write a short note on photosynthesis

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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.

Microbe Notes

Photosynthesis: Equation, Steps, Process, Diagram

Photosynthesis is defined as the process, utilized by green plants and photosynthetic bacteria, where electromagnetic radiation is converted into chemical energy and uses light energy to convert carbon dioxide and water into carbohydrates and oxygen.

• The carbohydrates formed from photosynthesis provide not only the necessary energy form the energy transfer within ecosystems, but also the carbon molecules to make a wide array of biomolecules.
• Photosynthesis is a light-driven oxidation-reduction reaction where the energy from the light is used to oxidize water, releasing oxygen gas and hydrogen ions, followed by the transfer of electrons to carbon dioxide, reducing it to organic molecules.
• Photosynthetic organisms are called autotrophs because they can synthesize chemical fuels such as glucose from carbon dioxide and water by utilizing sunlight as an energy source.
• Other organisms that obtain energy from other organisms also ultimately depend on autotrophs for energy.
• One of the essential requirements for photosynthesis is the green pigment ‘chlorophyll’ which is present in the chloroplasts of green plants and some bacteria.
• The pigment is essential for ‘capturing’ sunlight which then drives the overall process of photosynthesis.

Table of Contents

Interesting Science Videos

Photosynthesis equations/reactions/formula

• The process of photosynthesis differs in green plants and sulfur bacteria.
• In plants, water is utilized along with carbon dioxide to release glucose and oxygen molecules.
• In the case of sulfur bacteria, hydrogen sulfide is utilized along with carbon dioxide to release carbohydrates, sulfur, and water molecules.

Oxygenic Photosynthesis

The overall reaction of photosynthesis in plants is as follows:

Carbon dioxide + Water  + solar energy → Glucose + Oxygen

6CO 2 + 6H 2 O  +  solar energy   →   C 6 H 12 O 6 + 6O 2

Carbon dioxide + Water  + solar energy → Glucose + Oxygen + Water

6CO 2 + 12H 2 O+ solar energy    →    C 6 H 12 O 6 + 6O 2 + 6H 2 O

Anoxygenic Photosynthesis

The overall reaction of photosynthesis in sulfur bacteria is as follows:

CO 2 + 2H 2 S + light energy   →    (CH 2 O)  + H 2 O  + 2S

Video Animation: Photosynthesis (Crash Course)

Photosynthetic pigments

• Photosynthetic pigments are the molecules involved in absorbing electromagnetic radiation, transferring the energy of the absorbed photons to the reaction center, resulting in photochemical reactions in the organisms capable of photosynthesis.
• The molecules of photosynthetic pigments are quite ubiquitous and are always composed of chlorophylls and carotenoids.
• In addition to chlorophyll, photosynthetic systems also contain another pigment, pheophytin (bacteriopheophytin in bacteria), which plays a crucial role in the transfer of electrons in photosynthetic systems.
• Moreover, other pigments can be found in particular photosynthetic systems, such as xanthophylls in plants.

Image Source: Simply Science .

Chlorophyll

• Chlorophyll is the pigment molecule, which is the principal photoreceptor in the chloroplasts of most green plants.
• Chlorophylls consist of a porphyrin ring, which is bounded to an ion Mg 2+ , attached to a phytol chain.
• Chlorophylls are very effective photoreceptors because they contain networks of alternating single and double bonds.
• In chlorophyll, the electrons are not localized to a particular atomic nucleus and consequently can more readily absorb light energy.
• In addition, chlorophylls also have solid absorption bands in the visible region of the spectrum.
• Chlorophylls are found either in the cytoplasmic membranes of photosynthetic bacteria, or thylakoid membranes inside plant chloroplasts.

Bacteriorhodopsin

• Bacteriorhodopsin is another class of photosynthetic pigment that exists only in halobacteria.
• It is composed of a protein attached to a retinal prosthetic group.
• This pigment is responsible for the absorption of light photons, leading to a conformational change in the protein, which results in the expulsion of the protons from the cell.

Phycobilins

• Cyanobacteria and red algae employ phycobilins such as phycoerythrobilin and phycocyanobilin as their light-harvesting pigments.
• These open-chain tetrapyrroles have the extended polyene system found in chlorophylls, but not their cyclic structure or central Mg 2+ .
• Phycobilins are covalently linked to specific binding proteins, forming phycobiliproteins, which associate in highly ordered complexes called phycobilisomes that constitute the primary light-harvesting structures in these microorganisms.

Carotenoids

• In addition to chlorophylls, thylakoid membranes contain secondary light-absorbing pigments, or accessory pigments, called carotenoids.
• Carotenoids may be yellow, red, or purple. The most important are β -carotene, which is a red-orange isoprenoid, and the yellow carotenoid lutein.
• The carotenoid pigments absorb light at wavelengths not absorbed by the chlorophylls and thus are supplementary light receptors.

Factors affecting photosynthesis

Blackman formulated the Law of limiting factors while studying the factors affecting the rate of photosynthesis. This Law states that the rate of a physiological process will be limited by the factor which is in the shortest supply. In the same way, the rate of photosynthesis is also affected by a number of factors, which are namely;

• As the intensity of light increases, the rate of light-dependent reactions of photosynthesis and in turn, the rate of photosynthesis increases.
• With increased light intensity, the number of photons falling on a leaf also increases. As a result, more chlorophyll molecules are ionized, and more ATPs and NADH are generated.
• After a point, however, the rate of photosynthesis remains constant as the light intensity increases. At this point, photosynthesis is limited by some other factors.
• Besides, the wavelength of light also affects the rate of photosynthesis.
• Different photosynthetic systems absorb light energy more effectively at different wavelengths.

Carbon dioxide

• An increase in the concentration of carbon dioxide increases the rate at which carbon is incorporated into carbohydrates in the light-independent reactions of photosynthesis.
• Thus, increasing the concentration of carbon dioxide in the atmosphere rapidly increases the rate of photosynthesis up to a point after which it is limited by some other factors.

Temperature

• The light-independent reactions of photosynthesis are affected by changes in temperature as they are catalyzed by enzymes, whereas the light-dependent reactions are not.
• The rate of the reactions increases as the enzymes reach their optimum temperature, after which the rate begins to decrease as the enzymes tend to denature.

Process/ Steps of Photosynthesis

The overall process of photosynthesis can be objectively divided into four steps/ process:

1. Absorption of light

• The first step in photosynthesis is the absorption of light by chlorophylls that are attached to the proteins in the thylakoids of chloroplasts.
• The light energy absorbed is then used to remove electrons from an electron donor like water, forming oxygen.
• The electrons are further transferred to a primary electron acceptor, quinine (Q) which is similar to CoQ in the electron transfer chain.

2. Electron Transfer

• The electrons are now further transferred from the primary electron acceptor through a chain of electron transfer molecules present in the thylakoid membrane to the final electron acceptor, which is usually NADP + .
• As the electrons are transferred through the membrane, protons are pumped out of the membrane, resulting in the proton gradient across the membrane.

3. Generation of ATP

• The movement of protons from the thylakoid lumen to the stroma through the F 0 F 1 complex results in the generation of ATP from ADP and Pi.
• This step is identical to the step of the generation of ATP in the electron transport chain .

4. Carbon Fixation

• The NADP and ATP generated in steps 2 and 3 provide energy, and the electrons drive the process of reducing carbon into six-carbon sugar molecules.
• The first three steps of photosynthesis are directly dependent on light energy and are thus, called light reactions, whereas the reactions in this step are independent of light and thus are termed dark reactions.

Types/ Stages/ Parts of photosynthesis

Figure: Photosynthesis takes place in two stages: light-dependent reactions and the Calvin cycle. Light-dependent reactions, which take place in the thylakoid membrane, use light energy to make ATP and NADPH. The Calvin cycle, which takes place in the stroma, uses energy derived from these compounds to make GA3P from CO 2 . Image Source: OpenStax (Rice University) .

Photosynthesis is divided into two stages based on the utilization of light energy:

1. Light-dependent reactions

• The light-dependent reactions of photosynthesis only take place when the plants/ bacteria are illuminated.
• In the light-dependent reactions, chlorophyll and other pigments of photosynthetic cells absorb light energy and conserve it as ATP and NADPH while simultaneously, evolving O 2 gas.
• In the light-dependent reactions of photosynthesis, the chlorophyll absorbs high energy, short-wavelength light, which excites the electrons present inside the thylakoid membrane.
• The excitation of electrons now initiates the transformation of light energy into chemical energy.
• The light reactions take in two photosystems that are present in the thylakoid of chloroplasts.

