only difference is that –gonics relates to “the relative change in the free energy of the system” while –thermic relates to “the relative change in enthalpy of the system”.
• Photosynthesis: 6CO 2 + 6H 2 O → C6H 12 O 6 + 6O 2
• Respiration: C 6 H 12 O 6 6 + 6O 2 → 6CO 2 + 6H 2 O

Photosynthetic Membranes and Organelles

When we begin the discussion on this topic, it’s important that we know that no photosynthesis is possible without the pigment molecules that absorb light. The absorption of sunlight is the most vital step of photosynthesis.

We should also note that the energy of photons is different for every light of different wavelengths. And the energy needed for the photosynthesis to be conducted is of “a very specific wavelength range”.

For the absorption of lights of desired wavelengths, phototrophs organize their pigment molecules in the form of reaction center proteins . These proteins are located in the membranes of the organisms. Let’s learn how these pigment molecules reside inside the organism and how they make the membranes photosynthetic in nature.

• Prokaryotic photosynthetic organisms: These organisms have their pigment systems or photosystems located in the cell membranes or the thylakoid membranes in the cytosol itself. There are no special organelles called chloroplasts in the prokaryotes.
• Eukaryotic photosynthetic organisms (like green plants): These organisms have their pigment systems or photosystems located in the thylakoids of the chloroplast membranes. Eukaryotes have specialized organelles called chloroplasts (chlorophyll-containing plastids) in their cells.

Photosynthetic Pigments

There are 2 types of photosynthetic pigments in the oxygenic photosynthesizing organisms . They are as follows:

• Porphyrin-derivatives (Chlorophyll in plants and Phycobilin)

Carotenoids

Chlorophyll.

Chlorophyll is the green-colored pigment essential for photosynthesis. Let’s try to list its major characteristic features and roles of it.

• Nature: Lipid
• Location: Embedded in the thylakoid membrane
• Types: 9 types as identified by Arnoff and Allen in 1966 (chlorophyll-a, b, c, d, e, bacteriochlorophyll a, b, chlorobium chlorophyll-650,666). Bacteriochlorophylls are present in the anoxygenic photosynthetic organisms.
• Primary photosynthetic pigment: Chlorophyll-a
• Presence: In all oxygenic photosynthetic organisms
• Absorption range: Visible (blue and red) and IR (Infra-red)
• Ion important for its biological functioning: Magnesium ion (Mg 2+ )
• Structure: Chlorophyll-a, b, and d are “ chlorin ” derivatives; c is a “ porphyrin ” derivative.
• Chlorophyll Tail: Oxygenic photosynthetic organisms have a “ phytol ” tail in their chlorophyll; anoxygenic photosynthetic organisms have a “ geranyl ” tail in their bacteriochlorophylls.
• Main pigment for capturing and storing solar energy
• Photochemical reaction (chlorophyll-a is present in the photochemical reaction center i.e. PCRC. Chlorophyll a, b, c, and d play a role in resonance energy transfer.)

Carotenoid is the photosynthetic pigment essential for working in conjunction with chlorophyll. Let’s try to list its major characteristic features and roles of it.

• Nature: Lipid-soluble
• Types: More than 150
• Absorption range: 400-500nm
• Forms: Carotene (simple hydrocarbon, for example, beta carotene) and xanthophyll (oxygenated hydrocarbon, for example, lutein)
• In excitation and resonance energy transfer
• Photo-protection (work as a free-radical scavenger as well as a quencher)

Phycobilins

Phycobilins aren’t present in all the oxygenic photosynthetic organisms. They have a tetrapyrrole structure (no need for magnesium ion).

• Types: Phycoerythrobilin, Phycocyanobilins, Allophycocyanobilins When these pigment molecules combine with a water-soluble protein, they form the pigment-protein complex (phycobiliproteins, like phycoerythrin and phycocyanin).
• Location: Since these phycobiliproteins are water-soluble, they can’t exist in the membranes like chlorophyll and carotenoids. Therefore, phycobilin pigments as their pigment-protein complex aggregate into clusters and adhere to the membrane. These clusters are called phycobilisomes .
• Exceptional Note: These are the only pigments that are associated with protein molecules.
• Role: Resonance energy transfer

Organelle for Photosynthesis

What is chloroplast? In eukaryotes, photosynthesis occurs in chloroplasts as they are the designated organelles for the photosynthesis process. There are nearly 10-100 chloroplasts in a typical plant cell .

Inside chloroplasts are the thylakoids; the very specific site for the light capturing. The structure of this very unique part of the chloroplasts is briefly discussed here.

Thylakoid is a membrane-bound compartment in the chloroplasts of eukaryotic organisms. They are also present as such in the cytosol of cyanobacteria (cyanobacteria don’t have chloroplasts but they have simply thylakoids).

These thylakoids are the “primary site of the 1st stage of photosynthesis. i.e. “photochemical reaction” or popularly called “light-dependent reactions of photosynthesis”. The main components of the thylakoid are membrane, lumen, and lamellae. The chlorophyll molecules are present inside these thylakoid membranes.

Light-dependent Reactions

The first stage of photosynthesis is popularly called “light-dependent reactions” . We choose to call this stage the “1st stage: PHOTOCHEMICAL REACTION STAGE”. It is also called the “thylakoid reaction stage” or “hill’s reaction” .

This stage is marked by 3 essential steps of photosynthesis: Oxidation of water , reduction of NADP + , and ATP formation . The site where these reactions occur is the lamellar part of the chloroplast. The units of light-dependent reactions are quantosomes .

Let’s discuss this stage under some subheadings:

Wavelengths of light involved and their absorption

The white light that reaches Earth has subparts of different wavelengths together constituting the visible spectrum (390-760nm). But the photosynthetic organisms specifically use a subpart called PAR ( P hotosynthetically A ctive R adiation).

PAR ranges from 400-760nm. Blue light is 470-500nm while red light is 660-760nm). The green light (500-580nm) is reflected back by the plants and this is the reason that plants appear green in color. Blue-green light is not used, only blue light is used.

Absorption spectrum and action spectrum

• Absorption Spectrum: This is a pigment-specific entity or terminology. To find the absorption spectrum of a pigment, you need to plot “the amount of absorption of different wavelengths of light by that particular pigment” . The graph has the “wavelengths of light (in nanometers/nm)” on the X-axis and the “percentage of light absorption” on the Y-axis.
• Action Spectrum: To find the action spectrum of a pigment, you need to plot the “effectiveness of the different wavelengths of light in stimulating photosynthesis process” . The graph has the “wavelengths of light (in nanometers/nm)” on the X-axis and the “rate of photosynthesis (measured as oxygen released)” on the Y-axis. When you superimpose the action spectrum of photosynthesis with the absorption spectrum of the specific pigment, you can find the contribution of each different wavelength in the photosynthesis rate, photosynthetic efficiency, and photosynthetic productivity.

IMPORTANT NOTE: The absorption spectrum is calculated for any of the many pigments involved in photosynthesis. Contrastingly, the action spectrum is calculated only for the photochemical reaction performing pigment i.e. chlorophyll-a present at the reaction center. We identify the progress of photochemical reactions as the “evolution of oxygen gas” that primarily happens at the reaction center where only chlorophyll-a is present. Since the action is directly correlated to the specific excitation of chlorophyll-a molecule, the action spectrum is scientifically calculated only for this chlorophyll-a.

• Absorption spectrum of chlorophyll- a : 430 nm (blue), 660nm (red) {more absorbance at 660 nm)
• Absorption spectrum of chlorophyll-b: 430 nm (blue), 660nm (red) {more absorbance at 430 nm)

What actually happens in the Light-dependent reaction

Let’s briefly describe what actually happens here.

• 1 photon is absorbed by 1 molecule of the chlorophyll (P680) and simultaneously 1 electron is lost here.
• The electron flow of the photochemical reaction begins here.
• The electron is transferred to D1/D2 protein, then to a modified form of chlorophyll and “pheophytin”.
• After that, it’s transferred to plastoquinone A and then B.
• Initiates an electron flow down an electron transport chain.
• Ultimately aids the NADP reduction to NADPH.
• Creation of a proton gradient across the chloroplast membrane.
• Further on this proton gradient is exploited by the ATP synthase for the generation of ATP molecules.

Water photolysis

Now, if you are wondering how the first electron lost by the 1st chlorophyll is replenished to keep this cycle going, read on. The answer to this query is “photolysis of water molecules” . The chlorophyll molecule regains the lost electron when the “oxygen-evolving complex” in the thylakoid membrane carries out the photolysis of water. The chlorophyll molecule ultimately regains the electron it lost when a water molecule is split in a process called photolysis, which releases oxygen.

