Photosynthesis Variations on the Theme AP Biology 2007

Photosynthesis: Variations on the Theme AP Biology 2007 -2008

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

Leaf _________ Structure ______ ________ O 2 H O 2 ____ Gas exchange AP Biology O 2 CO 2 ________

Controlling water loss from leaves § Hot or dry days ________________________ u § gain H 2 O = stomates open § lose H 2 O = stomates close u adaptation to living on land, but… creates PROBLEMS! AP Biology

When stomates close… § Closed stomates lead to… O 2 build up from light reactions u CO 2 is depleted in Calvin cycle u § causes problems in Calvin Cycle O 2 xylem (water) CO 2 AP Biology O 2 CO 2 phloem (sugars) H 2 O The best laid schemes of mice and men… and plants!

Inefficiency of Ru. Bis. Co: CO 2 vs O 2 § Ru. Bis. Co in Calvin cycle u _________________________ § § § § AP Biology normally bonds C to Ru. BP CO 2 is the optimal substrate ____________ photosynthesis ____________ Ru. Bis. Co bonds O to Ru. BP O 2 is a competitive substrate _______________________ photorespiration

Calvin cycle when CO 2 is abundant 1 C ATP Ru. BP ADP G 3 P to make glucose 5 C Ru. Bis. Co 6 C unstable intermediate 5 C G 3 P 3 C AP Biology CO 2 C 3 plants 3 C PGA NADPH ATP NADP 3 C

Calvin cycle when O 2 is high O 2 Ru. BP Hey Dude, are you high on oxygen! It’s so sad to see a good enzyme, go BAD! AP Biology 5 C Ru. Bis. Co to mitochondria ––––––– lost as CO 2 without making ATP 2 C 3 C photorespiration

Impact of Photorespiration § Oxidation of Ru. BP short circuit of Calvin cycle u loss of carbons to CO 2 u § can lose 50% of carbons fixed by Calvin cycle u reduces production of photosynthesis § no ATP (energy) produced § no C 6 H 12 O 6 (food) produced u AP Biology if photorespiration could be reduced, plant would become 50% more efficient § strong selection pressure to evolve alternative carbon fixation systems

Reducing photorespiration § Separate carbon fixation from Calvin cycle u __________ § _________________________ w different cells to fix carbon vs. where Calvin cycle occurs w store carbon in 4 C compounds § different enzyme to capture CO 2 (fix carbon) w _____________ § different leaf structure u __________ § _________________________ § fix carbon during night w store carbon in 4 C compounds § perform Calvin cycle during day AP Biology

C 4 plants § A better way to capture CO 2 u 1 st step before Calvin cycle, fix carbon with enzyme __________ corn § store as 4 C compound u ______________________ § have to close stomates a lot § different leaf anatomy u sugar cane, corn, other grasses… AP Biology sugar cane

PEP (3 C) + CO 2 oxaloacetate (4 C) O 2 C 4 leaf anatomy light reactions CO 2 PEP carboxylase C 3 anatomy stomate § _____________ u ________ CO 2 Ru. Bis. Co ______________ § better than Ru. Bis. Co u u ______________ regenerates CO 2 in inner cells for Ru. Bis. Co AP Biology § keeping O 2 away from Ru. Bis. Co C 4 anatomy

Comparative anatomy C 3 Location, location! C 4 PHYSICALLY separate C fixation from Calvin cycle AP Biology

CAM (Crassulacean Acid Metabolism) plants § ________________ u ______________________ § close stomates during day § open stomates during night ______: open stomates & fix carbon in 4 C “storage” compounds u ______: release CO 2 from 4 C acids to Calvin cycle u § increases concentration of CO 2 in cells u AP Biology succulents, some cacti, pineapple It’s all in the timing!

CAM plants cacti succulents AP Biology pineapple

C 4 vs CAM Summary solves CO 2 / O 2 gas exchange vs. H 2 O loss challenge ___________ 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

Why the C 3 problem? We’ve all got baggage! § Possibly evolutionary baggage u Rubisco evolved in high CO 2 atmosphere § there wasn’t strong selection against active site of Rubisco accepting both CO 2 & O 2 § Today it makes a difference u u u AP Biology 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

It’s not so easy as it looks… Any Questions? ? AP Biology 2007 -2008