AP Biology Remember what plants need… Photosynthesis light reactions light ← sun H2O ← ground Photosynthesis: Variations on the Theme AP Biology 2007-2008 __________________ ___________ __________ O Calvin cycle What structures have plants evolved to APsupply Biology these needs? Controlling water loss from leaves Hot or dry days ________ ________ ________________________ ________________________ gain H2O = stomates open O2 H O 2 ________ Gas exchange AP Biology Gas exchange C CO2 ← air Leaf Structure ________ ________ O O2 lose H2O = stomates close CO2 adaptation to living on land, but… creates PROBLEMS! ________ ________ AP Biology 1 AP Biology When stomates close… Closed stomates lead to… Inefficiency of RuBisCo: CO2 vs O2 RuBisCo in Calvin cycle O2 build up → from light reactions CO2 is depleted → in Calvin cycle normally bonds C to RuBP CO2 is the optimal substrate causes problems in Calvin Cycle O2 xylem (water) _________________________ The best laid schemes of mice and men… and plants! _______________________ _______________________ photosynthesis _________________________ RuBisCo bonds O to RuBP CO2 AP Biology O2 phloem (sugars) CO2 O2 is a competitive substrate _______________________ _______________________ photorespiration AP Biology H2O Calvin cycle when CO2 is abundant 1C ATP RuBP CO2 5C RuBisCo ADP G3P to make glucose AP Biology O2 C3 plants 3C PGA NADPH ATP NADP ADP 3C RuBP unstable 6C intermediate 5C G3P 3C Calvin cycle when O2 is high Hey Dude, are you high on oxygen! It’s so sad to see a good enzyme, go BAD! 5C RuBisCo to mitochondria ––––––– lost as CO2 without making ATP 2C 3C photorespiration AP Biology 2 AP Biology Impact of Photorespiration Oxidation of RuBP Reducing photorespiration Separate carbon fixation from Calvin cycle short circuit of Calvin cycle loss of carbons to CO2 __________________________________________________ different cells to fix carbon vs. where Calvin cycle occurs store carbon in 4C compounds different diff t enzyme to t capture t CO2 (fix (fi carbon) b ) __________________________ different leaf structure can lose 50% of carbons fixed by Calvin cycle reduces production of photosynthesis no ATP (energy) produced no C6H12O6 (food) produced ___________________ __________________________________________________ if photorespiration could be reduced, plant would become 50% more efficient fix carbon during night store carbon in 4C compounds perform Calvin cycle during day strong selection pressure to evolve alternative carbon fixation systems AP Biology ___________________ AP Biology PEP (3C) + CO2 → oxaloacetate (4C) C4 plants A better way to capture CO2 corn ______________________ ______________________ have to close stomates a lot different leaf anatomy C4 leaf anatomy light reactions CO2 1st step before Calvin cycle, fix carbon with enzyme ___________________ store t as 4C compound d O2 sugar cane, corn, other grasses… AP Biology PEP carboxylase C3 anatomy t _________________________ ___________________________ ___________________________ regenerates CO2 in inner cells for RuBisCo ________ ________ ________ CO2 RuBisCo better than RuBisCo sugar cane stomate AP Biology keeping O2 away from RuBisCo C4 anatomy 3 AP Biology Comparative anatomy CAM (Crassulacean Acid Metabolism) plants Location, location,location! _______________________________ C3 C4 ____________________________________________ close stomates during day open stomates during night ______: open stomates & fix carbon in 4C “storage” compounds ______: release CO2 from 4C acids to Calvin cycle It’s all in the timing! increases concentration of CO2 in cells PHYSICALLY separate C fixation from Calvin cycle AP Biology CAM plants succulents, some cacti, pineapple AP Biology C4 vs CAM Summary solves CO2 / O2 gas exchange vs. H2O loss challenge cacti succulents AP Biology pineapple ___________ ___________ separate 2 steps of C fixation anatomically in 2 different cells separate 2 steps of C fixation temporally = 2 different times night vs. day AP Biology 4 AP Biology Why the C3 problem? We’ve all got baggage! Possibly evolutionary baggage It’s not so easy as it looks… Rubisco evolved in high CO2 atmosphere Any Questions?? there wasn’t strong selection against active site of Rubisco accepting both CO2 & O2 Today it makes a difference 21% O2 vs. 0.03% CO2 photorespiration can drain away 50% of carbon fixed by Calvin cycle on a hot, dry day strong selection pressure to evolve better way to fix carbon & minimize photorespiration AP Biology AP Biology 5
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