Many scientists had a doubt about the source of oxygen in photosynthesis. Some speculated the oxygen atom of the CO 2 gas is the source of oxygen post-photosynthesis. But it was the collective contribution of some 4 scientists that gave clarity on this topic.

C.B. Van Niel worked on purple photosynthetic bacteria ( Chromatium vinosum ) and found out that the source of oxygen is the oxidation of water molecules (‘indirect evidence’). While Ruben, Hassid, and Kamen carried out an isotopic study that gave ‘direct evidence’ of oxygen-evolving from H 2 O molecules and not CO 2 molecules.

Hydrolysis of 2 molecules of water leads to the evolution of 1 molecule of oxygen gas. The photosynthesis equation for light-dependent reactions (non-cyclic electron flow) or the chemical formula for photosynthesis:

2 H 2 O + 2 NADP+ + 3 ADP + 3 Pi + light → 2 NADPH + 2 H+ + 3 ATP + O 2

The photochemical reaction (or the light-dependent reactions) can be classified as:

• Cyclic reaction: Only 1 photosystem ( PS1 ) is involved. (Photon excites P700 in PS1, electron reaches Fe-S, then Ferredoxin, then Plastoquinone and then Cyt b6f complex and then Plastocyanin). Since in the solo involvement of PS1 here, the electron flow becomes cyclic. And this phosphorylation process is called cyclic phosphorylation. It happens in the stroma lamellae when light beyond 680nm is available.
• Non-cyclic reaction: Both photosystems (PS1 and PS2 ) are involved. (Photon excites P680 in PS2, the electron is lost and transferred to pheophytin, then sent on a roller coaster (Z-scheme). Within the z-scheme, the final redox reaction enables the reduction of NADP+ to NADPH. And the chemiosmotic potential generation via proton pumping proton across the membrane and into the thylakoid lumen ensures ATP synthesis.

Data Source: Akanksha Saxena of Biology Online

Light-Independent Reactions (Carbon-fixation Reaction)

Also called the carbon fixation process, the “light-independent reactions” is a misnomer as Science has now already proved that the second stage of photosynthesis isn’t really light-independent reactions. Though it doesn’t need direct light, indirect light is involved even in this process. We choose to label this stage of photosynthesis as the “2nd stage: CARBON-FIXATION REACTION STAGE ”, which is also called:

• Calvin Cycle or “stromal reaction” as it manifests in the stroma part of the chloroplast
• “C3 Cycle” or the “reductive pentose phosphate cycle”

Calvin cycle

The inputs for the Calvin cycle  in most plants come from the previously occurred photochemical reaction. In this cycle, the carbon dioxide produced is fixed to a glucose molecule. To be very specific, the Calvin cycle directly doesn’t produce glucose, rather it produces glyceraldehydes-5-phosphate (G-3-P). Glucose is formed after these G-3-P molecules move into the cytosol from the chloroplast .

It consists of primarily 3 steps as follows:

• Carboxylation: Acceptance of CO 2 by RuBP which is a 5-carbon compound and the CO2-acceptor). 2 molecules of 3-phosphoglycerate are generated as the result of the carboxylation process.
• Reduction: Generation of 3C/4C/5C/6C/7C molecules.
• Regeneration of RUBP: 3 molecules of RuBP are regenerated.

In totality, 3 molecules of CO 2 produce 1 molecule of G-3-P. This uses 9 ATPs and 6 NADPHs. And, 6 molecules of CO 2 produce 2 molecules of G-3-P which further produce 1 molecule of glucose. This uses 18ATPs and 12 NADPHs.

The main enzyme is RuBisCo . It’s a multi-enzyme complex with 8 large and 8 small subunits. The substrates for this enzyme are CO 2 , O 2 , and RuBP. An essential ion for the biological functioning of this enzyme: Mg 2+ . The role of RuBisCo is that it captures carbon dioxide gas from the atmosphere and utilizes the NADPH from the 1st stage (photochemical reaction/light-dependent reaction stage) to fix the CO 2 .

The equation of dark reaction of photosynthesis/light-independent reaction stage/2nd stage is: 3 CO 2 + 9 ATP + 6 NADPH + 6 H + → C 3 H 6 O 3 -phosphate + 9 ADP + 8 Pi + 6 NADP+ + 3 H 2 O

The simple carbon sugars formed via the C3 cycle are utilized by the biological systems to form complex organic compounds like cellulose, precursors for amino acids synthesis and thereby proteins, precursors for lipids, and the source of fuel for respiration.

Important Point To Note: It happens in all the photosynthetic organisms as the basic carbon-fixation step.

Carbon concentrating mechanisms

There are many carbon concentrating mechanisms to increase the carbon dioxide levels and the carbon fixation process like C4, CAM, etc.

• Doesn’t happen in all photosynthetic organisms. Rather it happens in conjunction with the C3 cycle in some 4% of angiosperm families.
• Most commonly angiosperm families that witness C4 cycle: Poaceae, Cyperaceae.
• First explained by: Hatch and Slack (hence also called the Hatch and Slack cycle). They worked on the maize plant.
• Role: Endow the ability to efficiently conduct photosynthesis in plants of the semi-arid regions by making them well adapted.
• Mechanism: By separation of photosynthesis stages in 2 types of cells (mesophyll cells and bundle sheath cells). The light reaction is restricted to the mesophyll cells and the CO 2 fixation happens in the bundle sheath cells. This phenomenon is also termed as “chloroplast dimorphism” in C4 plants. The Kranz anatomy is visible here.
• Why does the need arise in the first place? – In semi-arid regions or regions with very hot and dry environmental conditions, plants are forced to close their stomata in order to limit water loss. Under such harsh conditions, the intake of CO 2 decreases during the day as the stomata are forced closed. This might lead to no CO 2 intake and hence no CO 2 fixation (2nd stage of photosynthesis). But the 1st stage of photosynthesis keeps running as it doesn’t depend on stomata opening or closure. This means that a continuous oxygen evolution happens which can lead to oxygen saturation. As we know that RuBisCo enzymes use O 2 gas as substrate too, and this can lead to an increased rate of photorespiration by the oxygenase activity of RuBisCo. This further decreases the carbon fixation. This is a very big issue if not resolved. Hence, for situations like these, carbon concentrating mechanisms have evolved in some families of plants to concentrate and enrich the CO 2 concentration in the leaves of these plants under such conditions.
• Important enzyme for CO 2 concentration: PEP carboxylase
• CO 2 is first added to a three-carbon compound called phosphoenolpyruvate (PEP) in this cycle. This leads to the formation of a four-carbon (4C)  molecule called oxaloacetic acid or malate. This step happens in the mesophyll cells of the leaves.
• After that, these 4C compounds are transferred to the bundle sheath cells where the normal C3 cycle fixes them into glucose molecules.
• This CO 2 concentrating mechanism works on the “principle of separating the RuBisCo enzyme from the O 2 -generating photochemical reactions” in order to reduce the rates of photorespiration and simultaneously increase the rates of CO 2 fixation.
• This increases the photosynthetic capacity of the leaf/leaf photosynthesis.
• When the high light and high-temperature conditions are dominant, C4 plants prove more photosynthetically efficient than C3 plants as they produce more sugar molecules in such conditions.
• Examples of C4 plants: Many crop plants like wheat, maize, rice, sorghum, millet, and sugarcane.
• Number of ATPs required: 12 (for C-enrichment) + 18 (for C-fixation)= 30 ATPS for 1 glucose production
• Number of NADPH required: 18 NADPH for 1 glucose production
• Some plants resort to another mechanism called the CAM cycle in conjunction with the C3 cycle to fix carbon dioxide.
• Examples: xerophytes like cactus photosynthesis, and most succulents.
• Around 16,000 species of plants utilize the CAM mechanism
• Mechanism: Utilize PEP carboxylase to capture carbon dioxide. In contrast to the C4 cycle where there is a “spatial separation of the 2 processes of CO 2 reduction to PEP and PEP fixation to glucose”, CAM plants display a “temporal separation of the 2 listed processes”.

Land plants display different types of photosynthesis based on their requirements and environmental constraints. They are C3, C4 +C3, and CAM+ C3 types of photosynthesis.

Aquatic plants and algae display some extra features in the photosynthetic machinery. These features further refine and define the smooth functioning and efficiency of photosynthesis.

