Photosynthesis: The Process Of Photosynthesis Explained (With Diagrams) (2023)


Photosynthesis: The Process Of Photosynthesis Explained (With Diagrams)!

Photosynthesis is essentially the only mechanism of energy input in the living world. Photosyn­thesis (photos-light, synthesis-putting together) is an anabolic process of manufacture of organic com­pounds inside the chlorophyll containing cells from carbon dioxide and water with the help of sun­light as a source of energy. A simple equation of photosynthesis is as follows:


Photosynthesis: The Process Of Photosynthesis Explained (With Diagrams) (1)The source of molecular oxygen was water and not carbon dioxide as was believed earlier was experimentally proved first by Robert Hill (1937) and later confirmed by M.D. Kamen and S. Ruben (1945), employing tracer technique in which heavy isotopes of oxygen 18O were used. But this experi­mental proof was based on the suggestions of C.B. Van Niel’s work on bacterial photosynthesis (1930).

He suggested that in green plants H2O is the source of reduction and when split yields (H) and (OH), and O2 released by plants is derived from water, not from CO2. This splitting of water in light by green plants has come to be known as photolysis of water and the theory, Van Niel’s theory of photolysis of water.

Chloroplasts: Structures and Photosynthetic Pigments:

Chloroplast is the seat of pnotosynthesis and is best exemplified in the higher plants. A chloroplast is covered by an envelope of two membranes which are separated by periplastidial space of 10-20 nm. Internally a chloroplast contains a matrix or stroma in which are embedded a number of flattened membranous sacs called thylakoids or lamella. The external surface of thylakoids contains the photosynthetic pigments and serves the ends of light reaction.


The stroma, on the other hand, is concerned with the events of the dark reaction. In certain regions the thylakoids are stacked to form grana. The longer thylakoids that connect one granum to another extend through the stroma so these membranes are usually referred to as stroma thylakoids.

Photosynthetic Pigments:

The photosynthetic pigments present in thylakoid membranes con­sist largely of two kinds of green chlorophylls, Chlorophyll a (C55H72O5N4 Mg) and Chlorophyll b (C55 H70O6N4Mg). Also present are yellow to orange pigments classified as carotenoids.

There are two kinds of carotenoids, the pure hydrocarbon carotenes and the oxygen-containing xanthophylls. Certain carotenoids, especially violaxanthin, a xanthophyll, also exist in the chloroplasi envelope, giving it a yellowish colour. In most plants, including green algae, β-carotene and lutein are the most abundant carotenoids in the thylakoids.

Events of Photosynthesis:


Photosynthesis consists of two types of reactions: a light dependent one and a light-indepen­dent one. The light-dependent reaction is a photochemical reaction or light reaction as it came to be called, culminating in the generation of NADPH2, ATP and evolution of molecular oxygen.

The NADPH2 and ATP are energy-rich, having caught the electrons that became available when light impinged upon chlorophyll. They form the assimilatory power, utilised for CO2-fixation. The event of CO2-fixation is light independent reaction and is designated as dark reaction.

Light Reaction:

Light reaction consists of two phase: Phase I-Energy absorption (Absorption and retention of light by the photosynthetic pigments); and Phase II-Energy transduction (conversion of light energy absorbed in phase I into chemical energy-ATP and NADPH2 by photophosphorylation).

Phase I. Energy absorption:


Photosynthetic Units:

The events of light reaction are mediated through photosynthetic units, a photosynthetic unit being the smallest group of pigment molecules, together with their lipo-protein associate substances, able to bring about a photochemical act (Photoact).

The term, photochemical act, means absorption and migration of a light quantum by a trapping centre, as a result of which an electron is released. Emerson and Arnold thought that a photosynthetic unit (PSU) contained at least 2500 chlorophyll molecules, but recent work by Bessel Kock indicates that a photosynthetic unit con­tains only about 250 chlorophyll molecules. The occurrence of PSU as a distinct morphological entity was obtained by Park and his co-workers and they christened it quanta-some.

