Photosynthesis2_dark reactions.pdf

Photosynthesis: light (thylakoid) reactions & Photosynthesis involves a series of biochemical reactions: CO2 fixation (Calvin-Benson cycle) • Carbon (“dark”) reactions – synthesis of carbohydrates (sugar, output of photosynthesis) • Key learning objective: the key steps and strategies for carbon dio...

Photosynthesis: light (thylakoid) reactions & Photosynthesis involves a series of biochemical reactions: CO2 fixation (Calvin-Benson cycle) • Carbon (“dark”) reactions – synthesis of carbohydrates (sugar, output of photosynthesis) • Key learning objective: the key steps and strategies for carbon dioxide fixation Carbon (or “Dark”) Reactions (key points) • Require the products of the Light Reactions • Stroma (in Chloroplast) • Carbon dioxide fixation • Sugar formation • Enzyme mediated • Multiple variations (C3, C4, CAM) Melvin Calvin 8.2 The Calvin–Benson cycle proceeds in three phases C3 photosynthetic carbon reduction cycle (PCR): 3 molecules of Ribulose 1,5Bisphosphate + 3 molecules of Carbon dioxide 1 molecule Sugar : 3 C (triose phosphate) Net carbon gain or output Ribulose 1,5-Bisphosphate Carboxylase Oxygenase (Rubisco) O2 CO2 Ribulose 1, 5 bisphoshphate Rubisco Photorespiration Calvin cycle Carbon dioxide Released net carbon output in the presence of light WHAT IS PHOTORESPIRATION? • Rubisco also catalyses oxygenation of RuBP – photorespiration (in the presence of light) • CO2 and O2 are substrates for same active site on Rubisco - compete for RuBP HOW IS PHOTORESPIRATION INHIBITORY? • • • • RuBP wasted - thus energy to make it is lost Rubisco capacity diverted from carboxylation Some previously fixed CO2 is lost ATP and NADPH consumed in the conversion of C in phosphoglycolate back to useful metabolites • Why does PR exists - can we engineer more efficient photosynthesis? (see T+Z p219-220) Plants that do not photorespire - CO2 concentrating mechanisms C4 Photosynthesis - PCA • Discovered in tropical grasses (sugar cane + maize) • Recognized in many plant families • Evolved independently on several occasions C3 leaf cross section Mesophyll cells C4 leaf cross section Bundle sheath cells Mesophyll cells Figure 8.11 The C4 photosynthetic carbon cycle involves five successive stages in two different compartments Figure 8.12 C4 photosynthetic pathway in leaves of different plants (Part 2) Crassulacean Acid Metabolism (CAM) Figure 8.13 Crassulacean acid metabolism (CAM) Photosynthesis - environmental considerations Figure 9.6 Response of photosynthesis to light in a C3 plant