Example: Cyanobacteria photosynthesis – cyanobacteria have carboxysomes  that help in enriching the concentration of carbon dioxide around the RuBisCO enzyme. This directly increases the photosynthetic rates. The distinguished and specially enabled enzyme in the carboxysomes is called “carbonic anhydrase”. The carbonic anhydrase possesses the ability to evolve and release CO 2 from the dissolved hydrocarbonate ions (HCO-). As soon as the CO 2 is released, RuBisCo takes care that it doesn’t go to waste.

Order and Kinetics

There are innumerable reactions and processes involved in the biological mechanism of photosynthesis. Besides the normal flow of photosynthesis, there are some plant-specific and condition-specific additional steps that further complicate the mechanism.

Since every biological mechanism has a lot of enzymes, factors, cofactors, substrates, and entities involved, photosynthesis is no different.

Let’s try to list some kinetics-specific pointers that may help.

As discussed in the overview and starting of this article, the early photosynthetic organisms must have been primarily “anoxygenic” in nature. These bacteria used some other source than water molecules as their primary electron donors. Even the geological evidence aligns with this fact as the early atmosphere of Earth was highly reducing in nature. Some speculated organisms of the early evolutionary phase are :

• Green sulfur bacteria (Electron donor= hydrogen and sulfur)
• Purple sulfur bacteria (Electron donor= hydrogen and sulfur)
• Green nonsulfur bacteria (Electron donor= various amino and other organic acids)
• Purple nonsulfur bacteria (Electron donor= variety of nonspecific organic molecules)

After this, some filamentous photosynthetic organisms are expected to have evolved. This is scaled to be an occurrence of some 3.4 billion years old timeline. It is around 2 million years ago that oxygenic photosynthesis is believed to have evolved.

The modern and more commonly known photosynthesis in plants and most of the photosynthetic prokaryotes= Oxygenic (Electron donor= Water molecules)

Symbiosis and the origin of chloroplasts

There are some animal groups that have the ability to form and establish symbiotic relationships with photosynthetic organisms. By establishing such a relationship, these organisms can directly rely upon their photosynthetic partner for energy and food requirements. Some examples of such animal groups are:

• Sea anemones
• Marine mollusks (example: Elysia viridis & Elysia chlorotica )
• Fungi photosynthesis (Lichens)

When such symbiotic relationships are established, it’s sometimes observed that some genes of the plant cell’s nucleus get transferred to the animal cell . (Observed in some slugs).

Origin of Chloroplasts

Such symbiosis is popularly claimed to be the source of chloroplast evolution. As we notice many similarities between the photosynthetic bacteria and chloroplasts, the evolution of chloroplasts is often hinted to have occurred from these bacteria. Some of the common features between the 2 are:

• Circular chromosome
• Prokaryotic-type ribosome
• A similar set of proteins in the photosynthetic reaction center

It is for all these commonalities the “ endosymbiotic theory ” had been proposed for the evolution of chloroplasts and mitochondria in the eukaryotic cells. According to the endosymbiotic theory, the early eukaryotic cells are believed to have acquired the photosynthetic bacteria by the process of endocytosis). Those early eukaryotic cells after acquiring the photosynthetic bacteria transformed to be self-sustainable and became the “first plant cells”. (Mitochondria photosynthesis is true, they are associated with respiration!)

Photosynthetic eukaryotic lineages

Photosynthetic eukaryotic lineages include:

• Glaucophytes
• Chlorophytes
• Rhodophytes
• Cryptophytes (some clades)
• Haptophytes (some clades)
• Dinoflagellates & chromerids
• Euglenids—clade Excavata (unicellular)

Cyanobacteria and the evolution of photosynthesis

Almost all the prokaryotes carry out anoxygenic photosynthesis in contrast to cyanobacteria, which perform oxygenic photosynthesis. This ability to carry out oxygenic photosynthesis is speculated to have evolved at least 2450–2320 million years ago. The first photosynthetic cyanobacteria might not have been oxygenic as Earth’s atmosphere had no oxygen then.

This topic still requires more scientific study to bring out conclusive results. From the paleontological evidence, it is claimed that the 1st cyanobacteria evolved around 2000 Ma.

For the initial years of the Earth’s oxygen-rich environment (after the oxygen-evolving mechanism evolved), cyanobacteria are claimed to be the “principal primary producers of oxygen”. Even to date, cyanobacteria have been proven vital for marine ecosystems. They’re the primary producers of oxygen in oceans.

Cyanobacteria also fix nitrogen electrons fixation and play a role in biological nitrogen cycles.

Experimental History

We will list the long experimental history in deciphering the extensive photosynthesis process through the ages.

Discovery, Refinements, and Development of the concept

Find out the discovery, refinements, and development of photosynthesis as summarized in the table below:

C3 : C4 photosynthesis research

Several studies were conducted using isotopes of radioactive elements to identify the various aspects of the photosynthetic process. A number of organisms like Chlorella , Stellaria media, Cladophora, Spirogyra, Rhodopseudomonas , sulfur bacteria, green plants like maize, etc have been used to understand the photosynthesis process over the years. Gas exchange studies, isotopic studies, light spectrum studies, radioactive studies, plant anatomical and physiological studies, studies involving roles of carbon dioxide and water, etc have all together opened the gates for our deeper understanding of this topic.

The 3 main factors that directly affect the photosynthesis process are:

• Light irradiance and wavelength
• Carbon dioxide concentration

Temperature

Although there are many more corollary factors, these 3 are the most important ones.

Light intensity (irradiance), wavelength, and temperature

Light is an essential factor for photosynthesis. It directly affects the rate of it. There are 3 different parameters that we should look into:

• Sciophytes : Grow under “diffuse” light. Example: Oxalis
• Heliophytes: Grow under “direct: light. Example: Dalbergia
• Light quality: PAR as previously discussed is the quality or the fraction of light energy that is ‘photosynthetically active’ in nature. It ranges from 400-700nm in wavelength.
• Duration of light: This parameter doesn’t affect the rate of photosynthesis but affects the total photosynthetic output.

Carbon dioxide levels and photorespiration

Carbon dioxide concentration is the major factor in determining the rate of photosynthesis. There is no carbon-dioxide enriching system in C3 plants like the C4 plants. So, if you increase the concentration of CO 2 in the system, the photosynthetic rate of C3 plants will increase as the CO 2 concentration increases. On the other hand, the photosynthetic yield of the C4 plant won’t increase in such a scenario.

• CO 2 Compensation Point: A stage in CO 2 concentration when there’s no absorption of CO 2 by the illuminated plant part.

Featuring… “The curious case of RuBisCO and PEP Carboxylase”

Imagine an equal concentration (50-50%) of the two isotopes of carbon, C-12 and C-13, in the form of 12CO 2 and 13CO 2 , made available to both C3 and C4 plants. Now, can you tell which isotope of the carbon will be fixed more or less by the two types of photosynthetic organisms? Can you guess if there would be a “preferable” isotope between the two? Do you think C3 plants will fix the 12CO 2 and 13CO 2 equally or unequally? Or do you think the 12CO 2 and 13CO 2 incorporation would have a biased ratio in any of the two (C3/C4 plants)????

The answer to this lies in the major carbon fixing enzyme involved.

• C3 plants: Major C-fixing enzyme is RuBisCo and RuBisCo has a “discriminatory ability” to preferably fix 12CO 2 and not 13CO2. Hence, you will find more 12CO 2 fixed than 13CO 2 in the C3 plants.
• C4 plants: Major C-fixing enzyme is not RuBisCo but PEP Carboxylase . PEP Carboxylase has “no discriminatory ability”. So, you’ll find an almost equal proportion of 12CO 2 and 13CO 2 getting fixed in C4 plants. So, in comparison to C3 plants, the chances of getting 13CO 2 fixed are more in C4 plants.

Choose the best answer. 

Send Your Results (Optional)

• Rutherford, A.W., Faller, P. (Jan 2003). “Photosystem II: evolutionary perspectives”. Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences. 358 (1429): 245–253. doi:10.1098/rstb.2002.1186. PMC 1693113. PMID 12594932.
• Arnon, D.I., Whatley, F.R., Allen, M.B. (1954). “Photosynthesis by isolated chloroplasts. II. Photophosphorylation, the conversion of light into phosphate bond energy”. Journal of the American Chemical Society. 76 (24): 6324–6329. doi:10.1021/ja01653a025.
• Ehrenberg, R. (2017-12-15). “The photosynthesis fix”. Knowable Magazine. Annual Reviews. doi:10.1146/knowable-121917-115502. Retrieved 2018-04-03.
• El-Sharkawy, M.A., Hesketh, J.D. (1965). “Photosynthesis among species in relation to characteristics of leaf anatomy and CO 2 diffusion resistances”. Crop Sci. 5 (6): 517–521. doi:10.2135/cropsci1965.0011183x000500060010x.
• Earl, H., Said Ennahli, S. (2004). “Estimating photosynthetic electron transport via chlorophyll fluorometry without Photosystem II light saturation”. Photosynthesis Research. 82 (2): 177–186. doi:10.1007/s11120-004-1454-3. PMID 16151873. S2CID 291238.

