lec5mcdb1bSu09

lec5mcdb1bSu09 - Summary of C fixation Lecture 5...

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Lecture 5 • Photorespiration • C4, CAM photosynthesis • Plant nutrition • Transport of nutrients 1 Summary of C fixation 3 steps of Calvin cycle: 1) carboxylation 2) reduction/sugar production 3) regeneration of RuBP Takes 6 turns of cycle to make 1 sugar Competition between O 2 and CO 2 at first step -> photorespiration if O 2 used Photorespiration results in net loss of fixed C; involves metabolism in 3 organelles (plastid, peroxisome, and mitochondria) C4 plants avoid photorespiration by spatial separation of initial CO 2 fixation and Calvin cycle • Biofuels overview 3 Big Oil $$ is being invested in biofuels Rubisco is an oxygenase as well as a carboxylase. Can add O 2 to RuBP instead of CO 2 Products: 3PG and phosphoglycolate Reduces amount of CO 2 fixed, and limits plant growth. Called “Photorespiration” 4 Photorespiration higher at high temperature in light •Water splitting in light reactions produces O 2 •At high temperature, stomata start to close ->decreased gas exchange increases dissolved O 2 /CO 2 ratio •O 2 competes with CO 2 for binding to Rubisco Photorespiration: light-dependent, consumes O 2 , releases CO 2 Glycine Peroxisome CCC CCC 2 CC ox CCC Mitochondrion When/why is C3 pathway inadequate? Rubisco can join either CO 2 or O 2 to RuBP CO 2 -> productive cycle O 2 -> net loss of 1 Fxed CO 2 -> inhibits growth (“photorespiration”) Choice of CO 2 vs. O 2 determined by: afFnity of Rubisco relative levels of dissolved CO 2 and O 2
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Not all plants use C3 pathway Many tropical grasses show initial labeling in C4 cmpd: C4 fxation C4 pathway uses diFFerent initial incorporation step PEP carboxylase does not bind O 2 -> no competition •C4 compound synthesized in mesophyll, transported to adjacent bundle sheath cell where released and refxed via Calvin cycle •EFFectively “pumps” CO 2 into bundle sheath cells 9 Specialized anatomy and ultrastructure oF C4 plants •“Kranz” anatomy: bundle sheath cells ringed by mesophyll •Calvin cycle occurs in cells with high CO 2 /O 2 •Bundle sheath plastids have much more starch grains than mesophyll mesophyll bundle sheath Stomata are pores in leaf epidermis that allow gas exchange in/out Stomates surrounded by “guard cells”
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Paradox of gas exchange: Stomates open high CO 2 fluxes allows H 2 O loss (dehydration of leaf) Stomates closed conserve H 2 O losses low CO 2 fluxes Crassulacean acid metabolism (CAM): an adaptation to arid environments •Water loss minimized by reversing normal stomatal cycle •Initial C fxation at night via C4 pathway C4 and CAM plants both use C4 and C3 fxation C4 plants separate pathways spatially: 4C compound transported to adjacent cell layer CAM plants separate pathways temporally: 4C compound stored in vacuole at night, refxed during day Summary oF C fxation 3 steps oF Calvin cycle: 1) carboxylation 2) reduction/sugar production 3) regeneration oF RuBP Takes 6 turns oF cycle to make 1 sugar Competition between O 2 and CO 2 at active site oF
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lec5mcdb1bSu09 - Summary of C fixation Lecture 5...

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