According to the textbook:

• C4 Pathway (A): The primary CO2 acceptor in the mesophyll cells is a 3-carbon molecule, phosphoenol pyruvate (III) (PEP) .
• Light reaction (B): Involves the splitting of water to release oxygen, protons, and electrons () (I) .
• Photorespiration (C): In C3 plants, RuBisCO binds with O2 to form one molecule of phosphoglycerate and one molecule of Phosphoglycolate (IV) .
• Calvin cycle (D): The primary acceptor of CO2 is a 5-carbon ketose sugar, Ribulose 1,5-bisphosphate (II) (RuBP) .