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Last updated on July 15th, 2022

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Photosynthesis

What is photosynthesis.

It is the process by which green plants, algae, and certain bacteria convert light energy from the sun into chemical energy that is used to make glucose. The word ‘photosynthesis’ is derived from the Greek word phōs, meaning ‘light’ and synthesis meaning ‘combining together.’

Jan Ingenhousz, the Dutch-born British physician and scientist, discovered the process of photosynthesis.

Where does Photosynthesis Occur

Photosynthesis takes place mainly in the leaves of green plants and also in the stems of herbaceous plants as they also contain chlorophyll. Sometimes it also occurs in roots that contain chlorophyll like in water chestnut and Heart-leaved moonseed. Apart from plants, photosynthesis is also found to occur in blue-green algae.

What Happens During Photosynthesis

It involves a chemical reaction where water, carbon dioxide, chlorophyll, and solar energy are utilized as raw materials (inputs) to produce glucose, oxygen, and water (outputs).

Stages of the Process

Photosynthesis occurs in two stages:

1) The Light-dependent Reaction

• Takes place in the thylakoid membranes of chloroplasts only during the day in the presence of sunlight
• High-energy phosphate molecules adenosine triphosphate ( ATP ) and the reducing agent NADPH are produced with the help of electron transport chain

2) The Light-independent or Dark Reaction ( Calvin cycle )

• Takes place in the stroma of chloroplast in the absence of light that helps to fix carbon
• ATP and NADPH produced in the light reaction are utilized along with carbon dioxide to produce sugar in the form of glucose

Factors Affecting the Rate of Photosynthesis

• Intensity of Light: The higher intensity of light increases the rate of photosynthesis
• Temperature:  Warmer the temperature, higher the rate of photosynthesis. The rate is highest between the temperatures of 25° to 35° C, after which it starts to decrease
• Concentration of Carbon dioxide: Higher concentration of carbon dioxide increases the rate of photosynthesis until it reaches a certain point, beyond which no further effects are found   

Although all the above factors together interact to affect the rate of photosynthesis, each of them individually is also capable of directly influencing the process without the other factors and thus called limiting factors.

Importance of Photosynthesis

It serves two main purposes that are essential to support life on earth:

• Producing food for organisms that depend on others for their nutrition such as humans along with all other animals
• Synthesizing oxygen by replacing carbon dioxide in the atmosphere

Ans. Photosynthesis is an endothermic reaction because it absorbs the heat of the sun to carry out the process.

Ans. The oxygen in photosynthesis comes from splitting the water molecules.

Ans. Chlorophyll is the main light-absorbing pigment in photosynthesis.

Ans. The role of water is to provide oxygen in the form of oxygen gas to the atmosphere.

Ans. Sunlight is the source of energy that drives photosynthesis.

Ans. The easiest way to measure the rate of photosynthesis is to quantify the carbon dioxide or oxygen levels using a data logger. The rate of photosynthesis can also be measured by determining the increase in the plant ’s biomass (weight).

Ans. Photosynthesis is an energy-requiring process occurring only in green plants, algae, and certain bacteria that utilizes carbon dioxide and water to produce food in the form of carbohydrates. In contrast, cellular respiration is an energy-releasing process found in all living organisms where oxygen and glucose are utilized to produce carbon dioxide and water.

Ans. Glucose produced in photosynthesis is used in cellular respiration to make ATP.

Article was last reviewed on Tuesday, April 21, 2020

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What Is Photosynthesis?

The photosystem, the process.

Photosynthesis is the process used by plants to convert sunlight into chemical energy that can be used to fuel the plants’ growth. The process is fueled by the sun and powered by the chloroplasts in the plants’ leaves. The process begins with the sun’s light energy breaking down water molecules into oxygen and hydrogen. The oxygen is released into the air and the hydrogen is used to power the conversion of carbon dioxide into glucose, which is the plant’s food.

Plants, like every other organism, need energy to live, grow and repair. Unlike heterotrophs –  animals that consume food to synthesize energy – plants or autotrophs are self-reliant — they can make their own food, and therefore energy, by using the resources available in their surroundings. The resources include sunlight, water and carbon dioxide, and this incredible process is called  photosynthesis.

The process is known as photo synthesis because, while water and carbon dioxide are the major ingredients required to cook the food, it is light that ignites the stove, and sunlight is the most abundant light that illuminates the planet.

A photosynthetic organism using the energy of photons ( photo-) , makes its own food (- synthesis ).

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A plant is essentially a highly efficient solar panel. It is replete with molecular structures that have evolved to soak up and absorb as much sunlight as possible.

These structures are in separate compartments of the cell called chloroplasts. Inside the chloroplasts are small towers of discs called grana, each disc is called thylakoid. In the membrane or lining of the thylakoids are a dense grid of various light absorbing molecules, the most notable of them, chlorophyll. The pigments, like any other pigments, absorb light of certain wavelengths and reflect the rest.

This is how pigments or colors are radiated. Majority of plants are green because they (the abundance of chlorophyll) reflect wavelengths that we associate with green while absorbing wavelengths that fall into the red and blue zones. But there are other pigments that absorb different wavelengths in the visible spectrum. Carotenoids are pigments that absorb in the blue to green wavelengths, reflecting orange, yellow and red wavelengths.

This is why leaves take on shades from orange to red during autumn , as they start losing their chlorophyll come at that time of the year. Some deep-sea algae (rhodophyta) are red in color due to a photosynthetic pigment called phycoerythrin and phycocyanin which absorb in blue to green region and reflect back red wavelengths.

These pigments are part of larger protein machines called photosystems. Plants have two photosystems – photosystem I (PSI) and photosystem II (PSII), each having a key chlorophyll-a molecule embedded in it, along with other accessory pigments. These accessory pigments collect light energy and pass it onto the main chlorophyll-a molecule. The photosystems are the main light harvesting machines in the chloroplasts that convert light energy to chemical energy that the cell can use to do build its food. Some algae and photosynthetic bacteria only have single photosystems.

Solar energy is used by the chloroplast to trigger a chemical reaction between the two reactants – water and carbon dioxide.

Also Read: Why Are Plants Red In Color At The Bottom Of The Ocean?

A plant obtains water from its roots through osmosis . From there, the water ascends through the stem and is transported to every part of the plant that requires it for a myriad of processes other than photosynthesis.

To ensure that plant doesn’t dry out due to evaporation, they evolved special pores, called stomata about 400 million years ago. Stomata allowed them to respire, exchanging gases like oxygen and carbon dioxide. However, the crucial development of pores came at the expense of losing water unnecessarily

The stomata inhale carbon dioxide exhaled by animals, which reacts with water in the presence of sunlight to create sugar (glucose), their food. However, the reaction unfolds in two parts.

The first part is called a light-dependent or simply, a light reaction, in which light breaks down water to produce oxygen molecules. These molecules are the same oxygen molecules that we breathe. They are exhaled through the stomata and dispersed into the air. The light energy absorbed by a pigment can be either simply dissipated as heat or be converted into another form of energy. We witness the latter in plants. The light reaction converts solar energy into chemical energy; the reaction also produces ATP (Adenosine Tri-Phosphate) and NADP + (Nicotinamide Adenine Dinucleotide Phosphate), organic compounds that become sources of energy for subsequent metabolic processes.

When light breaks down water at PSII. This releases two protons (H+) and oxygen, and two energized electrons which the chlorophyll-a in PSII accepts. These electrons are passed from PSII to other middle protein structures, like cytochrome bf, to PSI. There these two electrons get energized at PSI due to light energy captured by the photosystem’s chlorophyll-a. These electrons are them funneled to making NADPH from NADP+.

While passing these energized electrons, protons (H+) were taken from outside the thylakoid to its inside. This creates a gradient of H+, more inside than outside. This electrochemical gradient fuels the production of ATP. This happens because of a protein machine called ATP synthase. H+ from inside can return to the outside only by passing through ATP synthase. When H+ pass through ATP synthase, it converts ADP to ATP. This is how light and water create energy for the cell.