Absorption of Light by Pigments:

All the light incident on photosynthesising surface is not used for photosynthesis. Much of it is lost and quite some is reflected back. Again, only a small fraction of the absorbed light is used to drive photosynthesis. It is estimated that in full sunlight, just about 3% is used for photosynthetic purpose.


Depending upon the pigment composition, the various plant groups absorb and utilise light of different spectral regions. Most green plants absorb light in the visible spectrum (390-700 nm), whereas purple bacteria employ wavelengths ranging from near ultraviolet to infrared (800-950 nm).

This range of the spectrum through which photosynthesis can take place is called photosynthetically active radiation. But the entire range is not employable in photosynthesis. The green plants, for example, absorb light maximally in the red and blue regions of the spectrum.

A study of the absorption spectra shows the quantitative relationship between the wavelength of light and its absorption by the pig­ment in question. Thus, we see that chlorophyll-a has its absorption peaks at 660 nm and 430 nm; chlorophyil-b at 648 nm and 456 nm; carotene at 478 nm and 449 nm and xanthophyll same as carotene.

Light Trap:

Chlorophyll-fl utilises the light that it absorbs on its own and also the light transferred to it by other pigments. This tunneling of light from other pigments to chlorophyll a has been called light trap or light sink. The light trap makes for a much better light-harvesting efficiency, for it ensures tunneling of light quanta towards one acceptor molecule of chlorophyll.


The light reaction actually consists of two photochemical reactions which are separated both in time and space. They are designated as Photoact I and Photoact II and the two reactions are mediated by two different systems whose composition differs in terms of pigments, electron carriers and light trap mechanisms. The mediating agencies of the two photoacts are respectively called Photosystem I and Photosystem II.


The concept of two photosystems originated in the work of Emerson and Lewis (1943). Working on the action spectrum for the pigments of Chlorella, they found that at wavelengths of light between 600 and 680 nm (wavelengths corresponding to the ‘red’ region of the spectrum) the evolution of oxygen was at its maximum.

But when light of wavelengths beyond 680 nm, a region of the spectrum referred to as ‘far-red’ was supplied; there was a drop in the evolution of oxygen, indicative of lessened photosynthesis efficiency. This observation had been christened the red drop.


His research group found that if light of shorter wave lengths was provided at the same time as the longer red wave­lengths, photosynthesis was even faster than when either of wavelengths alone was provided. This synergism or enhancement became known as the Emerson enhancement effect. These two observations the red drop effect and enhancement effect led to the first indication that the light reaction has two sites of action, one in the red region of the spectrum and the other in the far-red.

The explanation offered for these two effects is that the light reaction actually consists of two photoacts, photoact I and photoact II, mediated by two photosystems, photosystem I and photosystem II.

Photosystem I is driven by the far-red light and when it operates alone, it produces the red drop effect. But when it operates along with photosystem II, which functions in the red region, enhance­ment effect is produced. Physical separation of the two photosystems had been successfully carried out and their functions clarified.

Photosystem I has been located in the thylakoid membranes. It is made up of three forms of chlorophyll-a, one absorbing maximally at 683 nm, the second absorbing maximally at 695 nm and the third at 670 nm. The last of these has been called P-700. Photosystem II had been located in the stroma thylakoids. It is made up of two forms of chlorophyll-a with maximum absorption at 670 and 690 nm.

The second form is christened P-690. Each photosystem has three components: (i) a reaction centre made up of a special chlorophyll molecule—in photosystem I it is protein-bound chlorophyll- a molecule, P-700; in photosystem II it is P-690.

The reaction centres are the actual sites where light energy is converted to chemical energy, (ii) some electron carriers—in photosystem I, X, plastocyanin, cytochrome-f and ferrodoxin as the electron carrier; photosystem II has plastoquinone and cytochrome b-559. (iii) other chlorophyll and carotenoids, which merely serve to transfer the light absorbed by them to the active centres.


