C4 and CAM pathway MCQ - Practice Questions & Answers

Quick Facts

The C4 Pathway

The primary CO2 acceptor is 3-carbon molecule phosphoenolpyruvate (PEP) and is present in the mesophyll cells.
The CO2 is received in the form of bicarbonate ions.
The carbonic anhydrase hydrates CO2 and produce bicarbonate
The enzyme responsible for this fixation is PEP carboxylase or PEPcase.
It is important to register that the mesophyll cells lack the RuBisCO enzyme.
The C4 acid OAA is formed in the mesophyll cells.
It then forms other 4-carbon compounds like malic acid or aspartic acid in the mesophyll cells themselves, which are transported to the bundle sheath cells.
In the bundle sheath cells, these C4 acids are broken down to release CO2 and a 3-carbon molecule.
The 3-carbon molecule is transported back to the mesophyll where it is converted to PEP again, thus, completing the cycle
The CO2 released in the bundle sheath cells enters the C3 or the Calvin pathway, a pathway common to all plants.
The bundle sheath cells are rich in the enzyme Ribulose bisphosphate carboxylase-oxygenase (RuBisCO), but lack PEPcase.
Thus, the basic pathway that results in the formation of the sugars, the Calvin pathway, is common to the C3 and C4 plants.
In C4 plants:
For 1 CO2 molecule: 5 ATPs + 2 NADPH molecules are required
For 3 CO2 molecules (that is 1 triose phosphate): 15 ATPs + 6 NADPH molecules are required
For 6 CO2 molecules (that is 1 glucose): 30 ATPs + 12 NADPH molecules are required