One of these processes is the next part of the reaction itself. The two sources of energy fuel the light-independent or dark reaction. The energy breaks down the carbon dioxide molecules and reorganizes the constituents to form a molecule of glucose. The chloroplast then harvests energy by breaking down that glucose, just how mitochondria in animal cells produce energy by breaking down the food they consume. Photosynthesis as a combination of the two reactions can be summarized with this expression:

But if plants can produce ATP through photosynthesis, then why do they need to respire? First, photosynthesis can only happen in the daytime, when there is sun around to provide light. Plants still need energy when there is no light. Second, glucose is important molecule for the cell. It can be broken down or built up to make many other biomolecules like DNA , RNA, proteins, and even fats.

In this manner, we share a deep and indispensable symbiotic relationship with plants. The byproduct, or put more impolitely, the waste product exhaled by plants gives us life, whereas carbon dioxide, the waste product that we exhale, gives plants their life. American biologist and one of my favorite science communicators, Lynn Margulis, called this innocuous act of breathing, spirituality.

However, she believes that “the connection doesn’t stop at the exchange of gases in the atmosphere… The fact that we are connected through space and time shows that life is a unitary phenomenon, no matter how we express the fact.”

Also Read: How Do Plants Excrete?

• PHOTOSYNTHESIS - www2.estrellamountain.edu
• What is Photosynthesis.

Akash Peshin is an Electronic Engineer from the University of Mumbai, India and a science writer at ScienceABC. Enamored with science ever since discovering a picture book about Saturn at the age of 7, he believes that what fundamentally fuels this passion is his curiosity and appetite for wonder.

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What Would Happen If The Amount Of Oxygen On Earth Doubled?

Photosynthesis

Plants are autotrophs, which means they produce their own food. They use the process of photosynthesis to transform water, sunlight, and carbon dioxide into oxygen, and simple sugars that the plant uses as fuel. These primary producers form the base of an ecosystem and fuel the next trophic levels. Without this process, life on Earth as we know it would not be possible. We depend on plants for oxygen production and food.

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Photosynthesis: what it is, how it works and why it fuels life on earth

Photosynthesis is a biological process where plants and microbes use carbon dioxide, water and sunlight to create oxygen and energy.

In this expert guide, we look at photosynthesis – the process by which plants and other organisms convert carbon dioxide, water and light into food, creating oxygen along the way. So what exactly is photosynthesis, and how does it work?

What is photosynthesis?

Photosynthesis is one of nature's most important processes. Powered by sunlight, it uses carbon dioxide (CO2) to produce carbohydrates, the food that ultimately fuels most life on Earth.

The process also releases a byproduct, oxygen (O2), which has become vital for burning carbs to generate energy through aerobic respiration.

Photosynthesis enables an organism to harness solar power to synthesise its own food – carbohydrate molecules – from carbon dioxide in the atmosphere.

Which species can photosynthesise?

Besides plants and microbes such as cyanobacteria, photosynthetic organisms also include algae – a diverse group containing everything from single-celled species like Chlorella to multicellular seaweeds that form kelp forests.

In plant cells, photosynthesis occurs inside chloroplasts – descendants of cyanobacteria that were integrated into plants over 2 billion years ago through endosymbiosis.

When and where does photosynthesis take place?

While you might think it only happens during daytime, the process actually has two stages: light reactions that capture solar energy (the 'photo' half of the word) and dark reactions that can occur day or night to make food ('synthesis').

Dark reactions take place in a microbe or chloroplast's internal fluid, whereas light reactions occur in membranes called thylakoids that are folded to maximise the surface area exposed to sunlight. Leaves are often large, flat structures to catch light, but other green parts of a plant (like stems) also photosynthesise.

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Why do leaves have pores?

Leaf pores are a double-edged sword: the openings allow a plant to suck-up the carbon dioxide it needs to make food, but they also let precious water escape. While early plants had to cope with losing water passively through pores that were just holes, modern species have structures called stomata that actively adjust an aperture's size in response to physiological or environmental cues. Stomata therefore help a plant to minimise water loss, balanced with gaining carbon for photosynthesis.

Do aquatic plants have stomata?

The gas exchange that occurs via stomata for land plants is different for aquatic plants. If an aquatic plant were covered in tiny holes, it would drown. So in water lilies and other flowering plants with floating leaves, only the upper side of a leaf has stomata – and those pores are permanently open because aquatic plants don't really need to worry about losing moisture. It seems to work well, as floating plants have emerged at least 13 times independently by convergent evolution.

Some flowering plants are even adapted to a submerged lifestyle: seagrasses lack stomata completely and carry out photosynthesis, but they must rely on bacteria in shallow ocean sediments to provide a source of carbon in the absence of CO2.

How do organisms capture light?

The cells of photosynthetic organisms convert light to chemical energy through photosystems – a combination of pigments, enzymes and other molecules. The core component of a photosystem is the green pigment chlorophyll.

After being hit by photons (particles that make up light), electrons in chlorophyll absorb enough energy to break free, which starts a chain reaction in which energised electrons are passed between intermediates to generate the molecules ATP and NADPH, whose chemical bonds carry the energy that's later released to make food. In plants, chlorophyll's lost electrons are replaced by splitting water, releasing oxygen.

Is chlorophyll the only pigment?

No, but it's the best at absorbing sunlight near the blue end of the visible spectrum, where photons have greater energy. Many organisms, especially aquatic species, also have accessory pigments such as phycobilin that are more efficient at harnessing photon energies toward the red end of the spectrum, allowing those wavelengths to be absorbed if water has filtered-out blue light. The distinctive colour of red algae is explained by the presence of large amounts of phycobilin.

So how is food made from carbon dioxide?

Like making drinks in a slushie machine that continually rotates and is refilled with flavouring and ice, photosynthetic cells produce carbohydrates by recycling them and replenishing the supply of carbon by regularly adding carbon dioxide (and usually water). That involves a circular series of reactions popularly called the Calvin cycle, which is driven by the energy-carrying molecules ATP and NADPH.

The first step in that cycle involves carbon fixation: adding or 'fixing' carbon dioxide (a volatile gas) to a stable compound, a reaction catalysed by the enzyme RuBisCO. Because it constitutes an estimated 3 per cent of the total mass in leaves, and most of Earth's biomass is plant matter, RuBisCO is arguably the most abundant protein on the planet. And because the enzyme's activity ends-up producing more than 90 per cent of organic carbon, RuBisCO is one of nature's most important proteins.

Do any animals photosynthesise?

The tiny polyps of corals form a symbiotic relationship with algae, providing a home and the nutrients required for photosynthesis and benefiting from food generated by the algae.

Some sea slugs are able retain the chloroplasts after grazing on algae, and these are engulfed by their own cells. They also assimilate some of the algal genes, enabling them to produce cholorophyll. The slugs have been recorded using the pilfered structures to sustain themselves for up to ten months, earning them the nickname ‘solar-powered sea slugs’.

So far, only one vertebrate is known to have a comparable association with algae – the spotted salamander ( pictured above ), a very striking amphibian found in the eastern United States and Canada. Soon after being laid, the salamander’s eggs are colonised by water-dwelling algae, which turn the gelatinous spheres green. The algae appear to thrive on wast, such as ammonia produced by the developing salamander embryos, and in turn provide oxygen and carbon. This enables the embryos to develop faster, increasing their survival rates. The algae more or less disappear by the time the larvae hatch, so the partnership appears temporary.

This Q&A originally appeared in BBC Wildlife , and was answered by Laurie Jackson.

Main image: Large green hosta leaves. © Maryna Terletska/Getty

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Photosynthesis: What is it and how does it work?

Photosynthesis is essential for almost all life, and it’s the primary source of oxygen in the atmosphere.

Holly Spanner

Photosynthesis is all around us. It's happening under our feet, above our heads and in the sunlit zones of aquatic environments. But what exactly is photosynthesis? Why is it so important? And, when did it evolve? Answers to these questions, and more, are below.

For those who missed it, check out these five astonishing plant adaptations or find out whether plants are conscious.

What is photosynthesis?

Photosynthesis is the process by which carbohydrate molecules are synthesised. It's used by plants, algae and certain bacteria to turn sunlight, water and carbon dioxide into oxygen and energy, in the form of sugar. It’s probably the most important biochemical process on the planet.

Essentially, it takes the carbon dioxide expelled by all breathing organisms and reintroduces it into the atmosphere as oxygen.