Photosystem I takes part in both cyclic and non-cyclic photophosphorylations. PS-I can carry on cyclic photosphosphorylation independently. Normally it drives an electron from photosystem II to NADP+.

Photosystem II picks up electron released during photolysis of water. The same is extruded on absorption of light energy. As the extruded electron passes over cytochrome complex, sufficient energy is released to take part in the synthesis of ATP from ADP and inorganic phosphate. This photophos- phorylation is noncyclic. PS II can operate only in conjunction with PS I.

Phase II: Energy Transduction:

The excited molecules of P-700 and P-690 transducer their energies to generate ATP and NADPH2. Molecular oxygen is also produced but it escapes out of the photosynthetic system. ATP and NADPH2, together constitute the assimilatory power and are employed in the fixation of CO2 in the dark reac­tion.


Photophosphorylation is the light driven or light energised synthesis of ATP. It was discovered by Arnon et al in 1954. Photophosphorylation is of two main types, cyclic and non-cyclic.

Cyclic Photophosphorylation:

It is a process of photophosphorylation in which an electron expelled by the excited photocentre is returned to it after passing through a series of electron carriers. Cyclic photophosphorylation is performed by photosystem I only. Its photocentre P700 extrudes an electron with a gain of 23 kcal/ mole of energy after absorbing a photon of light (hv).

After losing the electron the photocentre becomes oxidised. The expelled electron passes through a series of carriers including X, ferredoxin, plasto­quinone, cytochrome complex and plastocyanin before returning to photocentre.


While passing between ferredoxin and plastoquinone and /or over the cytochrome complex, the electron loses sufficient energy to form ATP from ADP and inorganic phosphate.

Halobacteria or halophile bacteria also perform photophosphorylation but ATP thus produced is not used in synthesis of food. These bacteria possess purple pigment bacteriorhodopsin attached to plasma membrane. As light falls on the pigment, it creates a proton pump which is used in ATP synthe­sis.

Noncyclic Photophosphorylation:

It is the normal process of photophosphoryla­tion in which the electron expelled by the excited photocentre does not return to it. Non-cyclic photo­phosphorylation is carried Out in collaboration of both photosystems I and II. Electron released during photolysis of water is picked up by photocentre of PS II called P600.

The same is extruded out when the photocentre absorbs light energy (hv). The extruded electron has an energy equivalent to 23 kcal/mole. It passes through a series of electron carriers— Q, PQ, cytochrome complex and plas­tocyanin. While passing over cyto­chrome complex, the electron loses suf­ficient energy for the synthesis of ATP.

The electron is handed over to photocentre P700 of PSI by plastocya­nin. P700 extrudes the electron after ab­sorbing light energy. The extruded elec­tron passes though X, Fe-S centre A (ferredoxin), and NADP-reductase which combines it with NADP+. The letter then combines with H+ (released during photolysis) with the help of NADP-reductase to form NADPH.

ATP synthesis is not direct. The energy released by electron is actually used for pumping H+ ions across the thylakoid membrane. It creates a proton gradient. The gradient triggers the coupling factor to synthesize ATP from ADP and inorganic phosphate.

Oxygen Evolution:


The oxygen that is evolved during photosynthesis comes from water and it is part of the photoact II mediated by PSIL. The light energy trapped by this system excites P-690 and two electrons are ejected. The energy is used to remove two electrons from the hydrogen of the water and boost them to a higher energy level.

At this point, the molecular oxygen which escapes from the photosynthetic system is formed. The electrons pass through the carriers plastoquinone (PQ), cytochrome b-559, cytochrome-F, plastocyanin and finally end up in P-700 to take it back to the ground state.

Thus in photosystem II, the electrons that brings the excited chlorophyll molecule to the ground state comes from photolysis of water. Another aspect of oxygen evolution during photosynthesis is its relationship to the presence of certain ions in the medium such as CI”, Mn2+ and bicarbonate.

