Pre-Medical NEET (UG) Coaching

 : ATP and NADPH, the products of light reaction are used in synthesis of food. The ®rst CO2 ®xation product in C3 plant is 3-phosphoglyceric acid or PGA. The CO2 acceptor molecule is RuBP (ribulose bisphosphate). The cyclic path of sugar formation is called Calvin cycle on the name of Melvin Calvin, the discover of this pathway. Calvin cycle proceeds in three stages. (1) Carboxylation : CO2 combines with ribulose 1, 5 bisphosphate to form 3 PGA in the presence of RuBisCo enzyme (present in stroma) (2) Reduction : Carbohydrate is formed at the expense of ATP and NADPH. It involves 2ATP for phsophorylation and 2NADH2 for reduction per CO2 molecule ®xed. (3) Regeneration : The CO2 acceptor ribulose 1, 5-bisphosphate is formed again. 6 turns of Calvin cycles and 18 ATP molecules are required to synthesize one molecule of glucose. (Fig. 13.8) 6CO2 + 6 RuBP + 18ATP + 12NADPHC6H12O6 + 6RuBP + 18ADP + 18Pi + 12NADP Input Output 6CO2 — One Glucose 18 ATP — 18 ADP 18NADPH — 12 NADP The C4 pathway : C4 plants such as maize, sorghum, sugarcane have special type, of leaf anatomy, they tolerate higher temperatures. In this pathway, oxaloacetic acid (OAA) is the ®rst stable product formed. It is 4 carbon atoms compound, hence called C4 pathway (Hatch and Slack Cycle). The leaf has two types of cells : mesophyll cells and Bundle sheath cells (Kranz anatomy). Initially CO2 is’ taken up by phosphoenol pyruvate (PEP) in mesophyll cell and changed to oxaloacetic acid (OAA) in the presence of PEP carboxylase. Oxaloacetate is reduced to maltate/asparate that reach into bundle sheath cells. The decarboxylation of maltate/asparate occurs with the release of CO2 and formation of pyruvate (3C). In high CO2 concentration RuBisCo carboxylase and not as oxygenase, hence the photosynthetic losses are prevented. RuBP operates now under Calvin cycle and pyruvate transported back to mesophyll cells and changed into phosphoenol pyruvate to keep the cycle continue