Figure: Light-dependent reactions of photosynthesis in the thylakoid membrane of plant cells. Image Source: Wikipedia (Somepics) .

Photosystem II

• Photosystem II is a group of proteins and pigments that work together to absorb light energy and transfer electrons through a chain of molecules until it finally reaches an electron acceptor.
• Photosystem II has a pair of chlorophyll molecules, also known as P680 as the molecules best absorb light of the wavelength 680 nm.
• The P680 donates a pair of electrons after absorbing light energy, resulting in an oxidized form of P680.
• Finally, an enzyme catalyzes the splitting of a water molecule into two electrons, two hydrogen ion, and oxygen molecules.
• This pair of electrons then are transferred to P680, causing it to return to its initial stage.

Photosystem I

• Photosystem I is a similar complex like photosystem II except for that photosystem I have a pair of chlorophyll molecules known as P700 as they best absorb the wavelength of 700 nm.
• As photosystem I absorb light energy, it also becomes excited and transfers electrons.
• The now oxidized form of P700 then accepts an electron from photosystem II, restring back to its initial stage.
• The electrons from photosystem I are then passed in a series of redox reactions through the protein ferredoxin.
• The electrons finally reach NADP + , reducing them to NADPH.

2 H 2 O + 2 NADP +  + 3 ADP + 3 P i  + light → 2 NADPH + 2 H +  + 3 ATP + O 2

Video Animation: The Light Reactions of Photosynthesis (Ricochet Science)

2. Light independent reactions (Calvin cycle)

Light independent reactions of photosynthesis are anabolic reactions that lead to the formation of a sex-carbon compound, glucose in plants. The reactions in this stage are also termed dark reactions as they are not directly dependent on the light energy but do require the products formed from the light reactions.

Figure: Overview of the Calvin cycle pathway. Image Source: Wikipedia (Mike Jones) .

This stage consists of 3 further steps that lead to carbon fixation/ assimilation.

Step 1: Fixation of CO 2 into 3-phosphoglycerate

• In this step, one CO 2 molecule is covalently attached to the five-carbon compound ribulose 1,5-biphosphate catalyzed by the enzyme ribulose 1,5-biphosphate carboxylase, also called rubisco.
• The attachment results in the formation of an unstable six-carbon compound that is then cleaved to form two molecules of 3-phosphoglycerate.

Step 2: Conversion of 3-phosphoglycerate to glyceraldehydes 3-phosphate

• The 3-phosphoglycerate formed in step 1 is converted to glyceraldehyde 3-phosphate by two separate reactions.
• At first, enzyme 3-phosphoglycerate kinase present in the stroma catalyzes the transfer of a phosphoryl group from ATP to 3-phosphoglycerate, yielding 1,3-bisphosphoglycerate.
• Next, NADPH donates electrons in a reaction catalyzed by the chloroplast-specific isozyme of glyceraldehyde 3-phosphate dehydrogenase, producing glyceraldehyde 3-phosphate and phosphate (Pi).
• Most of the glyceraldehyde 3-phosphate thus produced is used to regenerate ribulose 1,5-bisphosphate.
• The rest of the glyceraldehyde is either converted to starch in the chloroplast and stored for later use or is exported to the cytosol and converted to sucrose for transport to growing regions of the plant.

Step 3: Regeneration of ribulose 1,5-biphosphate from triose phosphates

• The three-carbon compounds formed in the previous steps are then converted into the five-carbon compound, ribulose 1,5-biphosphate through a series of transformations with intermediates of three-, four,-, five-, six-, and seven-carbon sugar.
• As the first molecules in the process, if regenerated, this stage of photosynthesis results in a cycle (Calvin cycle).

3 CO 2 + 9 ATP + 6 NADPH + 6 H +     →     glyceraldehyde-3-phosphate (G3P) + 9 ADP + 8 P i  + 6 NADP +  + 3 H 2 O

A G3P molecule contains three fixed carbon atoms, so it takes two G3Ps to build a six-carbon glucose molecule. It would take six turns of the cycle to produce one molecule of glucose.

Video Animation: The Calvin Cycle (Ricochet Science)

Products of Photosynthesis

The outcomes of light-dependent reactions of photosynthesis are:

The products of light-independent reactions (Calvin cycle) of photosynthesis are:

• glyceraldehyde-3-phosphate (G3P) / Glucose (carbohydrates)

The overall products of photosynthesis are:

• Glucose (carbohydrates)
• Sulfur (in photosynthetic sulfur bacteria)

Photosynthesis Examples

Photosynthesis in green plants or oxygenic bacteria.

• In plants and oxygenic bacteria like cyanobacteria, photosynthesis takes place in the presence of green pigment, chlorophyll.
• It takes place in the thylakoids of the chloroplasts, resulting in products like oxygen gas, glucose, and water molecules.
• Most of the glucose units in plants are linked to form starch or fructose or even sucrose.

Photosynthesis in sulfur bacteria

• In purple sulfur bacteria, photosynthesis takes place in the presence of hydrogen sulfur rather than water.
• Some of these bacteria like green sulfur bacteria have chlorophyll whereas other purple sulfur bacteria have carotenoids as photosynthetic pigments.
• The result of photosynthesis in these bacteria are carbohydrates (not necessarily glucose), sulfur gas, and water molecules.

Importance of photosynthesis

• Photosynthesis is the primary source of energy in autotrophs where they make their food by utilizing carbon dioxide, sunlight, and photosynthetic pigments.
• Photosynthesis is equally essential for heterotrophs, as they derive their energy from the autotrophs.
• Photosynthesis in plants is necessary to maintain the oxygen levels in the atmosphere.
• Besides, the products of photosynthesis contribute to the carbon cycle occurring in the oceans, land, plants, and animals.
• Similarly, it also helps maintain a symbiotic relationship between plants, animals, and humans.
• Sunlight or solar energy is the primary source of all other forms of energy on earth, which is utilized through the process of photosynthesis.

Artificial photosynthesis

Artificial photosynthesis is a chemical process that mimics the biological process of utilization of sunlight, water and carbon dioxide to produce oxygen and carbohydrates.

Image Source: Phys .

• In artificial photosynthesis, photocatalysts are utilized that are capable of replicating the oxidation-reduction reactions taking place during natural photosynthesis.
• The main function of artificial photosynthesis is to produce solar fuel from sunlight that can be stored and used under conditions, where sunlight is not available.
• As solar fuels are prepared, artificial photosynthesis can be used to produce just oxygen from water and sunlight, resulting in clean energy production.
• The most important part of artificial photosynthesis is the photocatalytic splitting of a water molecule, resulting in oxygen and large quantities of hydrogen gas.
• Further, light-driven carbon reduction can also be performed to replicate the process of natural carbon fixation, resulting in carbohydrates molecules.
• Thus, artificial photosynthesis has applications in the production of solar fuels, photoelectrochemistry, engineering of enzymes, and photoautotrophic microorganisms for the production of microbial biofuel and biohydrogen from sunlight.

Video Animation: Learning from leaves: Going green with artificial photosynthesis

Photosynthesis vs. Cellular respiration

Image Source: Khan Academy .

Video Animation: Photosynthesis vs. Cellular Respiration Comparison (BOGObiology)

FAQs (Revision Questions)

Where does photosynthesis occur? Photosynthesis occurs in the thylakoid membrane of the chloroplasts.

What are the products of photosynthesis? The products of photosynthesis are carbohydrates (glucose), oxygen, and water molecules.

What are the reactants of photosynthesis? The reactants of photosynthesis are carbon dioxide, water, photosynthetic pigments, and sunlight.

How are photosynthesis and cellular respiration related? Photosynthesis and cellular respiration are essentially the reverses of one another where photosynthesis is an anabolic process resulting in the formation of organic molecules. In contrast, cellular respiration is a catabolic process resulting in the breaking down of organic molecules to release energy.

