Question

What is C4 pathway?
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Answer 1

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. Hence it is called C4 cycle. It is a process of carbon dioxide fixation.

Answer 2

NCERT based Answer for Class 11 Biology Students

C4 pathway:

 This mechanism builds up CO2 concentration in the vicinity of RuBisCO in the bundle sheath cells.

 High concentration of CO2 near RuBisCO favours carboxylation and suppresses photorespiration.  

 In C4 pathway, CO2 enters the mesophyll cells and combines with phosphoenol pyruvate (3 carbon compound) to form C4 acid, oxalo acetic acid (OAA). This reaction is catalysed by the enzyme phosphoenol pyruvate carboxylase (PEPcase).  

 The reaction occurs in cytosol of mesophyll cells and is called fixation of CO2 or carboxylation.

 C4 acid is transported from cytosol of mesophyll cells to bundle sheath cells chloroplasts.

 C4 acid is decarboxylated to release CO2 near RuBisCO in bundle sheath cells.

 The other product of decarboxylation reaction, pyruvate (C3 compound) is transported back to mesophyll cells, where it is used in regenerating phosphoenol pyruvate.

Note: C4 photosynthetic pathway is more efficient than C3 pathway due to absence of photorespiration.

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