Photosynthesis is the synthesis with the help of light. It is the process by which green plants use sunlight to manufacture food from simple molecules like CO₂ and H₂O. Photosynthesis is sometimes called carbon assimilation and is represented by following equation;

                   6CO2 + 6H2O ------> C6H12O6 + 6O2
                                                        Sunlight energy

During the process of photosynthesis, the light energy is converted into chemical energy and is stared in the organic matter which is usually carbohydrates and O₂ is a byproduct of photosynthesis. CO₂ and H₂O are the raw materials for this process. CO₂ is absorbed from the air and H₂O from the soil. Chloroplasts are the sites of photosynthesis.

 Photosynthesis is important to mankind by the following points. It maintains equilibrium of O₂ in the atmosphere. It provides food either directly as vegetable or indirectly as meat or milk of animals which in turn are fed on plants.

Photosynthesis apparatus: 

 The chloroplast in green plants constitutes the photosynthetic apparatus, Typically, the chloroplast of higher plant is discoid or ellipsoidal in shape. The chloroplast is bounded by two unit membrane and consisting of lipid and protein. Internally, chloroplast is filled with matrix known as stroma in which grana are embedded. Each granum consist disk shaped grana lamellae placed one above the other like the stack of coin. In cross-section, these lamella are paired to form sac- like structure and called as thalakoid. The grana lamellae or thalakoid consists of alternating layers of lipid and proteins. Some of the grana lamellae or thalakoid of granum connected with thalakoid of granum connected with thalakoid of other granum by stroma lamellae or fret membrane. The pigments are present in the thalakoid membrane. Chlorophyll and other photosynthetic pigments are confined to grana which are the sites of primary photo chemical reactions. The stroma is gel like and contains soluble enzymes of dark reactions. Besides necessary enzymes, some ribosomes and DNA have also been found in chloroplast which give them a partial genetic autonomy.

Photosynthetic pigments:

  There are three photosynthetic pigments. They are;

1. Chlorophylls

2. Carotenoid

3. Phycobillins

 Chlorophyll and carotenoids are insoluble in water and can be extracted only with organic solvent. Phycobillins are soluble in water. Carotenoid includes carotene and xanthophylls. Different pigment absorbs light of different wavelength. They show the property of inflorescence.

1. Chlorophylls: Chloroplast pigment present on the chloroplast membrane. The chloroplast pigments are so arranged that they are at right angle to the source of light for maximum absorption. Chlorophyll a and Chlorophyll b are two main pigments present in the chloroplast. Chloroplast a is more abundant green plants than chlorophyll b.

2. Carotenoid: These are of two types;

a. Carotenes ( Orange)

b. Xanthophyll (yellow)

 Carotenoids absorb in blue violet region and appear to be yellow, orange, red or brown pigments. In carotene, ß carotene is the most important as it gives orange colour to the carrot. They give colour to flower or fruit. They act as accessor pigments that absorb light energy and transfer it to chlorophyll a. All pigment except chlorophyll a is known as accessory pigments.

3. Phycobillins: It absorbs maximum absorption in green parts of the spectrum. Phycocyanin absorbs in the orange part of the spectrum.

Distribution of photosynthetic pigments in plant kingdom:

Location of photosynthetic pigments in chloroplast:

 The photosynthetic pigments are located in grana portions of chloroplast in higher plants. A number of molecular models of the chloroplast showing the arrangement of pigment molecules have been given by different workers and it usually held that chlorophyll molecules form a monomolecular layer between the alternative protein and lipid layer in grana lamellae. The  hydrophilic head of the chlorophyll molecules remain embedded in the protein layer while the lipophilic phytol tail in the lipid layer. The other pigments are present along with the chlorophyll molecules. Other workers have included chlorophyll molecules in the fret membrane or stroma lamellae in their model of chloroplast.

Mechanism of photosynthesis:

                6CO2 + 6H2O ------> C6H12O6 + 6O2
                                                     Sunlight energy

  Photosynthesis is an oxidation reduction process in which water is oxidized and CO₂ is reduced to carbohydrate level. The deduction of CO₂ to carbohydrate need assimilatory power such ATP and NADH₂ . Reduction of CO₂ occurs in dark but the production of assimilatory power is light dependent. Hence the process of photosynthesis consist of two phases.

1. Light dependent phase ( light reaction or Hill reaction)

2 Light independent phase ( dark reaction or Blackman reaction)

Photosystems and Reaction center:

  There are two types of photosystems, Photosystem I (PSI) and photosystem II (PSII), Each photosystem has one primary pigment molecule known as reaction center which collect all the energy collected by other accessory pigment molecule. The reaction center uses these energy to start chemical reaction to convert light energy into chemical energy. Primary pigment molecule is a special type of chlorophyll a. In PSI, it is P700 and PSII, it is P680 where Pmeans pigment and the figure wavelength of light, it absorbs.

