lec11 Membranes

lec11 Membranes - Alternative mechanisms of carbon fixation...

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Alternative mechanisms of carbon fixation have evolved in hot, arid climates On hot, dry days, plants close their stomata – Conserve water but limits access to CO 2 Causes oxygen to build up from light reactions In non-adapted plants this causes Photorespiration
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The Importance of Photosynthesis: A Review Light reactions: • Are carried out by molecules in the thylakoid membranes • Convert light energy to the chemical energy of ATP and NADPH • Split H 2 O and release O 2 to the atmosphere Calvin cycle reactions: • Take place in the stroma • Use ATP and NADPH to convert CO 2 to the sugar G3P • Return ADP, inorganic phosphate, and NADP+ to the light reactions O 2 CO 2 H 2 O Light Light reaction Calvin cycle NADP + ADP ATP NADPH + P 1 RuBP 3-Phosphoglycerate Amino acids Fatty acids Starch (storage) Sucrose (export) G3P Photosystem II Electron transport chain Photosystem I Chloroplast Figure 10.21
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Photorespiration: An Evolutionary Relic In photorespiration – O 2 substitutes for CO 2 in the active site of the enzyme rubisco The photosynthetic rate is reduced Wasteful process Reduces sugar output from photosynthesis How did this wasteful process arise ? – Evolutionary relic from a time with more CO 2 in atmosphere than O 2 • Rubisco’s active site wouldn’t need to be specific for CO 2 if there wasn’t much O 2 around!
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Adaptations to Bypass Photorespiration • C 4 plants— separate CO 2 fixation by space • CAM plants—separate CO 2 fixation by time
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C4 Plants • C 4 plants pump CO 2 into the Calvin Cycle Alternative mode of carbon fixation to bypass rubisco • Use enzyme PEP carboxylase to incorporate CO 2 into four carbon compounds in mesophyll cells • PEP carboxylase—much higher affinity for CO 2 than for O 2 These four carbon compounds Are exported to bundle sheath cells, where they release CO 2 used in the Calvin cycle
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4 leaf anatomy and the C 4 pathway CO 2 Mesophyll cell Bundle- sheath cell Vein (vascular tissue) Photosynthetic cells of C 4 plant leaf Stoma Mesophyll cell C 4 leaf anatomy PEP carboxylase
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This note was uploaded on 11/02/2008 for the course BILD 1 taught by Professor Unk during the Spring '08 term at UCSB.

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lec11 Membranes - Alternative mechanisms of carbon fixation...

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