Bio-Synthetic Phase (Dark Reaction)

BIOSYNTHEI1C PHASE (DARK REACTION)

The process by which carbon dioxide is reduced to carbohydrates and the process is termed as carbon fixation; it makes use of the ATP and NADPH produced in the light phase. This process occurs in the stroma of chloroplasts with the help of series of enzyme-catalysed reactions.

Calvin cycle consists of three phases namely:

Carboxylation

Reduction

 Regeneration of RuBP.

In Calvin cycle, RuBP carboxylase, which is the main enzyme of photosynthesis, functions as oxygenase at high temperatures and high oxygen concentration. It catalyses the oxidation of RuBP into one molecule of a 3Carbon compound, phosphoglycerate and one molecule of a 2Carbon compound, phosphoglycolate. This process is considered as energy utilization process as

There is no sugar or ATP formed.

Carbon dioxide is released and ATP is consumed.

The plants which employ this mechanism of carbon fixation show the presence of two types of photosynthetic cells, i.e., mesophyll cells and bundle sheath cells (Kranz anatomy).The chloroplasts are dimorphic, i.e., those in the mesophyll cells are granal and those in the bundle sheath cells are agranal. Phosphoenolpyruvate (PEP) is the primary carbon dioxide acceptor, present in the mesophyll cells and the reaction is catalysed by the enzyme phosphoenolpymvate carboxylase (PEP-case). The first stable product is oxaloacetic acid (OAA), which is a 4 Carbon compound. OAAis converted into malic acid and transported to bundle sheath cells, where it is decarboxylated into pyruvic acid. It is an adaptive mechanism to avoid photo- respiration; the decarboxylation of malic acid in the bundle sheath cells maintains a high concentration of carbon dioxide near RuBisCO, so that it functions as carboxylase only and not as oxygenase.