plant carbon metabolism

Plant carbon - Carbon metabolism Carbon fixation via photosynthesis is the basis of nearly all life on earth(only a tiny part of planetary

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Carbon metabolism Carbon fixation via photosynthesis is the basis of nearly all life on earth (only a tiny part of planetary ecosystems are based on chemosynthesis) about 160-170 billion tons of C fixation per year; maybe 100 billion tons by land plants heterotrophs (“consumers”) completely dependent on plants (“primary producers”); production of heterotroph biomass is much less than C fixation (2nd law of thermodynamics) But photosyntheis is an inefficient process (in terms of using solar energy) and puts great demands on land plants for water: CO 2 is quite scarce in the atmosphere (380 parts per million) the primary enzyme in the Calvin cycle -- the main photosynthetic ‘engine’ -- does not work well when CO 2 concentration is low in the leaf. not all wavelengths of sunlight are used in photosynthesis Transpirational water loss in plant leaves } inside leaf } outside air 45,000 ppm H 2 0 ~100 ppm CO 2 5,000 ppm H 2 0 Gradient of 40,000 ppm Gas space (100% Humidity) { 380 ppm CO 2 Gradient of 280 ppm Gradient for H 2 0 is ~ 150X that for CO 2 Plants can reduce water loss by adjusting or evolutionarily modifying stomates…. Control of stomata Open Closed Stomate opening and closing is a function of adjusting internal hydrostatic pressure…. Stomata opening and closing affected by: photoperiodism CO 2 concentration light water balance Control of stomata Control of internal hydrostatic pressure in guard cells is a function of osmotic movement of water, mediated by potassium (K + ) concentrations Direction of bending during osmotic changes due to orientation of cellulose microfibrils in cell walls.
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Evolutionary modification of stomata Stomatal crypts are stomates located in small ‘pits’ on the underside of leaves. Common in desert plants. Closing stomata or placing them in pits increase diffusion distance, and hence reduce transpirational water loss… but also decrease CO 2 uptake and carbon fixation Evolutionary modification of photosynthesis ‘light reactions’ localized in thylakoid membranes use energy in photons to produce ATP and reduce NADP + to NADPH ‘splits’ water, releases O 2 , hydrogen ions, and electrons ‘dark reactions’ ( carbon reactions ) localized in chloroplast stroma combines CO 2 with ribulose -1,5- bisphosphate (RuBP), using ATP and NADPH made in light rections -- carbon fixation (Calvin Cycle) stroma thylakoids Recall the basic reactions of photosynthetic process in chloroplasts: C fixation Recall the basic reactions of photosynthetic process in chloroplasts: Evolutionary modification of photosynthesis Calvin cycle (carbon reactions) sugar (output) CO 2 ATP reduction NADPH RuBP NADPH delivers electrons for the reduction step 3-C compound ATP Cycle has to ‘turn’ at least 6 times (fixing 6 CO 2 molecules) to produce one 6-carbon sugar end product.
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This note was uploaded on 07/31/2008 for the course BIO 5B taught by Professor Chappell during the Spring '07 term at UC Riverside.

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Plant carbon - Carbon metabolism Carbon fixation via photosynthesis is the basis of nearly all life on earth(only a tiny part of planetary

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