Transcript CO 2
Photosynthesis: Variations on the Theme AP Biology Remember what plants need… Photosynthesis light reactions light sun H2O ground Calvin cycle CO2 air What structures have plants evolved to APsupply Biology these needs? O C O vascular bundle xylem (water) Leaf Structure phloem (sugar) cuticle epidermis palisades layer spongy layer O2 H O 2 CO2 stomate Transpiration Gas exchange AP Biology O2 H 2O CO2 guard cell Controlling water loss from leaves Hot or dry days stomates close to conserve water guard cells excess H2O = stomates open conserve H2O = stomates close adaptation to living on land, but… creates PROBLEMS! AP Biology When stomata close… Closed stomata lead to… O2 build up inside the plant from light reactions CO2 is depleted /can’t enter for the Calvin cycle causes problems in Calvin Cycle xylem (water) phloem (sugars) AP Biology O2 CO2 H2O RuBisCo Carbon Fixation enzyme in the Calvin Cycle Fixes Carbon Dioxide onto RuBP • When the concentration of oxygen is high/ CO2 is low, RuBisCo will fix Oxygen onto RuBP in a process called photorespiration. •The products of fixed O2 are not particularly healthy for the plant, and most assuredly don’t lead to the formation of glucose, like the fixation of CO2 does. AP Biology Calvin cycle when CO2 is abundant 1C RuBP ATP CO2 5C RuBisCo ADP 6C unstable intermediate 5C NADPH NADP 3C C3 plants ATP Leave to make glucose AP Biology 3C PGA 2 PGAL 3C ADP Calvin cycle when O2 is high O2 RuBP 5C RuBisCo 2C 3C 1 PGA photorespiration AP Biology to mitochondria ––––––– without making ATP Impact of Photorespiration Oxidation of RuBP short circuit of Calvin cycle reduces production of photosynthesis no C6H12O6 (food) produced if photorespiration could be reduced, plant would become 50% more efficient pressure to evolve alternative carbon fixation systems AP Biology Reducing photorespiration Separate carbon fixation from Calvin cycle C4 plants PHYSICALLY separate carbon fixation from Calvin cycle different cells to fix carbon vs. where Calvin cycle occurs store carbon in 4C compounds different enzyme to capture CO2 (fix carbon) PEP carboxylase CAM plants separate carbon fixation from Calvin cycle by TIME OF DAY fix carbon during night store carbon in 4C compounds perform Calvin cycle during day AP Biology C4 plants are so named because instead of forming PGA’s in the beginning of the Calvin cycle, they form a four-carbon compound OAA (Oxaloacetate) Several thousand species use the C4 pathway. AMP Instead of being fixed by rubisco, CO2 combines with a 3Carbon molecule PEP to form OAA, using the fixing enzyme PEP carboxylase. • OAA is then converted to malate, and that is shuttled to the bundle sheath cells. • Here, malate is converted to pyruvate and CO2. • The pyruvate moves back to the mesophyll cells where one ATP is broken down to form AMP (not ADP) which is required to convert the pyruvate back to PEP (to help continue the cycle) • The overall effect of this process is to move CO2 from mesophyll cells to the bundle sheath cells, in order to make photosynthesis more efficient. AP Biology C4 plants A better way to capture CO2 1st step before Calvin cycle, fix carbon with enzyme PEP carboxylase corn store as 4C compound adaptation to hot, dry climates have to close stomates a lot sugar cane, corn, other grasses… AP Biology sugar cane PEP (3C) + CO2 oxaloacetate (4C) C4 leaf anatomy O2 light reactions CO2 PEP carboxylase C3 anatomy stomate PEP carboxylase enzyme bundle sheath cell CO2 RuBisCo higher attraction for CO2 than O2 better than RuBisCo fixes CO2 in 4C compounds regenerates CO2 in inner cells for RuBisCo AP Biology keeping O2 away from RuBisCo C4 anatomy AP Biology CAM (Crassulacean Acid Metabolism) plants Adaptation to hot, dry climates separate carbon fixation from Calvin cycle by TIME close stomates during day open stomates during night at night: open stomates & fix carbon in 4C “storage” compounds in day: release CO2 from 4C acids into normal Calvin cycle increases concentration of CO2 in cells AP Biology succulents, some cacti, pineapple CAM plants cacti succulents AP Biology pineapple AP Biology AP Biology C4 vs CAM Summary solves CO2 / O2 gas exchange vs. H2O loss challenge C4 plants CAM plants 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 Characteristics of Photosynthesis in C3, C4 and CAM plants Characteristic C3 Plant C4 Plant CAM Plant Photorespiration Yes Little none Rubisco present Yes Yes Yes PEP Carboxylase present No Yes Yes into OAA via PEP carboxylase, then to malic acid which moves from mesophyll cell to bundle sheath cell and then releases CO2. into OAA via PEP carboxylase, then to malic acid which moves into vacuole (during night). CO2 released during the day. Initial CO2 fixation directly into Calvin Cycle via Rubisco Secondary CO2 fixation ---------------- In bundle sheath cell using Rubisco In “mesophyll”* cell using Rubisco – in morning mesophyll cells bundle sheath cells Mesophyll cells Site of Calvin cycle AP Biology