Light or Hill Reaction:

 Photo-phosphorylation is the process in which the light energy is converted into chemical energy in the form of ATP.

 Light reaction is the first step in photosynthesis occurring in grana of chloroplast and needs the utilization of light energy. It consists of following three phases:

(a) Photolysis of water:

The light energy trapped by chlorophyll molecule decomposes water into its constituent elements, called photolysis of water.

H2­O       →(←)        4H+ + OH-

4OH-           →          2H2 + 4e­­- +2O2

(b) Photo-phosphorylation:

The electrons produced during the photolysis of water pass via 2 photosystems (PS –I and II). Each photo system has its own trap center and a primary pigment molecule.

It is the process of synthesis of ATP from ADP using light energy.

ADP+ ip         → (light)      ATP

It is of further two types:

1)     Non-cyclic photo-phosphorylation:

High energy electrons released from P680 of PS-II are accepted by primary electron acceptor. The electrons pass via a series of electron acceptor i.e. PQ- cytochorome complex- PC and finally to P700 of PSI.

Again, the electrons given out by P700 of PS-I are taken up by primary pigment molecule and are ultimately passed to NADP through Fd. The electrons combine with   ions and reduce NADP to NADP H2.

The net result of non-cyclic photo phosphorylation is the formation of 1oxygen (as a waste), 2 NADP H2 and 1 ATP molecule.

2)    Cyclic photo-phosphorylation:

High energy electrons expelled from P700 of PS-I are taken up by primary pigment molecule, when the pass through series of electron acceptors i.e. Fd-PQ-Cytochorome complex-PC and finally to the same pigment molecule from which they have been originated.

There is formation of 2ATP molecules at the end.

(c)  Photo reduction:

Chloroplast contains naturally occurring electron acceptor NADP. With addition of H+ from photolysis, it is reduced to NADP H2.

NADP + 2 H+ + 2e-       →(light)      NADPH2

Difference between Non-cyclic and cyclic photo- phosphorylation:

Dark reaction or Blackman’s reaction:

 This is the second step in the mechanism of photosynthesis. The chemical processes of photosynthesis occurring independent of light is called dark reaction. It takes place in the stroma of chloroplast. The dark reaction is purely enzymatic and it is slower than the light reaction. The dark reactions occur also in the presence of light. In dark reaction, the sugars are synthesized from CO2. The energy poor CO2 is fixed to energy rich carbohydrates using the energy rich compound, ATP and the assimilatory power, NADPH2 of light reaction. The process is called carbon fixation or carbon assimilation. Since Blackman demonstrated the existence of dark reaction, the reaction is also called as Blackman’s reaction. In dark reaction two types of cyclic reactions occur

1. Calvin cycle or C₃ cycle

2. Hatch and Slack pathway or C₄ cycle

Calvin cycle or C₃ cycle:

It is a cyclic reaction occurring in the dark phase of photosynthesis. In this reaction, CO₂ is converted into sugars and hence it is a process of carbon fixation. The Calvin cycle was first observed by Melvin Calvin in chlorella, unicellular green algae. Calvin was awarded Nobel Prize for this work in 1961. Since the first stable compound in Calvin cycle is a 3 carbon compound (3 phosphoglyceric acid), the cycle is also called as C₃ cycle. The reactions of Calvin’s cycle occur in three phases.

1. Carboxylative phase

2. Reductive phase

3. Regenerative phase

1. Carboxylative phase:

Three molecules of CO2 are accepted by 3 molecules of 5C compound , ribulose diphosphate to form three molecules of an unstable intermediate 6C compound. This reaction is catalyzed by the enzyme, carboxy dismutase

3 CO₂ + 3 Ribulose diphosphate    →   3 unstable intermediate 6 carbon compound

                                                         Carboxy

                                                        dismutase                                    

The three molecules of the unstable 6 carbon compound are converted by the addition of 3 molecules of water into six molecules of 3 phosphoglyceric acid.This reaction is also catalyzed by the enzyme carboxy mutase.

3 unstable intermediate 6 C compound + 3 H₂O  →   Carboxy dismutase3 phosphoglyceric acid

                                                                                 Carboxy

                                                                               Dismutase

3 phosphoglyceric acid (PGA) is the first stable product of dark reaction of photosynthesis and since it is a 3 carbon compound, this cycle is known as C3 cycle.

