Primary CO2 acceptor in C4 plants is PEP or phosphoenol pyruvate, combination of CO2 with PEP is called initial fixation and occurs in mesophyll cells. The enzyme required is PEP carboxylase. PGA (Phosphoglyceric Acid) is the first stable product of Calvin cycle or C3 cycle or reductive pentose pathway or photosynthetic carbon reduction. RuBP was formerly known as RuDP (Ribulose Diphosphate). It is first CO2 acceptor in C3 plants. Oxaloacetic acid is the first product of C4 cycle. It is unstable compound which is changed to maloic acid/aspartic acid for transport to bundle sheath cells.