BIO203H5 Lecture Notes - Lecture 5: Photosynthetic Efficiency, C4 Carbon Fixation, C3 Carbon Fixation
Document Summary
Calvin (c3) cycle: ribulose 1,5-biphosphate caroxylase oxygenase (rubisco, most abundant protein on earth (~40%, c3 fixation= first step- carbon from carbon dioxide added to 5c rubp, yields 2 x 3c (3-phosphoglycerate) molecules (hence c3 cycle, principle output g3p. Rubisco forms 3-pga most efficiently when there"s low o2 & high. Atmosphere has high o2 and low co2. Up to 25% of fixed c lost at ~20 c. If plants could utilize co2 more efficiently, they could: reduce time/extent of stomatal opening, increase water use efficiency, achieved to some extent in c4 plants (eg. sugar cane) C3 plants: co2 is fixed in the mesophyll chloroplasts by rubisco, how can the carboxylase reaction of rubisco be favoured to increase photosynthetic efficiency. C4 plants: turbo-charging co2 (excluding o2, trap co2 & deliver to localized rubisco, increases co2:o2 ratio around rubisco, reduces photorespiration, permits more prolonged stomatal closure, enhanced water use efficiency, fixation. There are other mechanisms for spatially concentrating co2 with rubisco.
