AP Biology Photosynthesis: Variations on the Theme

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

AP Biology Review: C3 Photosynthesis During regular photosynthesis, CO 2 is trapped into a 3- carbon compound by Rubisco C3 Photosynthesis This 3 carbon compound then goes through the calvin cycle to produce glucose (eventually)

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

AP Biology Controlling water loss from leaves Hot or dry days stomata close to conserve water guard cells Plenty of H 2 O = stomata open low H 2 O = stomata close adaptation to living on land, but… this creates PROBLEMS!

AP Biology When stomata close… xylem (water) phloem (sugars) H2OH2O O2O2 CO 2 O2O2 Closed stomata lead to… Build up of O 2 from light reactions Less CO 2 in Calvin cycle causes problems in Calvin Cycle

AP Biology Inefficiency of RuBisCo: CO 2 vs O 2 RuBisCo in Calvin cycle carbon fixation enzyme normally bonds C to RuBP CO 2 is the optimal substrate reduction of RuBP building sugars when O 2 concentration is high RuBisCo bonds O to RuBP O 2 is a competitive substrate oxidation of RuBP breakdown sugars photosynthesis photorespiration

AP Biology 6C unstable intermediate 1C CO 2 Calvin cycle when CO 2 is abundant 5C RuBP 3C ADP ATP 3C NADP NADPH ADP ATP T3P to make glucose 3C G3P 5C RuBisCo C3 plants

AP Biology Calvin cycle when O 2 is high 5C RuBP 3C 2C ––––––– lost as CO 2 without making ATP photorespiration O2O2 RuBisCo Its so sad to see a good enzyme, go BAD!

AP Biology Competing Reactions Rubisco can react with CO 2 (Carboxylase Reaction) – good for glucose output

AP Biology Competing Reactions Rubisco can also react with O 2 (Oxygenase Reaction) not good for glucose output Even though CO 2 is eventually regenerated, it wastes time and energy (occupies the active sites on Rubisco)

AP Biology Photorespiration When Rubisco reacts with O 2 instead of CO 2 Occurs under the following conditions: High O 2 concentrations High heat Photorespiration is estimated to reduce photosynthetic efficiency by up to 50%

AP Biology Why high heat? When it is hot, plants close their stomata to conserve water They continue to do photosynthesis use up CO 2 and produce O 2 creates high O 2 concentrations inside the plant photorespiration occurs

AP Biology Impact of Photorespiration Oxidation of RuBP short circuit of Calvin cycle loss of carbons to CO 2 can lose 50% of carbons fixed by Calvin cycle reduces production of photosynthesis no ATP (energy) produced no C 6 H 12 O 6 (food) produced if photorespiration could be reduced, plant would become 50% more efficient strong selection 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 different leaf structure 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 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 different leaf anatomy sugar cane, corn, other grasses… sugar cane corn

AP Biology Comparison Light intensity is directly related to temperature C4 Plants (CAM and C4 Pathway) are able to do more photosynthesis at high temperatures

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 stomata

AP Biology Comparative anatomy C3C4 PHYSICALLY separate C fixation from Calvin cycle

AP Biology CAM ( Crassulacean Acid Metabolism ) plants Adaptation to hot, dry climates separate carbon fixation from Calvin cycle by TIME close stomata during day open stomata during night at night: open stomata & fix carbon in 4C storage compounds in day: release CO 2 from 4C acids to 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 Why the C3 problem? Possibly evolutionary baggage Rubisco evolved in high CO 2 atmosphere there wasnt strong selection against active site of Rubisco accepting both CO 2 & O 2 Today it makes a difference 21% O 2 vs. 0.03% CO 2 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 A second look inside a leaf… Gas exchange & water flow CO 2 in O 2 out H 2 O out photosynthesis gas exchange water loss xylem (water) O2O2 CO 2 for Calvin cycle waste from light reactions for light reactions phloem (sugars) H2OH2O O2O2 CO 2

AP Biology C4 photosynthesis CO 2 O2O2 O2O2 Outer cells light reaction & carbon fixation pumps CO 2 to inner cells keeps O 2 away from inner cells away from RuBisCo Inner cells Calvin cycle glucose to veins PHYSICALLY separated C fixation from Calvin cycle