AP Biology Photosynthesis: Variations on the Theme

AP Biology Remember what plants need…  Photosynthesis  light reactions  light H2OH2O  Calvin cycle  CO 2 What structures have plants evolved to supply these needs?  sun  ground  air O O C

AP Biology Leaf Structure H2OH2O CO 2 O2O2 H2OH2O phloem (sugar) xylem (water) stomate guard cell palisades layer spongy layer cuticle epidermis O2O2 CO 2 Transpiration vascular bundle Gas exchange

AP Biology Controlling water loss from leaves  Hot or dry days  stomates close to conserve water  guard cells  excess H 2 O = stomates open  conserve H 2 O = stomates close  adaptation to living on land, but… creates PROBLEMS!

AP Biology When stomata close… xylem (water) phloem (sugars) H2OH2O O2O2 CO 2  Closed stomata lead to…  O 2 build up inside the plant  from light reactions  CO 2 is depleted /can’t enter  for the Calvin cycle  causes problems in Calvin Cycle

AP Biology 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 O 2 are not particularly healthy for the plant, and most assuredly don’t lead to the formation of glucose, like the fixation of CO 2 does.

AP Biology 6C unstable intermediate 1C CO 2 Calvin cycle when CO 2 is abundant 5C RuBP 3C PGA ADP ATP 3C NADP NADPH ADP ATP Leave to make glucose 3C 2 PGAL 5C RuBisCo C3 plants

AP Biology Calvin cycle when O 2 is high 5C RuBP 3C 2C to mitochondria ––––––– without making ATP photorespiration O2O2 RuBisCo 1 PGA

AP Biology Impact of Photorespiration  Oxidation of RuBP  short circuit of Calvin cycle  reduces production of photosynthesis  no C 6 H 12 O 6 (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 CO 2 (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 C 4 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 C 4 pathway. Instead of being fixed by rubisco, CO 2 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 CO 2. 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 CO 2 from mesophyll cells to the bundle sheath cells, in order to make photosynthesis more efficient. AMP

AP Biology C4 plants  A better way to capture CO 2  1st step before Calvin cycle, fix carbon with enzyme PEP carboxylase  store as 4C compound  adaptation to hot, dry climates  have to close stomates a lot  sugar cane, corn, other grasses… sugar cane corn

AP Biology C4 leaf anatomy PEP (3C) + CO 2  oxaloacetate (4C) CO 2 O 2 light reactions C4 anatomy C3 anatomy  PEP carboxylase enzyme  higher attraction for CO 2 than O 2  better than RuBisCo  fixes CO 2 in 4C compounds  regenerates CO 2 in inner cells for RuBisCo  keeping O 2 away from RuBisCo bundle sheath cell RuBisCo PEP carboxylase stomate

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 CO 2 from 4C acids into normal Calvin cycle  increases concentration of CO 2 in cells  succulents, some cacti, pineapple

AP Biology CAM plants succulents cacti pineapple

AP Biology

C4 vs CAM Summary C4 plants separate 2 steps of C fixation anatomically in 2 different cells CAM plants separate 2 steps of C fixation temporally = 2 different times night vs. day solves CO 2 / O 2 gas exchange vs. H 2 O loss challenge

AP Biology Characteristics of Photosynthesis in C 3, C 4 and CAM plants CharacteristicC 3 PlantC 4 PlantCAM Plant PhotorespirationYesLittlenone Rubisco presentYes PEP Carboxylase presentNoYes Initial CO 2 fixation directly into Calvin Cycle via Rubisco into OAA via PEP carboxylase, then to malic acid which moves from mesophyll cell to bundle sheath cell and then releases CO 2. into OAA via PEP carboxylase, then to malic acid which moves into vacuole (during night). CO 2 released during the day. Secondary CO 2 fixation In bundle sheath cell using Rubisco In “mesophyll”* cell using Rubisco – in morning Site of Calvin cyclemesophyll cellsbundle sheath cellsMesophyll cells