2H2O———> O2 + 4H+ + 4e

Both the photosystems working in unison produce, for every two turns, two molecules of NADPH2, three ATPs, and a molecule of oxygen from two water molecules.

Dark or Blackman’s Reaction:


Phase III: Energy Stabilisation:

The production of carbon dioxide to a carbohydrate is the essence of this phase and this is accomplished through the employment of the assimilatory power (ATP and NADPH2) generated in the light reaction. This energy stabilisation is a dark reaction.

It does not require light, instead assimilatory power (ATP and NADPH2) produced during photochemical phase is used here in fixation and reduction of CO2. The enzymes required for the process are present in the matrix or stroma of the chloroplast. There are two main pathways for the biosynthetic or dark phase—Calvin cycle and C4 dicarboxylic acid cycle. The plants exhibiting the two are respectively called C3 and C4 plants.

Calvin cycle (the photosynthetic carbon reduction cycle or the C3 photosynthetic pathway).

This cycle was discovered by Calvin, Benson and their colleagues using unicellular algae Chlo- rella pyrenoidosa and Scenedesmus obliques and radioactive isotope of 14C with a half-life of more than 5000 years.

Phases of Calvin Cycle:


Calvin cycle is divided into the following three phases—carboxylation, glycolytic reversal and regeneration of RuBP.

1. Carboxylation. It requires ribulose—1, -biphosphate or RuBP as acceptor of carbon dioxide and RuBP carboxylase or rubisco as enzyme. The enzyme was previously called carboxydismutase.

Carbon dioxide combines with ribulose-1, 5-biphosphate to produce a transient intermediate compound called 2-carboxy 3-keto 1,5-biphosphoribotol. The intermediate splits up immediately in the presence of water to form the two molecules of 3-phosphoglyceric acid or PGA. It is the first stable product of photosynthesis.

2. Glycolytic Reversal:

The processes involved in this step or phase are reversal of processes found during glycolysis part of respiration. Phosphoglyceric acid or PGA is further phosphorylated by ATP with the help of enzyme triose phosphate kinase. It give rise to 1,3-diphosphoglyceric acid.

Diphosphoglyceric acid is reduced by NADPH through the agency of enzyme triose phosphate dehydrogenase. It produces glyceraldehyde 3-phosphate or 3-phosphoglyceraldehyde.

Glyceraldehyde-3 phosphate is a key product which is used in synthesis of both carbohydrates and fats. For forming carbohydrates, say glucose, a part of it is changed into its isomer called dihy- droxyacetone-3-phosphate. The enzyme that catalyses the reaction is phosphate isomerase.

The two isomers condense in the presence of enzyme aldolase forming fructose 1, 6-diphos­phate.

Fructose 1, 6-diphosphate (FDP) loses one phosphate group, forms fructose 6-phosphate (F 6-P) which is then changed to glucose-6-phosphate (G 6-P). The latter can produce glucose or become part of sucrose and polysaccharide.

As glucose is a six carbon compound, six turns of Calvin cycle are required to synthesise its one molecule.

3. Regeneration of RuBP:

Fructose 6-phosphate (F 6-P) and glyceraldehyde 3-phosphate (GAP) react to form erythrose 4-phosphate (E 4-P) and xylulose 5-phosphate (X 5-P). Erythrose 4-phosphate combines with dihydroxy acetone 3-phosphate to produce sedoheptulose 1 : 7- diphosphate (SDP) which loses a molecule of phosphate and gives rise to sedoheptulose 7-phosphate (S 7-P).

Sedoheptu­lose 7-phosphate reacts with glyceraldehyde 3-phosphate to produce xylulose 5-phosphate (X 5-P) and ribose 5-phosphate. (R 5-P). Both of these are changed to their isomer ribulose 5-phosphate (Ru 5-P). Ribulose 5-phosphate picks up a second phosphate from ATP to become changed into ribulose 1,5 biphosphate (RuBP).

