Question

Write the differences between C3 and C4 cycles.

Answer 1

Hint: Both C3 and C4 cycles occur during the dark reaction of the photosynthesis and have the common end product, the main difference lies in the first stable carbon product.

Complete solution:
Let us differentiate between the C3 and C4 cycles in a tabular form to have a better understanding of the contrasting points:

Sl.No	C3 Cycle	C4 Cycle
1.	The first stable product of C3 cycle is a 3-carbon compound called phosphoglyceric acid	The first stable product of C4 cycle is a 4-carbon compound called oxaloacetic acid
2.	The carbon dioxide acceptor in C3 cycle is RuBP (Ribulose-1,5-bisphosphate) which is a 5-carbon compound	The carbon dioxide acceptor in C3 cycle is PEP (Phosphoenol Pyruvate) which is a 3-carbon compound
3.	The first enzyme used in carboxylation is RUBISCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase)	The first enzyme used in carboxylation is PEP carboxylase
4.	It is inhibited in the increased concentration of oxygen	Concentration of oxygen has no effect on C4 cycle
5.	It is inhibited in the increased concentration of oxygen	Concentration of oxygen has no effect on C4 cycle
6.	It requires 18 ATP molecules to synthesize one molecule of glucose	It requires 30 ATP molecules to synthesize one molecule of glucose
7.	It is completed within the mesophyll cells only	Only the initial steps of C4 cycle are performed in mesophyll cells, the rest are performed in bundle sheath cells
8.	Only a single type of granal chloroplasts are involved in the C3 cycle	Granal (in mesophyll cells) and agranal (in bundle sheath cells) types of chloroplasts are involved in C4 cycle
9.	There is no involvement of secondary carbon dioxide acceptor	PEP is the primary carbon dioxide acceptor while RuBP is the secondary accetor
10.	It is less efficient in carbon dioxide fixation due to photorespiration	It is more efficient in carbon dioxide fixation

Note: The C3 plants have low optimum temperature requirements while C4 plants can withstand a high temperature.