Glycogen Metabolism, Gluconeogenesis, Photosynthesis PDF

# Glycogen Metabolism, Gluconeogenesis, Photosynthesis - Glycogen Metabolism - Gluconeogenesis - Photosynthesis ## Dark Reactions - Occurs in the stroma of the chloroplast - Light is still needed for the activation of some involved enzymes ### Calvin-Benson Cycle or C3 Photosynthesis: - Reduction...

# Glycogen Metabolism, Gluconeogenesis, Photosynthesis - Glycogen Metabolism - Gluconeogenesis - Photosynthesis ## Dark Reactions - Occurs in the stroma of the chloroplast - Light is still needed for the activation of some involved enzymes ### Calvin-Benson Cycle or C3 Photosynthesis: - Reduction of carbon from CO₂ in a more reduced state as hexose. - Uses NADPH and ATP produced from the light reactions. ## Dark Reactions of Photosynthesis ### 1. CO₂ Fixation: - Fixation of CO₂ with ribulose-1,5-bisphosphate (RubP) to form 2 molecules of 3-phosphoglycerate (3-PGA). - Catalyzed by RuBisCO. #### RuBisCO: - Ribulose-1,5-bisphosphate carboxylase-oxygenase. - Most abundant enzyme on earth. - Requires Mg²⁺ as cofactor. ### 2. Reduction of 3-PGA to GAP: - 3-phosphoglycerate → 1,3-bisphosphoglycerate → glyceraldehyde-3-P. - Requires ATP and NADPH. ### 3. Regeneration of RubP: - Proceeds in the same fashion as the reverse of the non-oxidative stage of PPP. ## Stoichiometry - To fix 1 molecule of CO₂: - 3 ATP and 2 NADPH molecules are required. - To synthesize a glucose molecule: - 6 rounds of Calvin cycle. - 18 ATP and 12 NADPH molecules are required. - 6 CO₂ + 18 ATP + 12 NADPH + 12 H₂O → C₆H₁₂O₆ + 18 ADP + 18 Pi + 12 NADP+ + 6 H+ ## RuBisCO: Oxygenase Reaction: - Rubisco also catalyzes a wasteful oxygenase reaction when [CO₂]/[O₂] is low. - Happens under hot dry condition. - Stomata cannot open to allow entry of CO₂. - A phenomenon called photorespiration. ## C4 Photosynthesis - Prevents oxygenase activity of RuBisCO by maintaining a high [CO₂]. - Uses a carbon-fixing enzyme PEP carboxylase in tandem with RuBisCO. ### C4 Photosynthesis Steps: 1. CO₂ is added to PEP to form OAA (PEP carboxylase). 2. MC export OAA to the BSC through malate. 3. In the BSC, malate is broken down to CO₂ and pyruvate. 4. CO₂ is converted to sugar via normal Calvin Cycle. ## CAM Plants - CAM- Crassulacean Acid Metabolism. - Observed in Crassulaceae (succulents). - Thick cuticles. - High water content. - Lower number of stomates. ### CAM Photosynthesis: - During daytime, stomates are closed to prevent evaporation of H₂O; no CO₂ can enter the plant. - During nighttime, stomates are open; CO₂ is fixed via C4 pathway. - The following daytime, malate is decarboxylated and CO₂ is fixed via the Calvin cycle. ## C4 Versus CAM Plants ### C₄ Plants - Spatial separation of carbon dioxide fixation and uptake. - Malate is the carrier of carbon dioxide. ### CAM Plants - Temporal cell separation - Malate is for storage carbon dioxide.