• Berg JM, Tymoczko JL, Stryer L. Biochemistry. 5th edition. New York: W H Freeman; 2002. Section 17.2, Entry to the Citric Acid Cycle and Metabolism Through It Are Controlled.Available from: https://www.ncbi.nlm.nih.gov/books/NBK22347/
• Nelson DL and Cox MM. Lehninger Principles of Biochemistry. Fourth Edition.
• Montero F. (2011) Photosynthetic Pigments. In: Gargaud M. et al. (eds) Encyclopedia of Astrobiology. Springer, Berlin, Heidelberg
• Lodish H, Berk A, Zipursky SL, et al. Molecular Cell Biology. 4th edition. New York: W. H. Freeman; 2000. Section 16.3, Photosynthetic Stages and Light-Absorbing Pigments.Available from: https://www.ncbi.nlm.nih.gov/books/NBK21598/

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Anupama Sapkota

1 thought on “Photosynthesis: Equation, Steps, Process, Diagram”

How can we say that 6 calvin cycles are needed to produce 1 glucose molecule why not 2?

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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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ENCYCLOPEDIC ENTRY

Photosynthesis.

Photosynthesis is the process by which plants use sunlight, water, and carbon dioxide to create oxygen and energy in the form of sugar.

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Learning materials, instructional links.

• Photosynthesis (Google doc)

Most life on Earth depends on photosynthesis .The process is carried out by plants, algae, and some types of bacteria, which capture energy from sunlight to produce oxygen (O 2 ) and chemical energy stored in glucose (a sugar). Herbivores then obtain this energy by eating plants, and carnivores obtain it by eating herbivores.

The process

During photosynthesis, plants take in carbon dioxide (CO 2 ) and water (H 2 O) from the air and soil. Within the plant cell, the water is oxidized, meaning it loses electrons, while the carbon dioxide is reduced, meaning it gains electrons. This transforms the water into oxygen and the carbon dioxide into glucose. The plant then releases the oxygen back into the air, and stores energy within the glucose molecules.

Chlorophyll

Inside the plant cell are small organelles called chloroplasts , which store the energy of sunlight. Within the thylakoid membranes of the chloroplast is a light-absorbing pigment called chlorophyll , which is responsible for giving the plant its green color. During photosynthesis , chlorophyll absorbs energy from blue- and red-light waves, and reflects green-light waves, making the plant appear green.

Light-dependent Reactions vs. Light-independent Reactions

While there are many steps behind the process of photosynthesis, it can be broken down into two major stages: light-dependent reactions and light-independent reactions. The light-dependent reaction takes place within the thylakoid membrane and requires a steady stream of sunlight, hence the name light- dependent reaction. The chlorophyll absorbs energy from the light waves, which is converted into chemical energy in the form of the molecules ATP and NADPH . The light-independent stage, also known as the Calvin cycle , takes place in the stroma , the space between the thylakoid membranes and the chloroplast membranes, and does not require light, hence the name light- independent reaction. During this stage, energy from the ATP and NADPH molecules is used to assemble carbohydrate molecules, like glucose, from carbon dioxide.

C3 and C4 Photosynthesis

Not all forms of photosynthesis are created equal, however. There are different types of photosynthesis, including C3 photosynthesis and C4 photosynthesis. C3 photosynthesis is used by the majority of plants. It involves producing a three-carbon compound called 3-phosphoglyceric acid during the Calvin Cycle, which goes on to become glucose. C4 photosynthesis, on the other hand, produces a four-carbon intermediate compound, which splits into carbon dioxide and a three-carbon compound during the Calvin Cycle. A benefit of C4 photosynthesis is that by producing higher levels of carbon, it allows plants to thrive in environments without much light or water. The National Geographic Society is making this content available under a Creative Commons CC-BY-NC-SA license . The License excludes the National Geographic Logo (meaning the words National Geographic + the Yellow Border Logo) and any images that are included as part of each content piece. For clarity the Logo and images may not be removed, altered, or changed in any way.

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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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Photosynthesis – Definition, Steps, Equation, Process, Diagram, Examples

What is photosynthesis, definition of photosynthesis.

Photosynthesis is the biological process by which plants, algae, and certain bacteria convert light energy into chemical energy, producing oxygen and organic compounds, primarily glucose, from carbon dioxide and water.

Experimental History

Where Does Photosynthesis take place?

Photosynthesis primarily occurs in the chloroplasts, specialized organelles found predominantly in plant leaves. These chloroplasts belong to a category of organelles known as plastids, which are membrane-bound structures responsible for various vital cellular functions.

Photosynthesis: a two-stage process

Photosynthesis, a fundamental biochemical process, can be delineated into two distinct stages. Contrary to the traditional classification of these stages as ‘light’ and ‘dark’ reactions, contemporary scientific understanding emphasizes that both stages are influenced by light, albeit in different capacities.

1. Photochemical Reaction Process (Light-Dependent Reactions):

2. carbon fixation process (light-independent reactions):.

While this stage does not directly utilize light energy, it is influenced by the products of the light-dependent reactions. The primary objective here is the conversion of inorganic carbon into organic compounds. This energy-consuming, endergonic process can manifest in two distinct pathways:

Photosynthesis equations/reactions/formula

Oxygenic photosynthesis in plants:.

In green plants, photosynthesis utilizes water and carbon dioxide, harnessing solar energy to produce glucose and oxygen. The overarching equation representing this process is:

6 CO 2 ​+12 H 2 ​ O +light energy→ C 6 ​ H 12 ​ O 6 ​+6 O 2​ +6 H 2 ​ O

This equation signifies that six molecules of carbon dioxide and twelve molecules of water, in the presence of light energy, yield one molecule of glucose, six molecules of oxygen, and six molecules of water. It’s noteworthy that while a triose (3-carbon molecule) is the direct product of photosynthesis, glucose, a hexose, is often depicted as the end product due to its foundational role in cellular processes. Additionally, the oxygen produced serves dual purposes: it is utilized by the plant during oxidative phosphorylation and is also released into the atmosphere, facilitating aerobic respiration in other organisms.

Anoxygenic Photosynthesis in Sulfur Bacteria:

CO 2 ​+2 H 2 ​ S +light energy→( CH 2 ​ O )+ H 2 ​ O +2 S

This equation illustrates that carbon dioxide and hydrogen sulfide, under the influence of light energy, produce carbohydrates, water, and elemental sulfur.

In summation, photosynthesis, whether in plants or sulfur bacteria, is a series of intricate reactions that convert simple molecules into energy-rich compounds, using light as the primary energy source. The specific reactants and products differ based on the organism, but the core principle of harnessing light energy remains consistent.

Types of Photosynthesis

Photosynthesis, the fundamental process by which organisms convert light energy into chemical energy, manifests in various forms across different species. These diverse pathways are tailored to the specific environmental conditions and metabolic needs of the organisms. Here, we delve into the primary types of photosynthesis and their distinctive characteristics.

In summary, photosynthesis, though universally critical for life on Earth, manifests in a myriad of forms, each tailored to the unique needs and environments of the photosynthesizing organisms.

Photosynthetic pigments

2. Carotenoids: Carotenoids, often yellow, red, or purple, work synergistically with chlorophyll. Their key characteristics are:

4. Bacteriorhodopsin: Exclusive to halobacteria, bacteriorhodopsin is a pigment that consists of a protein linked to a retinal prosthetic group. It plays a role in light absorption, leading to proton expulsion from the cell.

In summary, photosynthetic pigments are integral to the process of photosynthesis, ensuring efficient light absorption and energy conversion. Each pigment type has a distinct role and absorption spectrum, collectively ensuring that a broad range of light wavelengths is harnessed for photosynthesis.

Structure Of Chlorophyll

Factors affecting photosynthesis.

Photosynthesis, the process by which green plants and certain other organisms transform light energy into chemical energy, is influenced by a myriad of both external and internal factors. A comprehensive understanding of these factors is pivotal for optimizing the photosynthetic efficiency of plants.

1. Light Intensity, Quality, and Duration:

2. temperature:.

Temperature plays a pivotal role in enzymatic activities associated with photosynthesis. For C3 plants, the optimal temperature range lies between 20-25°C, while C4 plants exhibit peak efficiency between 30-45°C. Beyond these ranges, enzymatic activities can diminish, potentially hampering the photosynthetic rate.

3. Carbon Dioxide Concentration:

4. water availability:.

Water is indispensable for photosynthesis, participating in both light-dependent and light-independent reactions. A deficiency can impede the electron flow in Photosystem II and disrupt the Calvin cycle, thereby affecting the overall photosynthetic efficiency.

5. Genetic Factors:

Intrinsic genetic factors can also influence photosynthesis. Variations in genes encoding for photosynthetic machinery can lead to differences in their efficiency and functionality. Some species have evolved unique mechanisms to enhance photosynthetic efficiency, adapting to their specific environmental conditions.

