5 Alternative mechanisms of carbon fixation Plants evolved ~475 myaBig problem for terrestrial plants = dehydrationSo…different carbon fixation pathways have evolved in hot, arid (dry) climates
6 Remember Stomata Leaf “pores” usually on UNDERSIDE of leaf…why? Allows O2 out and CO2 in for photosynthesisAllows H2O out through transpiration (“plant sweating”)
7 On hot, dry days, plants close their stomata (“pores”) Conserving water but limiting access to CO2 reduces photosynthesisCausing oxygen to build upWHY???1.swf
8 All plants carry out photosynthesis by… adding carbon dioxide (CO2) to a RuBPCatalyzed by the enzyme RUBISCORuBP+CO2 breaks down IMMEDIATELY 3-PGA3-PGA + ATP 1,3-PGA reduced to G3P by NADPHThe process is called the Calvin cycle and the pathway is called the C3 pathway.
9 C3 Plants (“regular” calvin cycle) Plants add CO2 in carbon fixation to RuBP (by Rubisco)Occurs in MESOPHYLL cells of leafCalled C3 plants b/c 1st product of carbon fixation is a 3 carbon compoundEx: Rice, Wheat , SoyBut…on hot days, stomata close reduces sugar productionO2 builds upEx: droughts little water but HOT little sugar production
10 RUBISCO ribulose bisphosphate carboxylase oxygenase (RUBISCO) Can bind CO2Also has affinity for/can bind O2Depends on concentrations of CO2 and O2
11 So…why is this a problem? Light Reactions gives off O2More O2 dissolves in cytoplasm of cell at higher temperatures b/c more light rxns occuringTherefore,high light intensities andhigh temperatures (above ~ 30°C) favor the second reaction (oxygenase action) of Rubisco.
12 Photorespiration: An Evolutionary Relic? In photorespirationO2 substitutes for CO2 (competitive inhibitor) in active site of enzyme RubiscoRubisco adds O2 to Calvin cycle NOT CO2 product made and splits into 2-C can’t make G3P Occurs in light (photo) and uses O2 and releases CO2 (respiration)NO ATP is made it USES ATP NO SUGAR produced The photosynthetic rate is reduced
13 Why photorespirate? Ancient atmosphere = little O2 Current atmosphere = lots of O2Rubisco has an affinity to bind to O2 and CO2So…now it is inevitable that Rubisco will bind some O2 b/c there is so much O2 in air
14 Alternative mechanisms of carbon fixation Photosynthetic adaptations to MINIMIZE photorespiration and OPTIMIZE Calvin Cycle2 Types of plants have adaptated:C4 PlantsCAM Plants
15 C4 Plants Usually found in high daytime temps and intense sunlight Ex: Corn, Sugarcane, crab grassHave different leaf anatomy than C3 plantC3= uses mesophyll cellsC4= uses mesophyll AND bundle sheath cells
18 C4 Plants C4 plants minimize the cost of photorespiration Takes CO2 into MESOPHYLL CELL turns it to 3-C PEP molecule using PEP carboxylase (enzyme)3-C PEP turned into 4-C intermediates (different acids)ACIDS are then stored in plant for later moves into Bundle Sheath cell
19 ACIDS = 4-C oxaloacetic acid (OAA) then Malate (4C) exported to bundle sheath cellsMalate CO2 +pyruvic acidin “normal” Calvin cycle (C3 cycle)Pyruvic acid moves back to mesophyll cell reforms 3-C PEPBundle sheath cells deep in leaf tissue (little O2)Rubisco can bind CO2 better
22 C4 Plants Characteristics well adapted to (found in) habitats with:high daytime temperaturesintense sunlightlow water conditions (drought)Advantage = photorespiration not as much of a problemEx: crabgrass, corn (maize), sugarcaneToday, C4 plants represent about 5% of Earth's plant biomass and 1% of its known plant species.Despite this scarcity, they account for around 30% of terrestrial carbon fixation
23 CAM Plants think “AM” has to do with time of day Type of C4 plantIn CAM plants, C3/C4 pathways NOT separate in leaf anatomy but by TIMENO BUNDLE SHEATH CELLS…only mesophyll cellslike C3 plantsOpen their stomata at night, incorporating CO2 into organic acids first (at night)CAM = crassulacean acid metabolism
24 CAM Plants During night stomata open CO2 joins 3-C PEP 4-C oxaloacetic acid (OAA) 4-C malate in central vacouleIn morning (light) stomata closedAccumulated malate leaves vacoule broken down CO2 released Calvin cycle (C3) cycle is used to produce sugar using energy made in light rxns
26 CAM Plants Characteristics Usually found inhigh daytime tempscool night tempsintense sunlightLow soil moistureAdvantage = photosynthesis can occur during day when stomata are closedEx: pineapple and cacti
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