Biosynthetic Pathway (Dark reaction)

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The process by which carbon dioxide is reduced to carbohydrates by making use of the products of light reaction (ATP and NADPH₂) is called carbon dioxide fixation or biosynthesis phase. These series of reactions which catalyse the reduction of CO₂ to carbohydrates (also called fixation of CO₂) take place in the stroma of the chloroplast. These reactions are independent of light i.e. light is not necessary but can continue in light as well if products of the light reaction are available. Thus it is also called dark reaction. Depending on whether the first product of photosynthesis is a 3-carbon compound or a 4-carbon compound, there are two pathways;

C3 Cycle (Also Called Calvin Cycle After Its Discoverer, Melvin Calvin)

A CO₂ molecule combines with a five carbon acceptor molecule, ribulose-1, 5-bisphosphate (RuBP). This step makes a six-carbon compound that splits into two molecules of a three-carbon compound, 3-phosphoglyceric acid (3-PGA). This reaction is catalyzed by the enzyme RuBP carboxylase/oxygenase or rubisco. This enzyme is probably the most abundant protein on earth. In reduction stage, ATP and NADPH are used to convert the 3-PGA molecules into molecules of a three-carbon sugar, glyceraldehyde 3-phosphate (G3P). In the next step, PGA is reduced to 3-carbon carbohydrate called triose phosphate using NADPH₂ and ATP (from light reaction). Much of these molecules are then diverted from the C₃ cycle and used for synthesis of other carbohydrates such as glucose and sucrose. To complete the cycle, the initial 5-carbon acceptor molecule, RuBP is regenerated from the triose phosphates using ATP molecule thus the C₃ cycle continues to regenerate the CO₂-acceptor (RuBP).

C4 Cycle (Or Hatch and Slack Cycle)

The C₄ plants are generally found in the tropical and subtropical regions and they have evolved C₄ pathway as a mechanism to avoid the occurrence of photorespiration, which is considered a wasteful process. All those plants which exhibit C₄ pathway of photosynthesis are called C₄ plants, e.g. Zea maize (maize), Saccharum (sugar cane), Amaranthus, Digitalis, etc. Photorespiration (oxidation of RuBP in presence of O₂) is absent in these plants. So the photosynthetic rate is high.

The bundle sheath cells are large Chlorenchyma/parenchyma cells, arranged in a single layer around the vascular bundles like a wreath; this type of arrangement is known as Kranz anatomy. These are; they have dimorphic chloroplasts, light reactions and the carbon cycle occur separately in two different types of cells, the former taking place in mesophyll cells and the latter occurring in bundle sheath cells. This type of cellular arrangement does not allow the oxygen to come in contact with the Rubisco in the bundle sheath cells, hence oxygenation of RuBP is completely avoided. Added to this, they have evolved a carbon dioxide concentrating mechanism, called C₄ pathway.

The CO₂ acceptor in C₄ plants is phosphoenolpyruvate (PEP). PEP reacts with CO₂ to form oxaloacetic acid which is reduced by NADPH to form malic acid. The malic acid then reacts with RuBP to form pyruvic acid and PGA. The pyruvic acid is then phosphorylated by ATP to regenerate PEP while PGA is converted to triose phosphate as far as C₃ plants. It combines with CO₂ in presence of an enzyme Phosphoenolpyruvate carboxylase (PEPCase) and forms a C₄ acid, oxaloacetic acid (OAA). OAA fixation of CO₂ occurs in the cytosol of the mesophyll cells of the leaf. OAA is the first stable product of this cycle and hence the name C₄ pathway. OAA then travels from mesophyll cells to the chloroplasts of bundle sheath cell where it releases the fixed CO₂. C₃ cycle operates within these cells and this CO₂ immediately combines with RuBP and C₃ cycle continues producing sugars.

Difference between C3 and C4 Plants