Photosynthetic Membranes and Organelles

Photosynthesis, the fundamental process by which energy from sunlight is converted into chemical energy, is facilitated by specific pigment molecules that absorb photons of light. The efficiency of this process is contingent upon the absorption of light within a precise wavelength range, ensuring the optimal energy required for photosynthesis.

To achieve this specificity in light absorption, phototrophic organisms have evolved specialized structures known as reaction center proteins. These proteins house the pigment molecules and are strategically positioned within the membranes of the organisms to optimize light absorption.

Organelle for Photosynthesis

Process/ steps of photosynthesis – mechanisms of photosynthesis.

Photosynthesis, a cornerstone of plant physiology, is a systematic process that facilitates the conversion of light energy into chemical energy stored in organic molecules. This intricate mechanism can be elucidated through the following sequential stages:

In summation, photosynthesis is a meticulously orchestrated sequence of events, harmonizing light-dependent and light-independent reactions to produce organic compounds vital for plant sustenance and growth.

Types/ Stages/ Parts of photosynthesis

1. light-dependent reactions.

Photosynthesis, a pivotal biochemical process, encompasses a series of reactions that convert light energy into chemical energy. The light-dependent reactions, as the name suggests, are contingent on the presence of light and primarily transpire within the thylakoid membranes of chloroplasts. These reactions can be systematically delineated into the following stages:

1. Photon Absorption and Light Harvesting

The initial phase involves the capture of photons by chlorophyll molecules, accessory pigments, and associated proteins, collectively termed photoreceptors. These photoreceptors are organized into intricate assemblies, comprising a photosynthetic reaction center, core antenna complexes, and light-harvesting complexes (LHC). The core antenna and LHC, constituted by accessory pigments and chlorophyll-bound proteins, function as photon traps, capturing diverse wavelengths of light. Sequentially, the absorbed photon energy is channeled towards the photosynthetic reaction center.

2. Electron Transport Dynamics:

Photon absorption culminates in the elevation of an electron to a heightened energy state, engendering a negatively charged radical. This unstable state prompts the electron’s spontaneous transfer to a proximate acceptor molecule, rendering the photoreceptor positively charged. Subsequent replenishment of the photoreceptor is achieved through electrons derived either from water or another electron transfer chain.

3. Photophosphorylation:

Wavelengths of light involved and their absorption.

Within the broad spectrum of visible light lies a critical subset termed PAR, or Photosynthetically Active Radiation. This segment, ranging from 400 to 760 nm, is of paramount importance to photosynthetic processes. Within PAR, specific wavelengths play distinct roles:

Absorption spectrum and action spectrum

In summary, while the absorption spectrum provides insights into the light-absorbing capabilities of pigments, the action spectrum reveals the functional efficacy of these absorbed wavelengths in driving photosynthesis. Together, these spectra offer a comprehensive understanding of the intricate interplay between light and photosynthetic pigments.

What actually happens in the Light-dependent reaction

Water photolysis.

Water photolysis, also known as the oxygen-evolving process, is a fundamental mechanism within the photosynthetic pathway. This process is responsible for replenishing the electron deficit experienced by the chlorophyll molecule during the initial stages of photosynthesis. Here’s a comprehensive overview of water photolysis:

In summary, the Z-Scheme is a crucial component of the non-cyclic photochemical reactions in photosynthesis. It delineates the intricate electron flow through both photosystems, facilitating the production of essential energy molecules, NADPH and ATP, vital for the subsequent stages of photosynthesis.

Cyclic vs. Non-cyclic phosphorylation

2. Light independent reactions (Calvin cycle)

The Calvin Cycle, also known as the light-independent reactions, is a crucial phase of photosynthesis that operates in the stroma of chloroplasts. Contrary to its name, this cycle doesn’t directly rely on light; however, it is dependent on the ATP and NADPH produced during the light-dependent reactions.

In essence, the Calvin Cycle is a metabolic pathway that transforms carbon dioxide and other compounds into glucose, providing energy and structural integrity to plants. This intricate process underscores the importance of both light-dependent and light-independent reactions in sustaining life on Earth.

Order and kinetics of Photosynthesis

Following energy absorption, the second stage encompasses the transfer of these energized electrons through a series of photochemical reactions within the thylakoid membranes. This process operates at a slightly longer timescale, ranging from picoseconds to nanoseconds. As electrons shuttle through protein complexes, such as Photosystem II and Photosystem I, they undergo redox reactions that ultimately result in the generation of ATP and the reduction of NADP+ to NADPH.

In conclusion, the order and kinetics of photosynthesis encompass a meticulously orchestrated sequence of events, spanning a wide range of time scales from femtoseconds to seconds. This process showcases the remarkable efficiency of nature in harnessing light energy to sustain life on Earth, underlining its significance in the realm of biological science.

Carbon Concentrating Mechanisms in Photosynthesis

2. CAM Photosynthesis: Crassulacean Acid Metabolism (CAM) is another carbon concentrating mechanism predominantly found in xerophytes like cacti and succulents, with around 16,000 species employing this strategy.

Regulation of the cycle

Photosynthesis is not possible in the night, however, glycolysis, a process that utilizes the same reactions as those in the Calvin-Benson cycles, with the exception of the reverse, takes place. This means that certain steps in the cycle would be inefficient when they are allowed to take place in darkness, as they could impede the process of glycolysis. In this regard, certain enzymes in the Calvin-Benson cycle can be “turned off” (i.e. they become inactive) in darkness.

Products of Photosynthesis

These products, especially glucose and oxygen, are fundamental to the survival and growth of most organisms on Earth.

Process of Photosynthesis – Overview

Light-dependent reactions vs. light-independent reactions.

Photosynthesis, the process by which plants convert light energy into chemical energy, is delineated into two primary phases: the light-dependent reactions and the light-independent reactions. Each phase plays a distinct role in the overall process, and they are characterized by their dependency on light and their respective locations within the chloroplast.

Photosynthesis examples

Importance of photosynthesis.

In essence, photosynthesis is not just a biological process but a cornerstone of life on Earth. It interlinks various biogeochemical cycles, supports biodiversity, and ensures the continuity of energy flow in ecosystems.

What is Artificial photosynthesis?

Photosynthesis vs cellular respiration.

Evolution of Photosynthesis

The fossil record provides intriguing insights into the origins of photosynthesis. Fossils of filamentous photosynthetic organisms, dating back approximately 3.4 billion years, suggest that photosynthesis may have commenced around this time. These ancient organisms likely laid the foundation for the photosynthetic processes we observe today.

A pivotal moment in the evolution of photosynthesis occurred with the origin of chloroplasts. Chloroplasts, which bear striking similarities to photosynthetic bacteria, are thought to have originated through endosymbiosis. According to this theory, early eukaryotic cells engulfed photosynthetic bacteria, eventually forming the first plant cells. The evidence for this theory lies in the presence of chloroplast DNA , separate from the host cell’s nucleus, resembling the genetic makeup of cyanobacteria. This genetic legacy supports the notion that chloroplasts evolved from photosynthetic bacteria.

Early photosynthetic systems, particularly those of green and purple sulfur and green and purple nonsulfur bacteria, were likely anoxygenic and utilized various molecules, such as hydrogen, sulfur, and organic acids, as electron donors. These systems were consistent with the highly reducing conditions of the early Earth’s atmosphere. Archaea, including haloarchaea, can harness energy from the sun but do not perform oxygenic photosynthesis.

Quiz Practice

A) Chlorophyll B) Carotenoid C) Xanthophyll D) Phycobilin

MCQ 2: In which organelle does photosynthesis primarily occur in plant cells?

A) Nucleus B) Mitochondria C) Chloroplast D) Endoplasmic reticulum

A) Oxygen B) Carbon dioxide C) Glucose D) Water

A) Soil B) Air C) Water D) Other plants

MCQ 5: Which of the following colors of light is least effective in driving photosynthesis?

A) Red B) Blue C) Green D) Yellow

A) Produce ATP B) Convert glucose to starch C) Generate carbon dioxide D) Synthesize glucose

MCQ 7: In C4 plants, what is the primary function of mesophyll cells?

A) Storing water B) Capturing light energy C) Fixing carbon dioxide D) Producing oxygen

A) Oxygen B) Carbon dioxide C) Nitrogen D) Hydrogen

MCQ 9: Which environmental factor can limit the rate of photosynthesis?

A) High oxygen levels B) Low carbon dioxide levels C) Warm temperatures D) Bright sunlight

A) Generate oxygen B) Produce ATP C) Fix carbon dioxide D) Capture light energy

Where does photosynthesis take place?

