Photosynthesis is a complex process of synthesis of organic food molecule. It is complicated oxidation- reduction process. Here water is oxidized and CO2 is reduced to carbohydrates. It takes place in two steps. They are light reaction and dark reaction.

In light reaction ATP and NADPH, two are produced in dark reaction; CO2 is reduced with the help of ATP and NADPH2 to produce glucose.

In dark reaction, two types of cyclic reaction occur. They are Calvin cycle or C3 cycle and Hatch Slack cycle or C4 cycle.

C3 pathway involve set of carbon reaction which are catalyzed by the enzyme RuBisCO to synthesize 3- carbon compound, 3- Phosphoglycerate from 5- carbon compound, Ribulose bisphosphate.

C4 pathway also involves carbon fixation to synthesize 4- carbon compound oxaloacetic acid, C4 pathway occurs only in some plants to prevent loss of energy by photorespiration.

Calvin Cycle or C3 Cycle:

·         The Calvin cycle was observed by Melvin Calvin in Chlorella, unicellular algae.

·         C3 cycle occurs in the stroma of chloroplast.

·         In this reaction CO2 is converted into sugars. So, it is a process of carbon fixation. The primary CO2 acceptor is Ribulose Biphosphate, a 5- carbon compound.

·         CO 2 fixation is slow and less efficient. Fixation of one molecule of CO2 requires 3 ATP and 2NADH.

·         The first stable compound produced in Calvin cycle is a 3 Carbon compound, namely 3- Phosphoglycerate (PGA). Hence Calvin cycle is also called C3 cycle.

·         C3 cycle operates in plants like mango, apple etc.

Steps of Calvin Cycle:

1.       Carboxylation:

In the first step CO2 is accepted by a 5- carbon, RuBP (Ribulose 1,5 Bisphosphate) and two molecule of 3- carbon compound i.e. 3-PGA are formed. This reaction is catalyzed by an enzyme called RuBisCO. Formation of PGA is called carboxylation. The 3- Carbon compound formed is the first stable product of this pathway and hence the name C3 pathway.

2.       Reduction:

After carboxylation, reduction of PGA occurs by utilizing ATP and NADPH2 formed during photochemical reaction. Reduction results in the formation of G3P. Then 3 carbon compounds also as Triose phosphate are diverted from the Calvin cycle and act as precursors for the synthesis of sucrose and starch.

3.       Regeneration:

During which the CO2 acceptor RuBP is formed again so that the cycle continues G3P using an ATP molecule.


Hatch and Slack Cycle:

·         C4 is the alternative pathway of Calvin cycle (C3 cycle) taking place during the dark phase of photosynthesis.

·         In the C4 cycle, the first stable compound is 4- Carbon compound, namely Oxaloacetic acid (OAA). Hence it is called C4 cycle. The primary CO2 acceptor is Phosphoenole pyruvic acid (PEP).    It is a process of CO2 fixation. CO2 fixation is fast and more efficient. This pathway was worked out by Hatch and Slack in 1966. Hence the C4 cycle is also called as Hatch- Slack cycle.

·         Fixation of one molecule of CO2 requires 5 ATP and 3 NADH.

·         C4 cycle occurs in plants like maize, sorghum etc.

·         Chloroplasts are dimorphic in nature. Chloroplasts of bundle sheath are larger and contain starch grains.

·          In the leaves of these plants, the vascular bundles are surrounded by bundle sheath of larger parenchymatous cells.

·         The chloroplast 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 reaction, one taking place in chloroplasts of mesophyll cells and another in chloroplast of bundle sheath cells.

Steps of C4 pathway:

There are four steps in Hatch and Slack cycle:

         i.            Carboxylation: It takes place in the chloroplast of mesophyll cells. Phosphoenol pyruvate (PEP), a 3 carbon compound picks up CO2 and changes into 4 C- Oxaloacetate in the presence of water. This reaction is catalyzed by the enzyme, PEP Carboxylase.

PEP + CO2+H2O -------- oxaloacetate (4C) + H3PO4 (PEP carboxylase)

       ii.            Breakdown: Oxaloacetate breaks down readily into 4 C- Malate and Aspartate in the presence of the enzymes, transaminase and Malate dehydrogenase.

Oxaloacetate (4C) ------------ Aspartate (4C) +Malate (4C) (Transaminase, Malate dehydrogenase)

These compounds diffuse from the mesophyll cells into sheath cells.

      iii.            Splitting: In the sheath cells Malate and Aspartate split enzymatically to yield free CO2 and 3C- pyruvate. The CO2 is used in Calvin cycle in the sheath cells.

Malate --------------- CO2 + Pyruvate (Decarboxylation)

The second carboxylation occurs in the chloroplast of bundle sheath cells. The CO2 is accepted by 5C- compound, ribulose bisphosphate (RuBP) in the presence of enzyme; carboxydimutase and ultimalely yield 3- phosphoglyceric acid. Some of the 3 Phosphoglyceric acids is utilized in the formation of sugar and the rest to regenerate ribulose diphosphate.

     iv.            Phosphorylation: The pyruvate molecule is transferred to chloroplasts of mesophyll cells, where it is phosphorylated to regenerate PEP in the presence of ATP. This reaction is catalyzed by pyruvate phosphokinase and the phosphoenol pyruvate is regenerated.

Pyruvate + ATP+ Pi --------------- PEP + AMP + Pyrophosphate (Pyruvate phosphokinase)

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