Flexi Says: In C3 cycle of photosynthesis, the first stable product is a 3 carbon compound Phosphoglyceric Acid (PGA). In C4 cycle of photosynthesis, the first stable product is a 4 carbon compound Oxaloacetic Acid (OAA). The carbon dioxide acceptor in C3 cycle of photosynthesis is a 4 carbon compound RuBP, whereas there are two carbon dioxide acceptors in C4 cycle of photosynthesis - a 3 carbon compound, PEP (Phosphoenolpyruvate) and RuBP. The first enzyme in C3 cycle is RUBISCO, which has a high affinity towards oxygen. The first enzyme in C4 cycle is PEP carboxylase, which does not have any affinity for oxygen. C3 cycle requires 18 ATP molecules to synthesize one molecule of glucose, whereas C4 cycle requires 30 ATP molecules to synthesize one molecule of glucose. All the steps of C3 cycle occur in the mesophyll cells, whereas only the initial steps of C4 cycle occur in the mesophyll cells. Also, C4 cycle is more efficient than the C3 cycle of photosynthesis due to the absence of photorespiration.