Photorespiration (Respiration Associated with Photosynthetic Tissues):

It was discovered by Decker and Tio in 1959. Photorespiration is the light dependent utilization of oxygen and release of carbon dioxide by the photosynthetic organs of a plant. Normally photosyn­thetic organs do the reverse in the light i.e., uptake of CO2 and release of O2.

Therefore, photorespira­tion is difficult to demonstrate. It is inferred from (i) Decrease in the rate of net photosynthesis when oxygen concentration is increased from 2—3% to 21% (ii) Sudden increased evolution of CO2 when an illuminated green organ is transferred to dark.

The site for photorespiration is chloroplast. Peroxisome is required for completing the process. RuBP carboxylase is changed to RuBP oxygenase. This happens at high temperature and high oxy­gen concentration. At high temperature and high oxygen concentration, the affinity of RuBP carboxy­lase for CCX, decreases and the affinity for O2 increases.

High temperature occurs in tropical areas. Therefore, tropical plants are the major sufferers. At high temperature, RuBP carboxylase functions as oxygenase and instead of fixing carbon dioxide, oxidises ribulose 1, 5-biphosphate to produce phos- phoglyceric acid and phosphoglycolate.

Phosphoglycolate is hydrolysed to form glycolate. Glycolate usually passes into peroxisome of the mesophyll cell and forms glyoxylate. Glyoxylate is aminated and gives rise to amino acid glycine. Inside mitochondrion and even cytoplasm the two molecules of glycine condense to form a molecule of serine, CO2 and ammonia are released in the process. Serine can further be deaminated to form PGA.

The latter passes into chloroplasts for synthesis of photosynthetic products as well as photores­piration. Since photorespiration involves the synthesis of two-carbon compounds, it is also called C2 cycle.

Photorespiration does not produce energy or reducing power. Rather, it consumes en­ergy. Further, it undoes the work of photosynthesis. It may reduce photosynthesis upto50%. There­fore, photorespiration is a highly wasteful process.

This happens only in case of C3 plants. C4 plants have overcome the prob­lem of photorespiration by per­forming Calvin Cycle in the in­terior of leaves (bundle sheath cells) where both temperature and oxygen are lower. They have further ensured high CO2 sup­ply to cells performing Calvin cycle.

C4-Dicarboxylic Acid Pathway:

(Hatch Slack Pathway, C4 Pathway)

It was worked out by Hatch and Slack (1965,1967). Kortschak et al (1965) found that labelled carbon dioxide (14C02) assimilated by Sugarcane leaves first appeared in a 4-carbon compound oxalo-acetic acid (OAA or oxaloacetate).

Hatch and Slack found it a regular mode of CO2—fixation in a number of tropical plants, both monocots and dicots, eg., Maize, Sugarcane, Sorghum, Panicum, Pentiisetum, Atriplex, Amaranthus, Salsola etc. These plants are called C4 plants because of the first stable photosynthetic product being a 4-carbon-compound. Other plants are C3 plants. C4 plants often live in hot, arid and saline habitats. They have Kranz anatomy.

In Kranz anatomy, the mesophyll is undifferentiated and its cells occur in concentric layers around vascular bundles having large bundle sheath cells. The mesophyll and bundle sheath cells are con­nected by plasmodesmata or cytoplasmic bridges.

The chloroplasts of the mesophyll cells are smaller. They have well developed grana and a peripheral reticulum but no starch. The chloroplasts of the bundle sheath cells are larger. They have ill defined grana, a peripheral reticulum and starch grains.

In C4 plants, initial fixation of carbon dioxide occurs in mesophyll cells. The primary acceptor of CO2 is phosphoenol pyruvate or PEP. It combines with carbon dioxide in the presence of PEP carboxylase or pepco to form oxalo-acetic acid or oxaloacetate.

Malic acid or aspartic acid is translocated to bundle sheath cells through plasmodesmata. In­side the bundle sheath cells they are decarboxylated (and deaminated in case of aspartic acid) to form pyruvate and CO2.