What are the reactants of photosynthesis, how are photosynthesis and cellular respiration related.

Photosynthesis is the process by which atmospheric carbon dioxide is assimilated and converted to glucose and oxygen is released. CO2 and H2O are utilised in the process. In the cellular respiration, glucose is broken down to CO2 and energy is released in the form of ATP, which is utilised in performing various metabolic processes. Oxygen is utilised in the process. Energy is stored in the process of photosynthesis, whereas it is released in the process of cellular respiration. The process of cellular respiration and photosynthesis complement each other. These processes help cells to release and store the energy respectively. They are required to keep the atmospheric balance of carbon dioxide and oxygen concentrations.

What is the equation for photosynthesis?

Where does photosynthesis occur, what are the products of photosynthesis, why is photosynthesis important.

Photosynthesis is the main source of food on earth. It releases oxygen which is an important element for the survival of life. Without photosynthesis, there will be no oxygen on earth. The stored chemical energy in plants flows into herbivores, carnivores, predators, parasites, decomposers, and all life forms.

Which of these equations best summarizes photosynthesis? A. C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + Energy B. C6H12O6 + 6 O2 → 6 CO2 + 12 H2O C. 6 CO2 + 6 H2O → C6H12O6 + 6 O2 D. 6 CO2 + 6 O2 → C6H12O6 + 6 H2O E. H2O → 2 H+ + 1/2 O2 + 2e-

What are the raw materials of photosynthesis, which gas is removed from the atmosphere during photosynthesis.

Photosynthesis removes CO2 from the atmosphere and replaces it with O2.

Which of the following sequences correctly represents the flow of electrons during photosynthesis? A. NADPH → O2 → CO2 B. H2O → NADPH → Calvin cycle C. NADPH → chlorophyll → Calvin cycle D. H2O → photosystem I → photosystem II

What organelle does photosynthesis occur in, what are the inputs of photosynthesis, related biology study notes, a-z botany terms with definitions – botany glossary, marchantia – characteristics, structure, reproduction, classification, what are lichens – characteristics, types, structure, reproduction, hornworts – morphology, life cycle, importance, examples, liverworts – characteristics, morphology, reproduction, classification, mosses (bryopsida) – morphology, characteristics, reproduction, uses, examples, transgenic plants – examples, definition, procedure, application, mesophyll cells – definition, location, structure, function, microscopy, latest questions, leave a comment cancel reply.

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Photosynthesis

All you need to know about photosynthesis.

Photosynthesis is the process by which a plant produces its food by converting light energy into chemical energy. Plants use water, carbon dioxide and sunlight in the presence of chlorophyll to produce their food or energy in the form of sugar and release oxygen as the byproduct. Understanding the process of photosynthesis means a clear concept about the different cellular and chemical activities going on in the plant body. The word photosynthesis is coined from the Greek word for meaning light along with synthesis. This implies a synthesis i.e a chemical reaction using light energy. This is not always for green plants only but certain bacteria and prokaryotes also use this process to prepare their food. In green plants or algae, the synthesis takes place within an important organelle called chloroplast where the pigment chlorophyll is present. Chlorophyll occurs in their leaves, stems, flowers, sepals and even in plastids.

Factors Affecting Photosynthesis

Various factors influence/affect the photosynthesis process. These are:

Light Intensity: More the light, the more will be the rate of photosynthesis. Similarly, low light will lead to a low rate of photosynthesis.

The Concentration of CO 2 : A higher CO 2 concentration rate in a plant also accelerates the photosynthesis process. The required amount of CO 2 is 300-400 PPM.

Temperature: If the temperature is between the range of 25 to 35 degrees Celsius, photosynthesis takes place effectively.

Water: An essential amount of water is required for stomatal opening, and it’s a key factor in the process of photosynthesis.

Pollution: The increasing rate of polluting particles in the atmosphere block the pores of somatic cells, and the intake of carbon dioxide becomes difficult.

Photosynthesis Equation

Carbon dioxide and water are the two major factors involved in the photosynthesis reaction. It’s an endothermic reaction, and the products resulting from it are oxygen and glucose. The formula is:

\[6CO_{2} + 6H_{2}O = C_{6}H_{1}2O_{6} + 6O_{2}\]

However, some bacteria don’t produce oxygen as a by-product of photosynthesis. They are called anoxygenic photosynthetic bacteria, and those who do it are called oxygenic photosynthetic bacteria.

Photosynthetic Pigments

Four types of photosynthetic pigments are present in the leaves of the plants. They are:

Chlorophyll a

Chlorophyll b

Xanthophylls

Carotenoids

Structure of Chlorophyll

Chlorophyll is a green colour pigment found in plants that play a vital role in photosynthesis. It allows the plants to absorb the energy coming from the sunlight, which is essential for photosynthesis.

Process of Photosynthesis

The photosynthesis process occurs in plants. It takes place in chloroplasts at the cellular level that contains chlorophyll. Leaves have parts called the petiole, epidermis, and lamina that absorb sunlight.

Photosynthesis Steps:

The photosynthesis process takes place at two levels or steps. These are:

Light Reaction of Photosynthesis (or) Light-dependent Reaction

The process begins in the daylight, by gathering the light. The two types of photosystems convert light energy into ATP and NADPH. During their conversion, oxygen is produced, and water is used. The equation of this step is:

\[2H_{2}O + 2NADP + 3ADP + 3P_{i} = O_{2} + 2 NADPH + 3ATP\]

The Dark Reaction of Photosynthesis (or) Light-independent Reaction

This is also called carbon-fixing. It is not dependent on light and takes place in chloroplast where the products from the earlier step are used. Plants intake CO 2 and the Calvin Photosynthesis Cycle begins, where the six molecules of CO 2 are converted into sugar or glucose.

\[3CO_{2} + 6NADPH + 5H_{2}O + 9ATP = G3P + 2H + 6NADP + 9ADP + 8P_{i}\]

Importance of Photosynthesis

The photosynthesis process is very important for the survival of living beings, and to continue the food chain. It also produces oxygen, which is required for breathing.

FAQs on Photosynthesis

1. What is photosynthesis?

The process by plants, of producing nutrients essential for survival.

2. What happens in a photosynthesis process?

The nutrients and glucose required for plants, and oxygen required for the animals, is created during the process.

3. What is the photosynthesis reaction?

A reaction taking place in plants that results in the production of glucose and oxygen.

4. What is shown in a photosynthesis diagram?

The photosynthesis diagram shows how the plants take sunlight and use it to produce essential nutrients and oxygen.

5. Why is photosynthesis in plants important?

It helps in the survival of the plants, the creation of products essential for the survival of living beings, and also to maintain the environmental balance.

6. How long is the photosynthesis cycle?

The whole process of photosynthesis, right from absorbing the light up to the final stage, takes place in just 30 seconds!

7. What is included in the mechanism of photosynthesis?

The mechanism of photosynthesis is an oxidation (oxygen releasing) and reduction reaction. It produces glucose along with oxygen.

8. What are the two phases of photosynthesis?

Photosynthesis is a synthesis reaction in which solar energy gets converted to chemical energy. However, the few steps involved in the synthesis also occur without light and based on that there are two phases of photosynthesis. The light-dependent reactions and the light-independent phase. In the light-dependent phase which is the starting phase the molecule of chlorophyll pigment absorbs one photon from the sun rays and loses electrons and ultimately after several steps generate NADPH and ATP which are used in the second phase or the light-independent phase of the reaction. In the dark phase, the atmospheric carbon dioxide is captured by a photosynthetic enzyme that uses the NADPH formed in the first phase to produce 3 carbon sugars which ultimately gets converted to starch and sucrose. Thus the two phases occur subsequently to form the end products which are the food of the plant.

9. What is the efficiency of the photosynthetic reaction?

Photosynthesis is the most important reaction or synthesis for which lives and the planet can run. It converts light to chemical energy. The efficiency of the reaction is however only 3-6% and the absorbed light cannot be used. The unused light gets dissipated to the atmosphere as heat. The efficiency of the synthesis varies with temperature, light intensity and proportion of carbon dioxide in nature which can vary the efficiency to a maximum of 8% even. The efficiency of the two phases can be separately taken into count and they both separately contribute to the total efficiency of the process.