The rate of photosynthesis is affected by light intensity, the concentration of carbon dioxide, water supply, temperature and availability of minerals. The process takes place entirely in the chloroplasts, and it's the chlorophyll within the chloroplasts that make the photosynthetic parts of a plant green.

Photosynthesis is important too, elsewhere in the biosphere. Both marine and terrestrial plants remove carbon dioxide from the atmosphere, and some of this is precipitated back out, as shells made of calcium carbonate, or buried as organic matter in soil.

Without photosynthesis, the carbon cycle could not occur, and we would soon run out of food. Over time, the atmosphere would lose almost all gaseous oxygen, and most organisms would disappear.

How does photosynthesis work?

Plants require light energy, carbon dioxide, water and nutrients. These ingredients come from both the adjacent atmosphere and the soil.

Plants absorb sunlight through the two top layers of their leaves, the cuticle and epidermis. These layers are thin, so light can travel through them easily. Carbon dioxide is brought in from the atmosphere, and at the same time, water is drawn up from the soil, into the body of the living plant.

Just beneath the cuticle and epidermis are the palisade mesophyll cells. These specialised cells are vertically elongated and arranged closely together to maximise light absorption.

Below the palisade mesophyll cells is the spongy mesophyll tissue, which is loosely packed for efficient gas exchange. As gases move in and out of these cells, they dissolve in a thin layer of water that covers the cells.

Inside the palisade mesophyll cells are the chloroplasts, lots of them . They contain chlorophyll, molecules that don’t absorb green wavelengths of white light. Instead, they reflect it back to us, giving plants their green colour.

Inside the chloroplast is where the magic happens. A light-dependent reaction takes place, where energy from the light waves is absorbed and stored in energy-carrying ATP molecules.

Then, in a light-independent reaction (the Calvin Cycle), ATP is used to make glucose, a source of energy. Water is oxidised, carbon dioxide is reduced, and oxygen is released into the atmosphere.

Oxygen is released via stomata in the leaves, microscopic pores that open to both let in the carbon dioxide, and release oxygen (and water vapour).

What is the equation for photosynthesis?

Photosynthesising organisms form the base of the food chain.

Carbon dioxide + water (with light energy) = glucose + oxygen

As well as the light energy, carbon dioxide and water, plants also need nutrients, which they get from the soil. These nutrients are released again, or recycled, when the plant tissue dies and begins decomposing in the soil.

Oxygen in the form of gas molecules (O 2 ) is actually a by-product of photosynthesis, but it's responsible for the oxygen in the air that keeps us alive. Plants also release energy and water to the atmosphere through respiration.

6CO 2 + 6H 2 O → C 6 H 12 O 6 + 6O 2

The balanced equation takes it a little further. Six carbon dioxide molecules and six water molecules (the reactants) are converted into one sugar molecule (C 6 H 12 O 6) and six oxygen molecules, via the light energy captured by the chlorophyll.

Photosynthesis and the food chain

During photosynthesis, energy passes through the system, and you can think of photosynthesis as an energy flow system, tracing the path of solar energy through the ecosystem. This energy is stored by the primary producers, the photosynthesising organisms. As these organisms are eaten and digested by the primary consumers, chemical energy is released and this is used to power new biochemical reactions.

At each level of energy transformation throughout the food chain, some energy is lost as waste heat. In addition, a significant amount of the energy input to each organism is used in respiration, to maintain the body of that organism. This energy is not stored for use by other organisms higher up the food chain. This is one of the reasons why both the number of organisms and their total quantity of living tissue decrease as you go further up the food chain.

When did photosynthesis start?

The evolution of photosynthesis had immense consequences for the Earth. As organic matter from photosynthetic life was buried in the strata, carbon was removed from the atmosphere allowing oxygen to accumulate.

Evidence suggests that photosynthetic organisms were present around 3.2 to 3.5 billion years ago , in the form of stromatolites. Stromatolites are laminated microbial structures ( generally an alternation between light and dark laminae ), usually formed by cyanobacteria and algae, and are the oldest known fossils, and therefore the earliest evidence of life on Earth.

As this early oxygen diffused into the upper atmosphere (the stratosphere), solar radiation transformed the oxygen molecules into ozone, which created the stratospheric ozone layer. And of course, as the ozone layer absorbs most of the Sun's ultraviolet radiation (UV-B), it plays an important role in protecting human health, so it's unlikely that life would have flourished without this protective shield.

• Can any plants live without sunlight?
• Can photosynthesis be recreated in the lab?
• Can any animals photosynthesise?
• Is artificial photosynthesis a promising technology for future energy production?

To submit your questions email us at [email protected] (don't forget to include your name and location)

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Explain Photosynthesis

What Is the Source of Energy for Chemosynthesis?

Plants usually receive credit for being the base of food chains. Less well-known but equally important are algae, which also perform the important task of changing carbon dioxide to oxygen. Plant-like protists, one-celled organisms containing chloroplasts, make their own contribution to the food chain and the conversion of carbon dioxide to oxygen. What do these organisms have in common? They all perform photosynthesis.

The Photosynthesis Process

The photosynthesis process uses the sun's energy to combine carbon dioxide and water to form glucose, a sugar. Carbon dioxide enters plants through tiny pores in the bottoms of leaves or by diffusion through cell membranes in the case of algae and protists. Water enters by a variety of means, usually roots, but also by osmosis, which lets water pass through the cell membranes. The sun's energy, absorbed by the green chemical chlorophyll, fuels the chemical reaction that combines the carbon dioxide molecules with the water molecules to form glucose, one type of sugar, and release oxygen as a waste product. The glucose can be stored in fruits, roots and stems of plants and released through the reverse process of respiration, where oxygen is used to break the glucose down into carbon dioxide and water, releasing the stored energy.

The Equation for Photosynthesis

The photosynthesis equation is written as: 6H 2 O + 6CO 2 → C 6 H 12 O 6 + 6O 2 and is explained in words as the reaction of six water molecules with six carbon dioxide molecules yielding one glucose molecule and six oxygen molecules. Note that one oxygen molecule contains a pair of oxygen atoms.

Photosynthesis Definition

The word photosynthesis literally breaks down into "photo," Greek for "light," and "synthesis," a Greek word meaning "composition" or putting together. So, photosynthesis actually means to put together using light. Plants, algae and plant-like protists use sunlight to put together carbon dioxide and water to make sugar.

Importance of Photosynthesis

A chemical explanation of photosynthesis doesn't begin to convey the importance of this process. Earth's early atmosphere, composed of carbon dioxide and other gases spewed out of volcanoes, gradually changed to the modern oxygen-rich atmosphere by photosynthesizing blue-green algae. The conversion of carbon dioxide and water to sugar provides food not only for the plant but also for almost all animal life. While plants provide most of the food on land, algae and plant-like protists provide food for most aquatic food chains. Over time, many interdependent relationships between plants and animals have developed, like the pollination of plants by insects, birds or bats. Ultimately, however, many plants would survive without animals, but most animals cannot live without plants or other photosynthesizing organisms.

Photosynthesis vs. Chemosynthesis

It is hard to explain photosynthesis without a brief note regarding chemosynthesis. Chemosynthesis uses chemical reactions to release energy and form sugars. While photosynthetic reactions only have one equation, chemosynthetic reactions vary depending on the organism. One chemosynthetic reaction, performed by bacteria at deep-sea hydrothermal vents, combines hydrogen sulfide, oxygen and carbon dioxide to form formaldehyde (H-CHO, sometimes written as CH 2 O) and release sulfur and water. Other chemosynthetic bacteria oxidize methane or reduce sulfides to release energy. Chemosynthetic bacteria form the base of the food chain in deep ocean communities where sunlight doesn't penetrate. Chemosynthetic bacteria also occur in some hot springs on land.

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About the Author

Karen earned her Bachelor of Science in geology. She worked as a geologist for ten years before returning to school to earn her multiple subject teaching credential. Karen taught middle school science for over two decades, earning her Master of Arts in Science Education (emphasis in 5-12 geosciences) along the way. Karen now designs and teaches science and STEAM classes.

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• What is Photosynthesis

What Is Photosynthesis?

“Photosynthesis is the process used by green plants and a few organisms that use sunlight, carbon dioxide and water to prepare their food.”

The process of photosynthesis is used by plants, algae and certain bacteria that convert light energy into chemical energy. The glucose formed during the process of photosynthesis provides two important resources to organisms: energy and fixed carbon.

Read on to explore what is photosynthesis and the processes associated with it.