CO2 is again fixed inside the bundle sheath cells through Calvin cycle. RuBP of Calvin cycle is called secondary or final acceptor of CO2 in C4 plants. Pyruvate is sent back to mesophyll cells. Here, it is changed to phosphoenol pyruvate. Energy is required for this. The same is provided by ATP. The latter is changed into AMP (adenosine monophosphate).

Conversion of AMP to ATP requires double the energy than energision of ADP to ATP. Therefore, actual requirement of energy is equal to two molecules of ATP. This energy is in addition to 3 ATP required for fixation of one molecule of CO3 through Calvin cycle. Therefore, C4 plants consume 5 ATP molecules per molecule of CO2 fixed instead of 3 ATP molecules for C3 plants. For the formation of a glucose molecule, C4 plants require 30 ATP while C3 plants utilize only 18 ATP.


1. The C4-plants are considered to possess greater photosynthetic effi­ciency, for they can utilise CO, until a level of 5 ppm is reached but the Calvin cycle plants cannot utilise C02 if the level falls below 40-50 ppm.

2. C4-plants can utilise greater light intensi­ties and their temperature optima for photosynthesis exceeds those of C3 plants.

3. The chloroplasts of these plants seem to gener­ate more ATP which of course makes for improved cellular work.

4. The presence of extensive peripheral reticulum in the chloroplasts of these plants indirectly suggests quicker transport of products and therefore greater utilisation of light and CO2.

Crassulacean Acid Metabolism:

This pathway worked out by Ranson and Thomas (1960) and Rouham, Vines and Black (1973) is found in succulents, mostly members of Crassulaceae (Bryophylnm and Seditm) and a few members of Bromeliaceae, such as pineapple. It is a device designed to meet the pressures of heavy transpira­tion, arising from their xerophytic environment.

These plants obtain their CO2-requirements during the night time when they keep their stomata open and as CO2 build-up occurs, the cell sap turns acidic. This process is called as dark acidification. In the following daytime, the stomata remain closed, minimising transpirational losses, but with the advent of light, the CO2 absorbed during the preced­ing night is utilised for photosynthetic purposes, the process of light deacidification then occurs.

Thus, there is a time lag between absorption and reduction of CO2. This arrangement helps to lessen transpiration stress but is responsible for the extremely slow growth of these plants. Below is given a short account of this pathway.

Phase I: Dark acidification:

In this phase, the reserve starch is broken to phosphoenoplyruvate (PEP) through a number of respiratory reactions. PEP accepts the atmospheric CO2 absorbed by the plant and produces malic acid. This happens in darkness when the stomata are open.

Starch———– > PEP.

PEP + NADPH2 + CO2 from atmosphere –> Malic acid + NADP.

Phase II: Light deacidification:

During this phase, which occurs in light when the stomata are closed, malic acid is decarboxy- lated into pyruvic acid and CO2. The CO2 released is routed through Calvin cycle to synthesise a hexose. The pyruvic acid is used to build up starch whose stocks are depleted earlier.

Malic acid –> Pyruvic acid + NADPH2 + CO2 in plant

Calvin cycle events:

6 CO2 + 12 NADPH2 + 18 ATP + 11 H2O Fructose -6-phosphate + 12 NADP + 18 ADP + 17 Pi

The assimilatory power needed for Calvin cycle is provided by the events of light deacidification.

Principle or Law of Limiting Factors:

Optimum value of a factor is never con­stant. It depends upon the magnitude of other factors. We may con­tinue to increase the magnitude of one or more factors without in­fluencing the rate of re­action. In such cases it is found that a factor called limiting factor is holding the balance.

A limiting factor is defined as a factor which is defi­cient to such an extent that increase in its magnitude directly increases the rate of the process. The effect of limiting factors was studied by Blackmann in 1905. He formulated the principle of limiting factors which states that when a process is conditioned as to its rapidity by a number of separate factors, the rate of the process is limited by the pace of the slowest factor. In other words the rate of a physiological process is limited at a given time by one and only one factor which is deficient.

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