10. Which factors can affect the process of photosynthesis?

Photosynthesis is affected by many factors including the corollary ones. The main factors that affect the process are as follows:

Light: Its intensity and wavelength

Carbon dioxide: Its concentration in the atmosphere

Temperature: Favorable temperature is needed

Water: Suitable quantity of water is needed

Apart from these factors photosynthesis also depends upon the factor referred to as the corollary factor which is the surface area of the leaf available to absorb the sunlight. This is why if a plant is overshadowed by other big trees it cannot efficiently photosynthesize.

11. What are the actual stages of the process of photosynthesis?

The process of photosynthesis can be divided into four essential steps. They are as follows:

Transfer of solar energy in thylakoid membranes of chlorophyll.

Transfer of the electrons in the light reaction

The synthesis of electron transport chain and ATP in the thylakoid membrane.

Absorption and fixation of carbon dioxide from the atmosphere with the formation of resulting stable plant food.

Each of these steps takes a specific time to happen with the maximum time of one second taken by the last step.

12. How is photosynthesis essential for the ecosystem?

Photosynthesis is the procedure by which plants convert solar energy into chemical energy in the form of their food. Photosynthesis is an ideal process for a living organism that has not evolved the capacity to actively move around in order to obtain food. Photosynthesis is also important for the consumers of the food web and thus is the starting point of all food chains of the ecosystem. Photosynthesis is thus an essential living process for plants and the ecosystem.  All life on the earth depends upon photosynthesis, not only for the food but also for the oxygen in the atmosphere which is essential for them to respire. It is the process of photosynthesis that purifies the atmosphere by removing carbon dioxide from the environment and converting it to oxygen.

Biology • Class 11

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: Definition, photosynthetic pigments, stage of light and dark reaction

October 23, 2020 Gaurab Karki Botany 0

What is photosynthesis?

• The process of synthesis with the assistance of light is termed as photosynthesis.
• It is the mechanism by which sunlight is used by green plants to produce food from simple molecules like CO2 and H2O.
• 6CO 2 + 6H 2 O——— > C 6 H 12 O 6 + 6O 2
• The light energy is transformed into chemical energy during the photosynthesis process and is stored into the organic matter, which is normally carbohydrates, and O2 is a by-product of photosynthesis.
• The raw materials for this process are CO 2 and H 2 O.
• CO 2 from the air and H 2 O from the soil are absorbed.
• The following points make photosynthesis essential to humanity:
• It preserves the atmospheric equilibrium of O 2 .
• It provides food either directly as vegetables or indirectly as animal meat or milk which, in turn, is fed to plants.

Where does photosynthesis takes place?

• The photosynthesis occurs in chloroplasts in case of plants and blue-green algae.
• When microscopic observation of a leaf section is done, chlorophyll is not uniformly distributed in the cell.
• However, it is confined to organelles termed as chloroplasts.
• In plants, chloroplasts lie mainly in the cells of mesophyll, a layer that includes several air spaces and a very high concentration of water vapor.
• The exchange of gases takes place between interior of the leaf and the outside through microscopic pores, termed as stomata.
• Each mesophyll cell consists of 20-100 chloroplasts.
• The chloroplast is enclosed by outer and inner membranes.
• The inner membrane encloses a fluid filled region termed as stroma.
• Stroma consists of most of the enzymes needed to produce carbohydrate molecules.
• The third system of membranes is suspended in the stroma.
• This system forms an interconnected set of flat, disc like sacs called thylakoids.
• A fluid-filled internal space, the thylakoid lumen, is enclosed by the thylakoid membrane.
• Thylakoid sacs are arranged in stacks called grana (sing., granum) in some regions of the chloroplast.
•  Each granum is identical to a coin stack, with a thylakoid being each “coin.”
• Some thylakoid membranes extend from one granum to another.
• Thylakoid membranes are involved in ATP synthesis.

Photosynthetic pigments:

• Chlorophylls
• Phycobillins

i. Chlorophylls:

• Chlorophylls are present in thylakoid membranes.
• Thylakoid membranes consist of several kinds of pigments that absorb visible light.
• Various pigments absorb light from various wavelengths.
• Chlorophyll is the dominant photosynthesis pigment and it absorbs light mainly in the visible spectrum’s blue and red areas.
• Green light is not appreciably absorbed by chlorophyll.
• Some of the green light that strikes the plant is either scattered or reflected and hence the plants typically appear to be green in colour.
• There are two principal parts of a chlorophyll molecule, a complex ring and a long side chain.
• A porphyrin ring is ring structure that comprises of smaller rings made of carbon and nitrogen atoms.
• The porphyrin ring absorbs light energy.
• The chlorophyll porphyrin ring is remarkably similar to the red pigment hemoglobin component of the heme in red blood cells.
• However, unlike heme, that holds an atom of iron in the center of the ring, chlorophyll consists of an atom of magnesium in that position.
• The chlorophyll molecule also contains a long, hydrocarbon side chain that makes the molecule extremely nonpolar and ties up the chlorophyll in the membrane
• All chlorophyll molecules in the thylakoid membrane are associated with specific chlorophyll-binding proteins
• Each thylakoid membrane is filled with accurately oriented chlorophyll molecules and chlorophyll-binding proteins that facilitate the transfer of energy from one molecule to another.
• There are many kinds of chlorophyll.
• The chlorophyll ‘a’ is the most essential pigment that starts the light-dependent reactions of photosynthesis.
• Chlorophyll ‘b’ is the accessory pigment which also takes part in photosynthesis.
• It differs from chlorophyll ‘a’ only in a functional group on the porphyrin ring.
• The methyl group (-CH 3 ) in chlorophyll ‘a’ is replaced in chlorophyll ‘b’ by a terminal carbonyl group (-CHO).
• This difference shifts the wavelengths of light absorbed and reflected by chlorophyll ‘b’.
• It makes it to appear yellow-green, whereas chlorophyll ‘a’ appears bright green.

ii.  Carotenoids:

• Carotenes (orange)
• Xanthophyll (yellow)
• Carotenoids absorb different wavelengths of light than chlorophyll.
• Thus it expands the spectrum of light that provides energy for photosynthesis.
• Chlorophyll may be excited by light directly by energy passed to it from the light source or indirectly by energy passed to it from accessory pigments that have become excited by light.
• When a carotenoid molecule is excited, its energy can be transferred to chlorophyll a .
• In addition, carotenoids are antioxidants that inactivate highly reactive forms of oxygen generated in the chloroplasts hence termed as shield pigment.

iii. Phycobillins:

• It is water soluble pigment.
• It consists of 4 pyrrol rings that lacks Mg and phytol tail.
• It is responsible for maximum absorption in green parts of the spectrum.
• It is distributed in case of red and blue algae.
• Phycoerythrin are distributed in red algae and phycocyanin are distributed in red as well as blue algae.

What are the stages of photosynthesis ?

• Photosynthesis consists of two phases.
• Light dependent reaction (grana reaction or Hill reaction or Photochemical reaction or Primary photochemical reaction)
• Dark reactions (light independent reaction or stroma reaction or Blackman’s reaction or carbon-fixation reactions)

Stage I: Light dependent reaction/ Hill reaction:

• Light reaction occurs in grana of chloroplast and it requires the light for it to take place.
• Green plants uses light belonging to visible spectrum (380nm-760nm).
• In light reaction, the light energy is converted into chemical energy in the forms of ATP, NADPH+H + , and oxygen.
• According to Robert Hill and Bendall, light reaction comprises of two steps i.e. photolysis and photophosphorylation.
• Photophosphorylation

Photolysis:

• The light reaction initiates when the chlorophyll absorbs light. It causes one of its electrons to move to a higher energy state.
• The excited electron is transferred to an acceptor molecule and is replaced by an electron from water (H 2 O).
• It causes the decomposition of water into H + and OH – , hence termed as photolysis.