Site of Photosynthesis

Photosynthesis takes place in special organelles known as chloroplast. This organelle has its own DNA, genes and hence can synthesize its own proteins. Chloroplasts consist of stroma, fluid, and stack of thylakoids known as grana. There are three important pigments present in the chloroplast that absorb light energy, chlorophyll a, chlorophyll b, and carotenoids.

Also Read: Photosynthesis Process

Types of Photosynthesis

There are two different types of photosynthesis:

• Oxygenic photosynthesis
• Anoxygenic photosynthesis

Oxygenic Photosynthesis

Oxygenic photosynthesis is more common in plants, algae and cyanobacteria. During this process, electrons are transferred from water to carbon dioxide by light energy, to produce energy. During this transfer of electrons, carbon dioxide is reduced while water is oxidized, and oxygen is produced along with carbohydrates.

During this process, plants take in carbon dioxide and expel oxygen into the atmosphere.

This process can be represented by the equation:

6CO2+ 12H2O + LIGHT ENERGY → C6H12O6 + 6O2 + 6H2O

Anoxygenic Photosynthesis

This type of photosynthesis is usually seen in certain bacteria, such as green sulphur bacteria and purple bacteria which dwell in various aquatic habitats. Oxygen is not produced during the process.

The anoxygenic photosynthesis can be represented by the equation:

CO2 + 2H2A + LIGHT ENERGY → [CH2O] + 2A + H2O

Also Read:  Difference between Photosynthesis and Respiration

Photosynthesis Apparatus

The photosynthesis apparatus includes the following essential components:

Pigments not only provide colour to the photosynthetic organisms, but are also responsible for trapping sunlight. The important pigments associated with photosynthesis include:

• Chlorophyll: It is a green-coloured pigment that traps blue and red light. Chlorophyll is subdivided into, “chlorophyll a”, “chlorophyll b”, and “chlorophyll c”. “Chlorophyll a” is widely present in all the photosynthetic cells. A bacterial variant of chlorophyll known as bacteriochlorophyll can absorb infrared rays .
• Carotenoids: These are yellow, orange or red-coloured pigments that absorb bluish-green light. Xanthophyll and carotenes are examples of carotenoids.
• Phycobilins: These are present in bacteria and red algae . These are red and blue pigments that absorb wavelength of light that are not properly absorbed by carotenoids and chlorophyll.

Plastids are organelles found in the cytoplasm of eukaryotic photosynthetic organisms. They contain pigments and can also store nutrients. Plastids are of three types:

• Leucoplast: These are colourless, non-pigmented and can store fats and starch.
• Chromoplasts: They contain carotenoids.
• Chloroplasts: These contain chlorophyll and are the site of photosynthesis.

Antennae is the collection of 100 to 5000 pigment molecules that capture light energy from the sun in the form of photons. The light energy is transferred to a pigment-protein complex that converts light energy to chemical energy.

Reaction Centers

The pigment-protein complex responsible for the conversion of light energy to chemical energy forms the reaction centre.

Also Read: Photosynthesis

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photosynthesis

Photosynthesis requires sunlight, chlorophyll, water, and carbon dioxide gas. Chlorophyll is a substance in all green plants, especially in the leaves. Plants take in water from the soil and carbon dioxide from the air.

Photosynthesis starts when chlorophyll absorbs energy from sunlight. Green plants use this light energy to change water and carbon dioxide into oxygen and nutrients called sugars. The plants use some of the sugars and store the rest. The oxygen is released into the air.

Photosynthesis is very important because almost all living things depend on plants for food. Photosynthesis is also important because of the oxygen it produces. Humans and other animals need to breathe in oxygen to survive.

Some living things other than plants also make their own food through photosynthesis. They include certain types of bacteria and algae.

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What is photosynthesis?

April 28, 2020 By Emma Vanstone Leave a Comment

Green plants make sugar for growth by a process called photosynthesis . Photosynthesis is a process where light energy is converted to chemical energy in the form of sugars. It’s a process that provides the main source of oxygen in the atmosphere and is essential for almost all life on Earth.

Photosynthesis made simple

Plants use sunlight, water, and carbon dioxide to create oxygen and energy in the form of glucose.

The glucose molecules created by photosynthesis act as fuel for cells and are used for cellular respiration and fermentation.

Photosynthesis Equation

Carbon dioxide + water  (and light ) ———> glucose and oxygen

Where does photosynthesis happen?

Photosynthesis takes place in chloroplast cells, which contain a substance called chlorophyll. It is chlorophyll, which gives plants their green colour.

Chloroplasts are one of the organelles in a plant cell. We made a jelly model of plant cell to learn about organelles and plant cell structure.

Sunlight is also needed to make chlorophyll. If plants are kept in the dark, they can’t make chlorophyll and will have yellow leaves! You can see this in our multicoloured cress caterpillar .

What affects the rate of photosynthesis

Four factors affect the rate of photosynthesis . The faster it occurs, the more the plant grows.

Light  – the more light there is, the faster photosynthesis occurs.

Water  – lack of water slows photosynthesis down.

Temperature  – photosynthesis works best at around 30 degrees Celsius.

Carbon dioxide levels  – photosynthesis is faster if there is more carbon dioxide in the air.

Plants make the energy to grow through a process called respiration . This uses the sugar produced by photosynthesis and oxygen.

How are leaves adapted for photosynthesis?

Leaves are green as they contain lots of chlorophyll, which absorbs sunlight.

They are thin and have a large surface area. This means they can absorb a lot of sunlight, and gases such as oxygen and carbon dioxide can pass in and out of the leaf easily.

Leaves have veins – xylem and phloem, which transport water, minerals and sugars around the plant.

Do only plants photosynthesise?

Plants, algae, and some types of bacteria use photosynthesis to create energy.

A large percentage of the Earth’s oxygen comes from phytoplankton in the oceans, which contain chlorophyll and use photosynthesis to create energy.

What is chlorophyll?

Chlorophyll is a green pigment which absorbs energy from blue and red light waves and reflects green light waves, which is why plants look green!

Chlorophyll in a plant is found in an organelle called a chloroplast. This is where photosynthesis occurs.

Why is photosynthesis important?

Photosynthesis also allows plants to make energy for growth and repair, and it has an important ecological impact.

Plants incorporate the carbon from carbon dioxide into organic molecules ( carbon fixation ). This creates a carbon source for animals who cannot create their own and also removes carbon dioxide from the air, slowing down the rate at which it builds up in the atmosphere.

Photosynthesis also creates oxygen, which is needed for most life on Earth!

Do you have more questions? You might find the answers in my collection of science questions for kids .

Last Updated on January 26, 2024 by Emma Vanstone

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photosynthesis

Definition of photosynthesis

Did you know.

Photosynthesis Has Greek Roots

The Greek roots of photosynthesis combine to produce the basic meaning "to put together with the help of light". Photosynthesis is what first produced oxygen in the atmosphere billions of years ago, and it's still what keeps it there. Sunlight splits the water molecules (made of hydrogen and oxygen) held in a plant's leaves and releases the oxygen in them into the air. The leftover hydrogen combines with carbon dioxide to produce carbohydrates, which the plant uses as food—as do any animals or humans who might eat the plant.

Examples of photosynthesis in a Sentence

These examples are programmatically compiled from various online sources to illustrate current usage of the word 'photosynthesis.' Any opinions expressed in the examples do not represent those of Merriam-Webster or its editors. Send us feedback about these examples.

Word History

1898, in the meaning defined above

Dictionary Entries Near photosynthesis

photosynthate

photosynthetic ratio

Cite this Entry

“Photosynthesis.” Merriam-Webster.com Dictionary , Merriam-Webster, https://www.merriam-webster.com/dictionary/photosynthesis. Accessed 6 Sep. 2024.

Kids Definition

Kids definition of photosynthesis, medical definition, medical definition of photosynthesis, more from merriam-webster on photosynthesis.

Nglish: Translation of photosynthesis for Spanish Speakers

Britannica.com: Encyclopedia article about photosynthesis

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What is Plant Science?

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Plant science, a broad field encompassing botany, horticulture, agriculture, and related disciplines, can be broadly defined as the study of plants. It provides a fundamental understanding of plant biology, from their molecular structure to their ecological interactions.

Introduction

Plants play a pivotal role in our lives, serving as a source of food, medicine, materials, and oxygen. Plant science is thus crucial for addressing global challenges such as food security and environmental sustainability.

Agricultural advancements, driven by plant science research, have increased crop yields and improved food production efficiency.

