Photosynthesis |Key Points, Reaction, Cycle, Process & Diagram

Photosynthesis is defined as an anabolism process in which carbon dioxide and water is converted into glucose, the demanded organic compound in presence of sunlight and chlorophyll. The most important aspect of photosynthesis is conversion of light energy into bio-chemical energy.

Key Points on Photosynthesis

1. It occurs by reduction of CO₂ and oxidation of H₂O along with anabolism of glucose molecules.
2. The raw materials for photosynthesis are CO₂, H₂O, photosynthetic pigments and light energy.
3. 90% of total photosynthesis is performed by bacteria (BGA) and phytoplankton of sea and oceans. Also more than 90% of oxygen presented in atmosphere is contributed by these phytoplankton and deep ocean photosynthetic organisms.
4. In photosynthesis plant release oxygen during light reaction and consumes carbon dioxide during dark reactions.
5. Chloroplast is the apparatus for photosynthesis which contains grana and stroma. Light reaction takes place in grana and dark reaction in stroma.
6. Photosynthetic bacteria like green sulfur bacteria, red and green filamentous phototrophs, purple bacteria, heliobacteria and acidobacteria uses hydrogen sulfide (H₂S) instead of water, that’s why oxygen is not evolved during their photosynthesis process.

Photosynthesis equation/chemical formula in balanced form is

Byproduct

Byproduct are the secondary products that are produced along with main primary product. The byproduct of photosynthesis is oxygen. The main or primary product is glucose. Oxygen formed during photosynthesis is also called as waste product and are released in atmosphere which are taken by aerobic organism for performing cellular metabolism like respiration.

Purpose

The main purpose of photosynthesis is to manufacture glucose by utilizing solar energy, carbon dioxide and water. It occurs inside chloroplast and in presence of pigment chlorophyll. The formed glucose is food for plants and its different cells. The excess glucose is stored in the form of starch, a polysaccharide in different parts of plant like rhizomes, stem, root, fruits etc.

Photosynthesis Apparatus

Chloroplast is photosynthesis apparatus for photosynthesis. It contains grana and stroma. Grana is site for light reaction whereas stroma is site for dark reaction. Read more

Phases/Cycles of Photosynthesis

It occurs in mainly 2 cycles (phases), called as light and dark reactions.

1. Light Reaction
2. Dark reaction

Light Reaction

It occurs in presence of sunlight and inside grana of chloroplast. It is also called as grana reaction/ Hill reaction or photochemical reaction. It utilizes light energy for synthesizing assimilatory power like ATPs and NADPH₂. Evolution of oxygen by breakdown of water molecules also takes place in this phase.

First step of photosynthesis is excitation of electrons present in chlorophyll molecule. But the first chemical step of photosynthesis is the photolysis of water in presence of Mn, Ca and Cl.

There are 4-steps in which light reaction is completed. They are:

1. Absorption of Light Energy
2. Activation & Excitation of Antenna Pigment
3. Photolysis of Water and Evolution of Oxygen
4. Photophosphorylation

Absorption of Light Energy

The absorption of light energy by photosynthetic pigments takes place in this step and beginning of the process of photosynthesis starts. The incident light falling on green parts of plants are not totally absorbed. Majority of light falling on green surface are wasted in the form of reflection & transmission on the surface. Only an average about 1% of total light falling on the surface are utilized during photosynthesis.

Also, all light is not absorbed by pigments. The intensity of absorption of different wavelength of light differs due to their capacity to activate electrons in the pigments.

The photosynthetic pigments absorb light energy in the visible part of spectrum only. White light or sunlight is highly absorbed and most efficient while the green light is least absorbed and also least efficient. The decreasing order of absorption spectra for different wavelength of light can be shown as:

White>Blue>Red>Violet>Indigo>Orange>Yellow>Green.

In green region of spectra, there is least absorption of green light by the pigments. That’s why chloroplast appears green in color.

Photo system or Pigment system(PS)

The existence of 2 photosystem/pigment system came from ‘Red Drop Experiment’ carried by Emerson and Lewis in 1943. The 2 pigment systems are PSI and PSII. Each pigment system has core complex with chlorophyll-a and other accessory pigments. These pigments are collectively called as harvesting complex or antenna complex.