Photo-phosphorylation:

• Chlorophylls a and b and accessory pigment molecules are arranged with pigment-binding proteins in the thylakoid membrane into units called antenna complexes.
• The pigments and related proteins are organized as highly ordered groups comprising approximately 250 molecules of chlorophyll associated with particular enzymes and other proteins.
• Each antenna complex absorbs light energy and transfers it to its reaction system.
• This reaction system consists of chlorophyll molecules and proteins, including electron transfer components directly involved in photosynthesis.
• The light energy is converted into chemical energy in the reaction centers by a series of electron transfer reactions.
• Photosynthesis requires two types of photosynthetic units, referred to as photosystem I and photosystem II.
• Their reaction centers can be differentiated as they are related to proteins in a way that induces a slight difference in their absorption spectra.
• Normally, chlorophyll a has a strong absorption peak at about 660 nm.
•  In contrast, the reaction center of photosystem I consists of a pair of chlorophyll a molecules with an absorption peak at 700 nm and is referred to as P700 .
• The reaction center of photosystem II is made up of a pair of chlorophyll a molecules with an absorption peak of about 680 nm and is referred to as P680
• Photophosphorylation is further divided into two types:
• Photosystem II becomes activated when a pigment molecule in an antenna complex absorbs a photon of light energy.
• The energy is transferred to the reaction center, where it causes an electron in a molecule of P680 to move to a higher energy level.
• This energized electron is accepted by a primary electron acceptor (a highly modified chlorophyll molecule known as pheophytin ).
• Then it passes along an electron transport chain until it is donated to P700 in photosystem I.
• A pigment molecule in an antenna complex associated with photosystem I absorbs a photon of light.
• The absorbed energy is transferred from one pigment molecule to another until it reaches the reaction center, where it excites an electron in a molecule of P700.
• This energized electron is transferred to a primary electron acceptor, which is the first of several electron acceptors in a series.
• The energized electron is passed along an electron transport chain from one electron acceptor to another, until it is passed to ferredoxin , an iron-containing protein.
• Ferredoxin transfers the electron to NADP +  in the presence of the enzyme ferredoxin–NADP +   reductase .
• Although single electrons pass down the electron transport chain, two are needed to reduce NADP + . When NADP +   accepts 2 electrons, they unite with a proton (H + ); thus, the reduced form of NADP +   is NADPH, which is released into the stroma.
• P700 becomes positively charged when it gives up an electron to the primary electron acceptor.
• The missing electron is replaced by one donated by photosystem II.
• Noncyclic electron transport is a continuous linear process
• In the availability of light, there is a continuous, one- way flow of electrons from the ultimate electron source, H2O, to the terminal electron acceptor, NADP+.
• Water goes through enzymatically catalyzed photolysis to replace energized electrons donated to the electron transport chain by molecules of P680 in photosystem II.
• These electrons travel down the electron transport chain that connects photosystem II with photosystem I.
• Thus, they provide a continuous supply of replacements for energized electrons that have been given up by P700.
• As electrons are transferred along the electron transport chain that connects photosystem II with photosystem I, they lose energy.
• Some of the energy released is used to pump protons across the thylakoid membrane, from the stroma to the thylakoid lumen, producing a proton gradient.
• The energy of this proton gradient is utilized to produce ATP from ADP by chemiosmosis .
• ATP and NADPH, the products of the light-dependent reactions, are released into the stroma.
• Both are required by the carbon fixation reactions.
• Cyclic electron transport is the simplest light-dependent reaction.
• Only Photosystem I is involved in this reaction.
• The term cyclic is given for the pathway as the energized electrons that initiate from the P700 at the reaction finally return to P700.
• In the presence of light, electrons flow continuously through an electron transport chain within the thylakoid membrane.
• As the electrons pass from one acceptor to the other, the electrons lose energy.
• Some of the energy is used to pump protons across the thylakoid membrane.
• ATP synthase, an enzyme present in the thylakoid membrane uses the energy of the proton gradient to manufacture ATP.
• In the cyclic photophosphorylation, NADPH is not produced, H 2 O is not split, and oxygen is not generated.
• Hence, it cannot serve as basis for photosynthesis as NADPH is required to reduce CO 2 to carbohydrate.
• The importance of cyclic electron transport to photosynthesis in plants is unclear.
• Cyclic electron transport may take place in plant cells when there is too little NADP+ to accept electrons from ferredoxin.
• There is a proof that cyclic electron flow may aid to maintain the optimal ratio of ATP to NADPH required for carbon fixation as well as provide extra ATP to power other ATP-requiring processes in chloroplasts.
• The process is termed as photophosphorylation as the synthesis of ATP (i.e. the phosphorylation of ADP) is coupled to the transport of electrons that have been energized by photons of light.

Stage II: Dark reaction/Carbon fixation reactions:

• In carbon fixation, the energy of ATP and NADPH is used in the formation of organic molecules from CO 2 in carbon fixation.
• The carbon fixation reactions may be summarized as follows:
• 12NADPH + 18ATP + 6CO 2 —> C 6 H 12 O 6 + 12NADP + +18ADP +18P i + 6H 2 O
• Calvin cycle/C 3 cycle
• Hatch and Slack pathway or C 4 cycle

i. Calvin cycle/C 3 cycle:

• It comprises a series of 13 reactions.
• Uptake of CO 2
• Carbon reduction
• RuBP regeneration
• All 13 enzymes that catalyze steps in the Calvin cycle are located in the stroma of the chloroplast.
• These enzymes catalyze reversible reactions, degrading carbohydrate molecules in cellular respiration and synthesizing carbohydrate molecules in photosynthesis.
• The first phase of the Calvin cycle consists of a single reaction.
•  In this reaction, a molecule of CO2 reacts with a phosphorylated 5-carbon compound, ribulose bisphosphate (RuBP) .
• This reaction is catalyzed by the enzyme ribulose bisphosphate carboxylase/oxygenase , also termed as rubisco .
• The chloroplast contains abundant rubisco enzyme than any other protein.
• The product of this reaction is an unstable 6-carbon intermediate, which instantly breaks down into 2 molecules of phosphoglycerate (PGA) with 3 carbons each.
• The carbon that was initially the part of a CO2 molecule is now part of a carbon skeleton; i.e. the carbon has been “fixed.”
• The Calvin cycle is also known as the C3 pathway asthe product of the initial carbon fixation reaction is a 3-carbon compound.
• Plants that initially fix carbon in this way are called C3 plants.
• The second phase of the Calvin cycle involves two steps in which the energy and reducing power from ATP and NADPH (both produced in the light-dependent reactions) are used to convert the PGA molecules to glyceraldehyde-3-phosphate (G3P) .
• For every 6 carbons that enter the cycle as CO2, 6 carbons can leave the system as 2 molecules of G3P, to be used in carbohydrate synthesis.
• Each of these 3-carbon molecules of G3P is essentially half a hexose (6-carbon sugar) molecule.
• The reaction of 2 molecules of G3P is exergonic and leads to the formation of glucose or fructose.
• In some plants glucose and fructose are then joined to produce sucrose
• The plant cell also uses glucose to produce starch or cellulose.
• Even if, two G3P molecules are eliminated from the cycle, 10 G3P molecules remains, this represents a total of 30 carbon atoms.
• Through a series of 10 reactions that make up the third phase of the Calvin cycle, these 30 carbons and their associated atoms become rearranged into 6 molecules of ribulose phosphate.
• Each of them becomes phosphorylated by ATP to produce RuBP, the 5-carbon compound with which the cycle started.
• These RuBP molecules begin the process of CO2 fixation and eventual G3P production once again.
• In summary, the inputs required for the carbon fixation reactions are 6 molecules of CO2, phosphates transferred from ATP, and electrons (as hydrogen) provided by NADPH (but ultimately derived from the photolysis of water).
• In the end, the 6 carbons from the CO2 are accounted for by the harvest of a hexose molecule.
• The remaining G3P molecules are used to synthesize the RuBP molecules with which more CO2 molecules may combine.

ii. Hatch and slack pathway/ C 4 cycle:

• Several plant species found in hot, dry environments have adaptations that facilitate carbon fixation.
• C4 plants first fix CO2 into a 4-carbon compound, oxaloacetate.
• CAM plants initially fix carbon at night through the formation of oxaloacetate.
• These special pathways found in C4 and CAM plants precede the Calvin cycle (C3 pathway); they do not replace it.
• C 4 pathway:
• The C4 pathway efficiently fixes CO2 at low concentrations.
• In this pathway, CO2 is fixed through the formation of oxaloacetate, occurs not only before the C3 pathway but also in different cells.
• Leaf anatomy is usually typical in C4 plants.
• The photosynthetic mesophyll cells are closely associated with prominent, chloroplast-containing bundle sheath cells, which tightly surrounds the veins of the leaf.
• The C4 pathway occurs in the mesophyll cells, whereas the Calvin cycle takes place within the bundle sheath cells.
• The major component of the C4 pathway is PEP carboxylase.
• It is a peculiar enzyme that has an extremely high affinity for CO2 i.e. binds it effectively even at unusually low concentrations.
• PEP carboxylase catalyzes the reaction by which CO2 reacts with the 3-carbon compound phosphoenolpyruvate (PEP) and forms oxaloacetate.
• In a step that needs NADPH, oxaloacetate is converted to some other 4-carbon compound, malate and aspartate in presence of enzyme transaminase and malate dehydrogenase.
• The malate and aspartate then passes to chloroplasts within bundle sheath cells.
• Here, a different enzyme catalyzes the decarboxylation of malate and yields pyruvate (which has 3 carbons) and CO2.
• NADPH is formed, replacing the one used earlier.
• Malate + NADP+  —->   pyruvate + CO2 + NADPH
• The CO2 released in the bundle sheath cell combines with ribulose bisphosphate in a reaction catalyzed by rubisco and undergoes the Calvin cycle in the usual manner.
• The pyruvate formed in the decarboxylation reaction returns to the mesophyll cell, where it reacts with ATP to regenerate phosphoenolpyruvate.
• As the C4 pathway captures CO2 and provides it to the bundle sheath cells so efficiently, CO2 concentration within the bundle sheath cells is about 10 to 60 times as great as its concentration in the mesophyll cells of plants having only the C3 pathway.
• Photorespiration is negligible in C4 plants such as crabgrass because the concentration of CO2 in bundle sheath cells (where rubisco is present) is always high.
• The combined C3-C4 pathway involves the expenditure of 30 ATPs per hexose rather than the 18 ATPs used by the C3 pathway alone.
• The extra energy expense required to regenerate PEP from pyruvate is worthwhile at high light intensities because it ensures a high concentration of CO2 in the bundle sheath cells and allows them to carry on photosynthesis at a rapid rate.
• At lower light intensities and temperatures, C3 plants are favored.
• For example, winter rye, a C3 plant, grows lavishly in cool weather, while crabgrass cannot because it requires more energy to fix CO2.

Fixation of  CO 2 by CAM plants at night:

• Plants living in dry, or xeric , conditions have a number of structural adaptations that enable them to survive.
• Many xeric plants have physiological adaptations as well, including a special carbon fixation pathway termed as the crassulacean acid metabolism (CAM) pathway .
• Unlike most plants, CAM plants open their stomata at night, permitting CO2 while minimizing water loss.
• They use the enzyme PEP carboxylase to fix CO2, forming oxaloacetate, which is converted to malate and stored in cell vacuoles.
• During the day, when stomata are closed and gas exchange cannot take place between the plant and the atmosphere, CO2 is removed from malate by a decarboxylation reaction.
• Now the CO2 is available within the leaf tissue to be fixed into sugar by the Calvin cycle (C3 pathway).
• The CAM pathway is very similar to the C4 pathway but with important differences.
• C4 plants initially fix CO2 into 4-carbon organic acids in mesophyll cells.
• The acids are later decarboxylated to produce CO2, which is fixed by the C3 path- way in the bundle sheath cells.
• In other words, the C4 and C3 pathways occur in several locations within the leaf of a C4 plant.
• In CAM plants the initial fixation of CO2 occurs at night.
• Decarboxylation of malate and subsequent production of sugar from CO2 by the normal C3 photosynthetic pathway occur during the day.
• In other words, the CAM and C3 pathways occur at various times within the same cell of a CAM plant.
• Although it does not promote rapid growth the way that the C4 pathway does, the CAM pathway is a very successful adaptation to xeric conditions.
• CAM plants can exchange gases for photosynthesis and reduce water loss significantly.
• Plants with CAM photosynthesis survive in deserts where neither C3 nor C4 plants can.
• chloroplast
• dark reaction
• photosynthesis

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• Biology Article

Chloroplasts

Plants form the basis of all life on earth and are known as producers. Plant cells contain structures known as plastids which are absent in animal cells. These plastids are double-membraned cell organelles which play a primary role in the manufacturing and storing of food. There are three types of plastids –

• Chromoplasts- They are the colour plastids, found in all flowers, fruits and are mainly responsible for their distinctive colours.
• Chloroplasts- They are green coloured plastids, which comprise green-coloured pigments within the plant cell and are called chlorophyll.
• Leucoplasts- They are colourless plastids and are mainly used for the storage of starch, lipids and proteins within the plant cell.

Table of Contents

• Explanation

Let us have a detailed look at the chloroplast structure and function.

Chloroplast Definition

“Chloroplast is an organelle that contains the photosynthetic pigment chlorophyll that captures sunlight and converts it into useful energy, thereby, releasing oxygen from water. “

What is a Chloroplast?

Chloroplasts are found in all green plants and algae. They are the food producers of plants. These are found in mesophyll cells located in the leaves of the plants. They contain a high concentration of chlorophyll that traps sunlight. This cell organelle is not present in animal cells.

Chloroplast has its own extra-nuclear DNA and therefore are semiautonomous, like mitochondria. They also produce proteins and lipids required for the production of chloroplast membrane.

Also Read :  Plastids

Diagram of Chloroplast

The chloroplast diagram below represents the chloroplast structure mentioning the different parts of the chloroplast. The parts of a chloroplast such as the inner membrane, outer membrane, intermembrane space, thylakoid membrane, stroma and lamella can be clearly marked out.

Chloroplast Diagram representing Chloroplast Structure

 Structure of Chloroplast

Chloroplasts are found in all higher plants. It is oval or biconvex, found within the mesophyll of the plant cell . The size of the chloroplast usually varies between 4-6 µm in diameter and 1-3 µm in thickness. They are double-membrane organelle with the presence of outer, inner and intermembrane space. There are two distinct regions present inside a chloroplast known as the grana and stroma.

• Grana are made up of stacks of disc-shaped structures known as thylakoids or lamellae. The grana of the chloroplast consists of chlorophyll pigments and are the functional units of chloroplasts.
• Stroma is the homogenous matrix which contains grana and is similar to the cytoplasm in cells in which all the organelles are embedded. Stroma also contains various enzymes, DNA, ribosomes, and other substances. Stroma lamellae function by connecting the stacks of thylakoid sacs or grana.

The chloroplast structure consists of the following parts:

Membrane Envelope

It comprises inner and outer lipid bilayer membranes. The inner membrane separates the stroma from the intermembrane space.

• Intermembrane Space

The space between inner and outer membranes.

Thylakoid System (Lamellae)

The system is suspended in the stroma. It is a collection of membranous sacs called thylakoids or lamellae. The green coloured pigments called chlorophyll are found in the thylakoid membranes. It is the sight for the process of light-dependent reactions of the photosynthesis process. The thylakoids are arranged in stacks known as grana and each granum contains around 10-20 thylakoids.

It is a colourless, alkaline, aqueous, protein-rich fluid present within the inner membrane of the chloroplast present surrounding the grana.

Stack of lamellae in plastids is known as grana. These are the sites of conversion of light energy into chemical energy.

Chlorophyll

It is a green photosynthetic pigment that helps in the process of photosynthesis.

Also read:  Light-dependent Reactions

Functions of Chloroplast

Following are the important chloroplast functions:

• The most important function of the chloroplast is to synthesise food by the process of photosynthesis.
• Absorbs light energy and converts it into chemical energy.
• Chloroplast has a structure called chlorophyll which functions by trapping the solar energy and is used for the synthesis of food in all green plants.
• Produces NADPH and molecular oxygen (O 2 ) by photolysis of water.
• Produces ATP – Adenosine triphosphate by the process of photosynthesis.
• The carbon dioxide (CO2) obtained from the air is used to generate carbon and sugar during the Calvin Cycle or dark reaction of photosynthesis.

Also Refer:   Calvin Cycle

Frequently Asked Questions

Where does the photosynthesis process occur in the plant cell.

In all green plants, photosynthesis takes place within the thylakoid membrane of the Chloroplast.

List out the different parts of Chloroplast?

Chloroplasts are cell organelles present only in a plant cell and it includes:

• Inner membrane
• Outer membrane
• Thylakoid membrane

What is the most important function of chloroplast?

The most important function of chloroplast is the production of food by the process of photosynthesis.

Why is the chloroplast green?

Chloroplast contains a green pigment called chlorophyll which gives it a green colour.

How many types of plastids are there?

There are three types of plastids-chloroplast, chromoplast and leucoplast.

What is the stack of lamellae inside a plastid called?

The stack of lamellae or thylakoids inside a plastid is called grana.

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What is the definition of chloroplast

The chloroplast is an organelle that contains the photosynthetic pigment chlorophyll which uses sunlight to create energy which can be used by the plant.

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