Additionally, understanding plant-environment interactions can help us to develop sustainable agricultural practices that minimize environmental impact and preserve biodiversity.

Core Areas of Plant Science

• Plant Biology investigates the fundamental aspects of plant structure, function, and growth. It explores the anatomy of plant cells, tissues, and organs, as well as their physiological processes, such as photosynthesis, respiration, and water transport. This field also studies plant development, from germination to flowering and seed production.
• Plant Genetics is the study of plant heredity and variation. It examines the genetic material (DNA) of plants and how it is transmitted from one generation to the next. This knowledge is essential for plant breeding, where genetic manipulation is used to develop new plant varieties with desirable traits. ​​​​​​
• Epigenetics , a field that studies heritable changes in gene expression that do not involve alterations to the underlying DNA sequence, is playing an increasing role in plant breeding. Epigenetic modifications (e.g., DNA methylation) influence gene activity and phenotypic variation; thereby, epigenetic mechanisms can also be manipulated to improve crop traits 1 .
• Plant Ecology focuses on the interactions between plants and their environment. It explores how plants adapt to different habitats, compete with other organisms, and contribute to ecosystem function. This subfield of plant sciences also investigates the role of plants in nutrient cycling and climate regulation 2,3 .
• Plant Pathology is the study of plant diseases and their causes. It examines the various pathogens that can infect plants, including fungi, bacteria, and viruses. Plant pathologists work to understand the mechanisms of disease transmission and development, ultimately developing effective strategies for disease prevention and control 4 .

Read More:  Why is Plant Biology Crucial for Sustainable Agriculture?

Applications of Plant Science

Plant science has revolutionized diverse fields ranging from agriculture to pharmaceuticals, improving crop yields, developing new medicines, and promoting sustainability.

The study of plants has revolutionized agriculture by leading to significant advancements in crop production and pest management. Through plant breeding, scientists have developed high-yielding, disease-resistant, and drought-tolerant crop varieties.

Plant science has also been essential for environmental conservation 5 . By understanding plant-environment interactions, scientists can develop strategies for habitat restoration and invasive species control.

Similarly, plant research has also played a vital role in drug discovery . Many modern pharmaceuticals are derived from plant compounds, and plant scientists are actively exploring new plant species and compounds for potential medicinal applications 6 .

Challenges in Plant Science

Plant research is addressing critical challenges, with some of the hottest topics including climate change, sustainable practices, and the ethical implications of genetically modified organisms:

• Climate change poses significant challenges to plant agriculture. Rising temperatures can adversely affect crop growth and yield. Plant genetics offers promising strategies for improving crop resilience to challenging environmental conditions. By understanding and manipulating genetic and epigenetic mechanisms, scientists can develop crop varieties that are more tolerant to heat stress, drought, and other environmental challenges 7 .
• The use of genetically modified plants has raised ethical concerns . Balancing the potential benefits and risks of GM is a complex issue that requires careful consideration and ongoing research 8 .
• Plant science research is also focused on developing sustainable agricultural practices, including the use of biocontrol agents 9 and environmentally friendly pesticides 10 . These practices aim to reduce agriculture's environmental impact while maintaining high yields and quality.

Related Stories

• Identifying QTLs for Improved Nitrogen Use Efficiency in Eggplant
• Converting Dairy Manure into Livestock Feed with Black Soldier Flies
• Quorum Bio: Where Synthetic Biology Meets Sustainable Farming

Learn More Here:  How Biofortification Strengthens Food Security

Future Directions

Plant science is a dynamic field with exciting prospects for future research. Advances in genomics, biotechnology, and computational biology will continue to drive innovation. Using big data and artificial intelligence will also enable the analysis of complex plant systems and the development of predictive models for crop performance and disease resistance.

Interdisciplinary approaches will be crucial for addressing global challenges. For example, integrating plant science with climate science can help develop climate-resilient crops, while collaborations with social partners can inform agricultural policy and promote sustainable practices like organic farming.

Furthermore, synthetic biology and the engineering of plant genomes offer new possibilities for creating plants with enhanced traits, such as improved nutritional value and increased stress tolerance. These advancements will contribute to creating a more sustainable and resilient food system.

Continued research in plant science is essential to meet the growing demands of a global population and to ensure a sustainable future. Advances in technology and interdisciplinary collaborations will drive innovation in this field, leading to novel discoveries and breakthroughs.

By investing in plant science research, we can unlock plants' full potential to benefit society, directly impacting human populations and well-being.

• Vaschetto, Luis María. Exploring an emerging issue: crop epigenetics. Plant molecular biology reporter 33 (2015): 751-755.  https://link.springer.com/article/10.1007/s11105-014-0796-z
• Hobbie, S. E. (2015). Plant species effects on nutrient cycling: revisiting litter feedbacks. Trends in ecology & evolution , 30(6), 357-363.  https://www.cell.com/trends/ecology-evolution/fulltext/S0169-5347(15)00078-6
• Valladares, F., Laanisto, L., Niinemets, Ü., & Zavala, M. A. (2016). Shedding light on shade: ecological perspectives of understorey plant life. Plant Ecology & Diversity , 9(3), 237-251.  https://www.tandfonline.com/doi/full/10.1080/17550874.2016.1210262
• Tariq, M., Khan, A., Asif, M., Khan, F., Ansari, T., Shariq, M., & Siddiqui, M. A. (2020). Biological control: a sustainable and practical approach for plant disease management. Acta Agriculturae Scandinavica, Section B—Soil & Plant Science, 70(6), 507-524.  https://www.tandfonline.com/doi/full/10.1080/09064710.2020.1784262
• Thompson, J. D. (2020). Plant evolution in the Mediterranean: insights for conservation. Oxford University Press, USA.  https://academic.oup.com/book/31972
• Rijo, P., Athanassopoulos, C. M., & Carpinella, M. C. (2022). Considering plant metabolites and their synthetic derivatives as candidates for the development of drugs against multidrug resistant (MDR) tumors. Frontiers in pharmacology, 13, 1108252.  https://www.frontiersin.org/research-topics/24184/considering-plant-metabolites-and-their-synthetic-derivatives-as-candidates-for-the-development-of-drugs-directed-against-multidrug-resistant-mdr-tumors
• Vaschetto LM. Epigenetics in Crop Improvement: Safeguarding Food Security in an Ever-Changing Climate . SpringerNature (Cham, Switzerland). https://link.springer.com/book/9783031731754
• Vega Rodríguez, Angelo, et al. "Myths and realities about genetically modified food: A risk-benefit analysis." Applied Sciences 12.6 (2022): 2861.  https://www.mdpi.com/2076-3417/12/6/2861/review_report
• El-Saadony, M. T., Saad, A. M., Soliman, S. M., Salem, H. M., Ahmed, A. I., Mahmood, M., ... & AbuQamar, S. F. (2022). Plant growth-promoting microorganisms as biocontrol agents of plant diseases: Mechanisms, challenges and future perspectives. Frontiers in plant science, 13, 923880.  https://pubmed.ncbi.nlm.nih.gov/36275556/
• Vaschetto LM. RNAi Strategies for Pest Management: Methods and Protocols (2021). DOI: 10.1007/978-1-0716-1633-8 . Springer Science Business Media (SpringerNature group). New York, USA.

Further Reading

• All Agriculture Content
• How Biofortification Strengthens Food Security
• How can genetically modified organisms be safely integrated into organic farming practices to ensure food security?
• Bridging Genomics and Phenomics in Precision Agriculture

Last Updated: Sep 5, 2024

Dr. Luis Vaschetto

After completing his Bachelor of Science in Genetics in 2011, Luis continued his studies to complete his Ph.D. in Biological Sciences in March of 2016. During his Ph.D., Luis explored how the last glaciations might have affected the population genetic structure of Geraecormobious Sylvarum (Opiliones-Arachnida), a subtropical harvestman inhabiting the Parana Forest and the Yungas Forest, two completely disjunct areas in northern Argentina.

Please use one of the following formats to cite this article in your essay, paper or report:

Vaschetto, Luis. (2024, September 05). What is Plant Science?. AZoLifeSciences. Retrieved on September 06, 2024 from https://www.azolifesciences.com/article/What-is-Plant-Science.aspx.

Vaschetto, Luis. "What is Plant Science?". AZoLifeSciences . 06 September 2024. <https://www.azolifesciences.com/article/What-is-Plant-Science.aspx>.

Vaschetto, Luis. "What is Plant Science?". AZoLifeSciences. https://www.azolifesciences.com/article/What-is-Plant-Science.aspx. (accessed September 06, 2024).

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