2. Reductive phase

Six molecules of 3PGA are phosphorylated by 6 molecules of ATP (produced in the light reaction) to yield 6 molecules of 1-3 diphospho glyceric acid and 6 molecules of ADP. This reaction is catalyzed by the enzyme, Kinase

 3 Phospho

glyceric acid     + ATP    →     1,3 diphosphoglyceric acid   + ADP

                                      Kinase                      

Six molecules of 1, 3 diphosphoglyceric acid are reduced with the use of 6 molecules of NADPH2 (produced in light reaction) to form 6 molecules of 3 phospho glyceraldehyde. This reaction is catalysed by the enzyme, triose phosphate dehydrogenase.

1,3 diphospho

glyceric acid       +    NADPH₂            →          3 phosphoglyceraldehyde   + NADP + H₃PO₄

                                  Triose phosphate  Dehydrogenase

3. Regenerative phase

In the regenerative phase, the ribose diphosphate is regenerated. The regenerative phase is called as pentose phosphate pathway or hexose monophophate shunt.

C4 cycle or Hatch and Slack pathway:

It is the alternate pathway of C3 cycle to fix CO2. In this cycle, the first formed stable compound is a 4 carbon compound, oxaloacetic acid. Hence it is called C4 cycle. The pathway is also called as Hatch and Slack as they worked out the pathway in 1966 and it is also called as C4 dicarboxylic acid pathway. This pathway is commonly seen in many grasses, sugar cane, maize, sorghum and amaranthus.

               The C4 plants show a different type of leaf anatomy. The chloroplasts are dimorphic in nature. In the leaves of these plants, the vascular bundles are surrounded by bundle sheath of larger parenchymatous cells. These bundle sheath cells have chloroplasts. These chloroplasts of bundle sheath are larger, lack grana and contain starch grains. The chloroplasts in mesophyll cells are smaller and always contain grana. This peculiar anatomy of leaves of C4 plants is called Kranz anatomy. The bundle sheath cells are bigger and look like a ring or wreath. Kranz in German means wreath and hence it is called Kranz anatomy. The C4 cycle involves two carboxylation reactions, one taking place in chloroplasts of mesophyll cells and another in chloroplasts of bundle sheath cells. There are four steps in

Hatch and Slack cycle:

1. Carboxylation

2. Breakdown

3. Splitting

4. Phosphorylation

1. Carboxylation

        It takes place in the chloroplasts of mesophyll cells. Phosphoenolpyruvate, a 3 carbon compound picks up CO2 and changes into 4 carbon oxaloacetate in the presence of water. This reaction is catalysed by the enzyme, phosphoenol pyruvate carboxylase.    

2. Breakdown

Oxaloacetate breaks down readily into 4 carbon malate and aspartate in the presence of the enzyme, transaminase and malate dehydrogenase. These compounds diffuse from the mesophyll cells into sheath cells.

3. Splitting

In the sheath cells, malate and aspartate split enzymatically to yield free CO2 and 3 carbon pyruvate. The CO2 is used in Calvin’s cycle in the sheath cell. The second Carboxylation occurs in the chloroplast of bundle sheath cells. The CO2 is accepted by 5 carbon compound ribulose diphosphate in the presence of the enzyme, carboxy ismutase and ultimately yields 3 phosphoglyceric acid. Some of the 3 phosphoglyceric acid is utilized in the formation of sugars and the rest regenerate ribulose diphosphate.

4. Phosphorylation

The pyruvate molecule is transferred to chloroplasts of mesophyll cells where, it is phosphorylated to regenerate phosphoenol pyruvate in the presence of ATP. This reaction is catalysed by pyruvate phosphokinase and the phophoenol pyruvate is regenerated. In Hatch and Slack pathway, the C3 and C4 cycles of carboxylation are linked and this is due to the Kranz anatomy of the leaves. The C4 plants are more efficient in photosynthesis than the C3 plants. The enzyme, phosphoenol pyruvate carboxylase of the C4 cycle is found to have more affinity for CO2 than the ribulose diphosphate carboxylase of the C3 cycle in fixing the molecular CO2 in organic compound during Carboxylation.

Comparison of the plants of C₃ and C₄  cycle :

Factors affecting photosynthesis :
 I. External factors

1.Light:

     It is the most important factor of photosynthesis. Any kind of artificial light such as electric light can induce photosynthesis. Out of the total solar energy, only 1-2 % is used for photosynthesis and the rest is used for other metabolic activities. The effect of light on photosynthesis can be studied under three categories. 

 a. Light intensity: The rate of photosynthesis is greater in intense light than in diffused light.The rate of photosynthesis is directly proportional to light intensity.

b. Light quality (wavelength):Photosynthesis occurs only in the visible part of the light spectrum i.e., between 400 and 700 nm. The maximum rate of photosynthesis occurs at red light followed by blue light. The green light has minimum effect and photosynthesis cannot take place either in the infrared or in the ultraviolet light.

c. Light duration: In general tropical plants get 10-12 hours of light per day and this longer period of light favours photosynthesis.