Difference between Pigment system I and pigment system II

Pigment System I	Pigment System II
It absorbs light of wavelength more than 680nm and nearly 700nm.	It absorbs light of wavelength less than 680nm.
It’s reaction center is P700.	It’s reaction center is P680.
It is present in grana and intergranal lamella.	It is present in grana lamella only.
The amount of chlorophyll-a is higher than chlorophyll-b.	The amount of chlorophyll-b is higher than chlorophyll-a.
It is not involved in photolysis of water.	It is involved in photolysis of water along with release of oxygen.
It performs cyclic photophosphorylation independently.	It performs non cyclic photophosphorylation along with release of electron.

Activation & Excitation of Antenna Pigment

Chlorophyll-a is called antenna pigment and found universally in all green plants. After absorption of light by the chlorophyll, it is activated and excited to high energy level. It becomes unstable and releases energy along with electrons. Chlorophyll is oxidized here. As the electrons are released, Chlorophyll again returns back to its normal state.The released electrons are used for breakdown of water molecules.

Photolysis of Water and Evolution of Oxygen

The electron released from excited chlorophyll is used for splitting of water molecules and this process is called photolysis of water. This reaction is catalyzed by Mn⁺⁺ and Cl^–. Photolysis of water is the first chemical process in photosynthesis. 4 mole (or molecules) of water on splitting inside chloroplast produces 4H⁺ and 4OH^– ions. 4OH^– again reduces into 2H₂O, 4e^– and a O₂ molecules. Overall reaction produces 4H⁺, 4e^– and a oxygen molecule.

Due to release of oxygen molecules during photolysis of water, it conforms that oxygen during photosynthesis comes from water molecules not from carbon dioxide.

Further for its confirmation, Ruben, Hassid and Kamen used H₂O₁₈ in place of normal water. O₁₈ is the isotopes of oxygen. The oxygen evolved during the reaction was containing O₁₈ isotopes which proved that oxygen comes from water molecule not from other compounds. Also, Robert Hill worked on isolated chloroplast and proved O₂ is produced from H₂O not CO₂.

Photophosphorylation

It was discovered by Aron. During photolysis of water, H⁺, e^– and O₂ molecule are produced. Oxygen gets escape from chloroplast to the environment. But hydrogen ions(H⁺) and electrons(e^–) are used for production of assimilatory power like ATP and NADPH₂. ATP and NADPH₂ are collectively called as assimilatory power. These power(i.e. energy in the form of ATP & NADPH₂) are used in dark reaction for production of glucose from CO₂.

There are 2-types of photophosphorylation.

• Non cyclic photophosphorylation
• Cyclic photophosphorylation

Non Cyclic Photophosphorylation

It was discovered by Hill and Bendal in 1960. It occurs in granular thylakoid region of chloroplast. It requires external electrons which is supplied from photolysis of water. Hence, it involves photolysis of water. In this cycle, light energy is converted into chemical energy in the form of ATP and NADPH₂.

It occurs in several steps. First photo system I (PSI) is excited by photon having wavelength less than 680nm. As the PSI is excited, its reaction center P700 is also activated. The chlorophyll present in the reaction center P700 releases electron and become chl-a⁺. The electron is first accepted by Fes or FRS(ferredoxin reducing substance) and transfer to coenzyme NADP which finally produces NADPH₂.

The PSII/P680 also accept photons and becomes excited. As the chlorophyll in the reaction center P680 is excited, it also releases electron and changes to chl-a⁺. Then the released high energy electron is transferred to plastoquinone(PQ) to cytochrome series and then plastocyanin(PC). Finally electrons are accepted by chl-a⁺ of PSI.

During transfer of electron from PSII to PSI, energy from electron is released which is used by ADP to convert into ATP in presence of inorganic phosphate(ip). Similarly, 4H⁺ and 4e^– obtained from photolysis of water are utilized by NADP for reduction into NADPH₂. Since all electron are utilized in formation of assimilatory product, It loses electron and thus is unidirectional and noncyclic system.

Cyclic Photophosphorylation

It was discovered by Aron and Frenkel. In cyclic photophosphorylation, the electrons are recycled back to the system along with production of only ATP but not NADPH₂.

It only involves PSI. It does not require external electron and thus photolysis of water along with evolution of oxygen molecules does not takes place. Here, photons(light) are only converted into ATP. The light with wavelength more than 680 falling on the leaf surface are absorbed by PSI and reaction center P700 is activated. As the reaction center is activated, it releases electron and chl-a molecules changes to chl-a⁺. The released electron has high energy.

