Lec20.gas.exchange.slide

Lec20.gas.exchange.slide - But there’s trouble in...

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Unformatted text preview: But there’s trouble in paradise UBISCO can bind & catalyze a reaction with O 2- happens when partial pressure of CO 2 very low- it binds more O 2 than CO 2 ! O 2 concentrations in a leaf drop under water stress 1) stomata closed to prevent H 2 O loss but this causes 2) cessation of CO 2 exchange with the atmosphere 3) as CO 2 is used in the leaf, O 2 binding predominates eaction : uBP (5-C) + O 2-> 3-PGA (3-C) + phosphoglycolate (2-C) P-Glyco further metabolized in mitochondria/peroxisomes--> glyoxylate --> glycine --> serine + CO 2 ↑ OH C=O C-PO 4 Photorespiration! NO NET CARBON FIXATION HERE & PREVIOUSLY-FIXED C is LOST as CO 2 ! Very wasteful – up to 30% of fixed C may be lost Why hasn’t evolution gotten rid of RUBISCO? It’s a very old enzyme; evolved 3 BYA- when¡no¡O 2 in atmosphere- all plants were aquatic and single celled!- so no problem back then NO LAND PLANTS THEN (& NO STOMATA) No replacement has evolved from scratch; too complex? Photorespiration Plant needs higher Water Use Efficiency (WUE) ¡ Definition: the amount of carbon fixed per unit of H 2 O transpired ¡ Plants that fix more grams of carbon for each gram of water they lose have higher WUE ¡ Since photorespiration is caused by closed stomata, plant needs to evolve a way of fixing more carbon even when water is limiting. First product of CO...
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Lec20.gas.exchange.slide - But there’s trouble in...

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