2.Carbon dioxide:

CO2 is one of the raw materials required for photosynthesis. If the CO2 concentration is increased at optimum temperature and light intensity, the rate of photosynthesis increases. But, it is also reported that very high concentration of CO2 is toxic to plants inhibiting photosynthesis.

3. Temperature:

       The rate of photosynthesis increases by increase in temperature up to 40 ºC and after this, there is reduction in photosynthesis. High temperature results in the denaturation of enzymes and thus, the dark reaction is affected.

4. Water:

        Water has indirect effect on the rate of photosynthesis although it is one of the raw materials for the process. Water rarely acts as a limiting factor for photosynthesis. During water scarcity, the cells become flaccid and the rate of photosynthesis might go down.

5. Oxygen:

          Oxygen is a byproduct of photosynthesis and an increase in the O2 concentration in many plants results in a decrease in the rate of photosynthesis.

II Internal factors:

1. Leaf:

     The leaf characters such as leaf size, chlorophyll content, number of stomata. Leaf orientation and leaf age are some of the factors that are responsible for photosynthesis.

2. Chlorophyll content:

          It is very much essential to tarp the light energy. In 1929, Emerson found direct relationship between the chlorophyll content and rate of photosynthesis. In general, the chlorophyll sufficient plants are green in colour showing efficient photosynthesis. The chlorotic leaves due to irregular synthesis of chlorophyll or breakdown of chlorophyll pigment exhibit inefficient photosynthesis.

PHOTORESPIRATION

Photorespiration is carried out only in the presence of light. But the normal respiration is not light dependent and it is called dark respiration. In photorespiration, temperature and oxygen concentration play an important role. Photorespiration is very high when the temperature is between 25 and 30 ºC. The rate of photorespiration increases with the increase in the concentration of oxygen. Three cell organelles namely chloroplast, peroxisome and mitochondria are involved in the photorespiration. This kind of respiration is seen in plants like cotton, pulses, capsicum, peas, tomato, petunia soybean, wheat, oats, paddy, chlorella etc and it is absent in grasses.

 Mechanism:

1. In the presence of excess oxygen and low CO2 , ribulose 1,5 diphosphate produced in the chloroplast during photosynthesis is split into 2 phospho glycolic acid and 3 phospho glyceric acid by the enzyme, ribulose 1,5 diphosphate oxygenase

2. The 3 phospho glyceric acid enters the Calvin cycle.

3. In the next step, phosphate group is removed from 2 phosphoglycolic acid to produce glycolic acid by the enzyme, phosphatase.

4. Glycolic acid then it come out of chloroplast and enter the peroxisome. Here, it combines with oxygen to form glyoxylic acid and hydrogen peroxide. This reaction is catalyzed by the enzyme, glycolic acid oxidase. Hydrogen peroxide is toxic and it is broken down into water and oxygen by the enzyme, Catalase. Photorespiration is an oxidation process. In this process, glycolic acid is converted into carbohydrate and CO2 is released as the by product. As glycolic acid is oxidized in photorespiration, it is also called as glycolate metabolism.

5. The glyoxylic acid converted into glycine by the addition of one amino group with the help of the enzyme, amino transferase.

6. Now, the glycine is transported from the peroxisome into the mitochondria. In the mitochondria, two molecules of glycine condense to form serine and liberate carbon dioxide and ammonia.

 7. Amino group is removed from serine to form hydroxyl pyruvic acid in the presence of the enzyme, transaminase.

8. Hydroxy pyruvic acid undergoes reduction with the help of NADH to form glyceric acid in the presence of enzyme alpha hydroxyl acid reductase.

9. Finally, regeneration of 3 phosphoglyceric acid occurs by the phosphorylation of glyceric acid with ATP. This reaction is catalyzed by the enzyme, Kinase.

10. The 3 phosphoglyceric acid is an intermediate product of Calvin cycle. If it enters the chloroplast, it is converted into carbohydrate by photosynthesis and it is suppressed nowadays with the increased CO2 content in the atmosphere.

Significance of photorespiration:

1.Photorespiration helps in classifying the plants Generally, photorespiration is found in C₃ plants and absent in C₄ plants.

2. Carbon dioxide is evolved during the process and it prevents the total depletion of CO₂ in the vicinity of chloroplasts.

3. Photorespiration uses energy in the form of ATP and reduced nucleotides, but normal respiration yields ATP and reduced nucleotides.

4. It is believed that photorespiration was common in earlier days when CO₂ content was too low to allow higher rates.