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LIFE 102 (201)

Carbon fixation, photorespiration, C4 plants, CAM plants, cell signaling, receptors in the plasma membrane, intracellular receptors, transduction

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Colorado State University
Life Science
LIFE 102
Louis Bjostad

7 MarchConcept 104 Alternative mechanisms of carbon fixation have evolved in hot arid climatesDehydration is a problem for plants sometimes requiring tradeoffs with other metabolic processes especially photosynthesisOn hot dry days plants close stomata which conserves HO but also limits 2photosynthesisThe closing of stomata reduces access to CO and causes O to build up22These conditions favor a seemingly wasteful process called photorespirationPhotorespiration An Evolutionary RelicIn most plants C plants initial fixation of CO via rubisco forms a threecarbon 32compoundIn photorespiration rubisco adds O instead of CO in the Calvin cycle22Photorespiration consumes O and organic fuel and releases CO without producing ATP 22or sugarPhotorespiration may be an evolutionary relic because rubisco first evolved at a time when the atmosphere had far less O and more CO22Photorespiration limits damaging products of light reactions that build up in the absence of the Calvin cycleIn many plants photorespiration is a problem because on a hot dry day it can drain as much as 50 of the carbon fixed by the Calvin cycleC Plants4C plants minimize the cost of photorespiration by incorporating CO into fourcarbon 42compounds in mesophyll cellsThis step requires the enzyme PEP carboxylasePEP carboxylase has a higher affinity for CO than rubisco does it can fix CO even 22when CO concentrations are low2These fourcarbon compounds are exported to bundlesheath cells where they release CO that is then used in the Calvin cycle2CAM Plants
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