The released electron is again transferred to excited chl- a⁺ through different electron carrier. The electron is accepted by FRS and then transfer to FD(ferredoxin) to cytochrome-b6 to cytochrome f to PC(plastocyanin) and finally to chl-a⁺ again. In this way, the excited chlorophyll- a⁺ return back to ground state and became functional for another cycle along with production of ATP (only).

Non cyclic photophosphorylation Vs cyclic photophosphorylation

Non Cyclic Photophosphorylation	Cyclic Photophosphorylation
Electron does not recycle back to system.	Electrons recycle back to system.
Both pigment system PSI and PSII are involved.	Pigment system PSI is only involved.
One ATP and one NADPH2 molecule are produced by utilizing electrons(e–) & protons(H+).	Only 2 ATP molecules are produced by utilizing electrons(e–) without production of NADPH2.
Terminal electron accepter is NADP, which also accept proton.	Terminal electron accepter is PSI since electron recycle back to system.
It is commonly found in higher plants and cyanobacteria.	It is commonly found in bacteria, lower algae like Rhodophyceae and anoxygenic organism.

Dark reaction

It occurs in absence of sunlight and occurs inside stroma of chloroplast. It is also called as stroma reaction/ Blackman’s reaction or carbon-fixation reaction. It is enzymatic process and is slower than light reaction. It does not require light but utilize the assimilatory powers obtained from light reaction for fixation of CO₂ into glucose.

On the basis of initial product obtained in dark reaction, there are 3- alternative pathways which are:

1. Calvin Cycle (C3 Cycle)
2. Hatch and Slack Cycle (C4 Cycle)
3. Crassulacean Acid Metabolism Pathway (CAM Cycle)

Calvin Cycle

It was discovered by Melvin Calvin, Andrew Benson and James Bassham in Chlorella pyrenoidosa and Scenedesmus by using ¹⁴C isotope in carbon dioxide (14CO₂) with the help of radiography technique. They were awarded Nobel Prize for chemistry in 1961 for their discovery.

It is also called as C3 cycle as the first product obtained during this reaction is 3- carbon containing compound i.e. 3-phosophoglyceric acid (3-PGA). This is the common and universal pathway occurring in all photosynthetic plants. Plants performing this pathway are called as C3 Plants. About 85% of total plant performs this cycle.

Calvin cycle occurs in 3-steps.

1. Carboxylation
2. Glycolytic reversal
3. Regeneration of RuBP

Carboxylation

Here, CO2 combines with Ribulose 1, 5-biphosphate (RuBP) in presence of enzyme RuBiSCo to form 2-carboxy-3-keto 1, 5 diphosphorobitol. It is unstable compound and breaks down into 3-phosphoglyceric acid(3-PGA). 3-PGA is the first stable intermediate product during photosynthesis. Ribulose Biphosphate Carboxylase (RuBiSCo) enzyme is richly found in plants. In this step ATP and NADPH2 are not consumed.

Glycolytic Reversal

It is called so because it is the reversal steps of glycolysis that occurs during respiration. 3-phosphoglyceric acid obtained from carboxylation is phosphorylated by utilizing ATPs to produce 1,3-diphosphoglyceric acid. It is then reduced to 1,3-diphosphoglyceraldehyde (1,3-diPGAL) in the presence of enzyme triose phosphate dehydrogenase by utilizing NADPH₂.

Some molecules of 1,3-diPGAL isomerized to form dihydroxyacetone phosphate, which unite with each other and form fructose 1,6-diphosphate in presence of enzyme aldolase.

Regeneration of RuBP

Some of the molecules of 1,3-diphosphoglyceraldehyde forms glucose during glycolytic reversal while some of them are utilized for RuBP regeneration. The remaining 1,3-diPGAL undergoes several reactions with the regeneration of 6-molecules of RuBP.

The reaction process are shown in diagram.

Sedoheptulose 1,7-diphosphate loses one phosphate to form sedoheptulose 7-Phosphate in presence of enzyme phosphatase.

Note

In above diagram:
P = phosphate
ip = inorganic phosphate
dip = diphosphate
PGAL = phosphoglyceraldehyde
DHAP = dihydroxyacetone phosphate

Difference Between Light Reaction and Dark Reaction

Light Reaction	Dark Reaction
It is light dependent reaction.	It is light independent reaction.
It occurs in grana of chloroplast.	It occurs in stroma of chloroplast.
Photolysis of water takes place with release of oxygen.	Fixation of carbon dioxide into glucose takes place.
All the reaction involved in this process are non-enzymatic i.e. in absence of enzymes.	Reactions are processed by the actions of different enzymes.
It is non-cyclic and irreversible process.	It is cyclic and reversible process.
Solar energy is the driving force for reaction.	Assimilatory power (ATPs & NADPH2) obtained from light reaction are driving force/energy for reactions.
End products of this reaction are ATPs, NADPH2 and O2.	End product of this reaction are glucose compounds.

Factors Affecting Yield of Photosynthesis

There are many factors which affects the rate and yields of photosynthesis. Physical factors include light, temperature, intensity while chemicals factors include quantity of water and carbon dioxide. Biological factors may include chlorophyll.

Light

Quality of Light

Quality refers to particular wavelength of light in which maximum photosynthesis occurs. Most efficient light is red light followed by blue light. Green light is least efficient as it is reflected mostly by green leaves. Sunlight has combination of all wavelength of light so, maximum photosynthesis occurs in sunlight/white light.

Quantity of Light

Quantity refers to amount of light. Out of total light energy falling on the surface of leaf, only about 1% in average is utilized by plant.

Duration of Light

It does not affect the rate of photosynthesis.

Intensity of Light

Rate of photosynthesis is higher in intense light than in diffused light. However, exposure in high intense light for long duration cause photo-oxidation of chlorophyll resulting into functionless apparatus. This is called as solarization and mainly occurs during summer seasons in plant’s leaves.

Temperature

The temperature at which photosynthesis is maximum is called optimum temperature.  Optimum temperature for photosynthesis is 25°-30 °C. However, plants growing in hot and cold temperature belt have ability to perform photosynthesis in its surrounding temperature.

Blackman’s Law of Limiting Factors

Blackman in 1905, postulated the “Laws of limiting factors”. According to this postulate, the raw materials (i.e. CO₂, water, light) required for photosynthesis may be limited if required amount of that materials is not available for photosynthesis. Example: In a cloudy day, CO₂ are plenty but light may be limiting factor which may slows down the rate of photosynthesis. 

In general, CO₂ is called limiting factor. The rate of photosynthesis in all plants increases with increase in the concentration of CO₂ upto 500 ppm, while other factors ae not limiting. Higher concentration shows inhibitory effect.

Internal Factors or Leaf

Internal factors include leaf age, angle and orientation. Out of which leaf age plays great role in maximizing photosynthesis yield. The young age has maximum nos. of chloroplast and can perform maximum metabolism activities as compared to older leaf. Thus, young leaves maximize photosynthesis rate.

Significance of Photosynthesis

1. It is the process of producing or accumulating food by the green plant for themseves as well as for consumers like us.
2. Oxygen released during this process is used by aerobic living organism during respiration. Respiration helps in breaking complex organic matter(food) and thereby releasing energy which is utilized for cellular and physical activities.
3. About 90% of oxygen presented in atmosphere is contributed from photosynthesis mainly by blue green algae (BGA) and other photosynthetic plants making our atmosphere suitable for adapting living beings.
4. As soon as the evolution of photosynthesis occurred in plants, ancient reducing atmosphere was converted into oxidizing atmosphere.
5. It plays important role in recycling polluted air with fresh oxygen rich air in our daily life.
6. The excess glucose and sugar formed during photosynthesis is stored in plant’s part like rhizomes (like ginger, turmeric), roots (like radish, carrot) and fruits which is used as food by consumers like human.

Summary

In the reduction of CO₂ into glucose, energy in the form of ATP and NADPH₂ are required. The amount of energy required depend on the path of fixation CO₂. For fixation (formation) of 6 CO₂ molecule into a single glucose molecule in Calvin cycle, 18 ATP and 12 NADPH₂ are required. But in C4 cycle to produce a single glucose molecule, 30 ATP and 12 NADPH₂ are required.

FAQs on Photosynthesis

Reference

https://en.wikipedia.org/wiki/Photosynthesis
https://www.britannica.com/science/photosynthesis
https://en.wikipedia.org/wiki/Photophosphorylation
https://www.life.illinois.edu/govindjee/paper/gov.html