Objective: I can explain how C3, C4, and CAM pathways differ for photosynthesis

Objective: I can explain how C3, C4, and CAM pathways differ for photosynthesis Agenda: Bell Ringer Notes over C4 and CAM plants

Photosynthesis:Variations on the Theme

Remember what plants need… • Photosynthesis • light reactions • _____ • H2O • Calvin cycle • ____  sun  ground air

xylem (water) phloem (sugar) cuticle epidermis palisades layer spongy layer stomate guardcell vascular bundle Leaf Structure O2 H2O CO2 O2 H2O CO2 Transpiration

O2 CO2 O2 CO2 H2O A second inside a leaf… • Gas exchange & water flow • _______ • O2 out  • H2O out  for Calvin cycle waste from light reactions _________________ photosynthesis xylem (water) phloem (sugars) gas exchangewater loss

Controlling water loss from leaves • Hot or dry days • ____________________________ • guard cells • gain H2O = stomates open • lose H2O = stomates close • adaptation to living on land, but… creates PROBLEMS!

Closed stomates • Closed stomates lead to… • O2 builds up  from light reactions • CO2 is depleted  in Calvin cycle • ______________________________ The best laidschemes ofmice and men…and plants!

Inefficiency of Rubisco: CO2 vs O2 • Rubisco in Calvin cycle • carbon fixation enzyme • normally bonds CtoRuBP • reduction ofRuBP • __________________ • when O2 concentration is high • Rubisco bonds OtoRuBP • O2 is alternative substrate • oxidation of RuBP • ________________________ photosynthesis photorespiration

1C 3C CO2 RuBP 5C 6C unstable intermediate ATP ADP 3C ATP PGA ADP 3C NADPH NADP Calvin cycle review Rubisco G3P to make glucose G3P

1C 3C CO2 RuBP 5C G3P to make glucose 3C PGA G3P Calvin cycle review Rubisco C3 plants

O2 RuBP 5C 3C 2C to mitochondria ––––––– lost as CO2 without making ATP Calvin cycle with O2 Rubisco A good enzyme,gone BAD! photorespiration

Impact of Photorespiration • Oxidation of RuBP • __________________________ • loss of carbons to CO2 • can lose 50% of carbons fixed by Calvin cycle • reduces production of photosynthesis • no C6H12O6 (food) produced • if photorespiration could be reduced, plant would become 50% more efficient • strong selection pressure to evolve alternative systems

Reducing photorespiration • Separate carbon fixation from Calvin cycle • C4 plants • physically separate carbon fixation from actual Calvin cycle • _______________________________ • PEP carboxylase stores carbon in 4C compounds • different leaf structure • _________________________ • separate carbon fixation from actual Calvin cycle by time of day • fix carbon (capture CO2) during night • store carbon in organic acids • ______________________________

C4 plants • A better way to capture CO2 • 1st step before Calvin cycle, fix carbon with enzymePEP carboxylase • store as 4C compound • ______________________________ • have to close stomates a lot • different leaf anatomy • sugar cane, corn, other grasses…

C4 Plants corn sugar cane

O2 PEP carboxylase light reactions • PEP carboxylase enzyme • higher affinity for CO2 than O2 (better than Rubisco) • _______________________ • regenerates CO2in inner cells for Rubisco phosphoenolpyruvate (3C) + CO2 oxaloacetate (4C)

Location,location, location! Comparative anatomy • Separate reactions in different cells • light reactions • _________________ • Calvin cycle C3 C4

CO2 O2 O2 CO2 C4 photosynthesis Physically separated C fixation from Calvin cycle • Outer cells • light reaction & carbon fixation • pumps CO2 to inner cells • keeps O2 away from inner cells • away from Rubisco • Inner cells • Calvin cycle • glucose to veins

CAM (Crassulacean Acid Metabolism) plants • Different adaptation to hot, dry climates • separate carbon fixation from Calvin cycle by time • close stomates during day • _______________________ • at night, open stomates & fix carbon in “storage” compounds • organic acids: malic acid, isocitric acid • in day, close stomates & release CO2 from “storage” compounds to Calvin cycle • _________________________________ • succulents, some cacti, pineapple It’s all inthe timing!

CAM plants

C4 vs CAM Summary solves CO2 / O2 gas exchangevs. H2O losschallenge C4 plants separate 2 steps of C fixation anatomically in 2 different cells CAM plants separate 2 steps of C fixation temporally at2 different times

We’ve all gotbaggage! Why the C3 problem? • Possibly evolutionary baggage • Rubisco evolved in high CO2 atmosphere • there wasn’t strong selection against active site of Rubisco accepting both CO2 & O2 • Today it makes a difference • _____________________________ • 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

Objective: I can explain how C3, C4, and CAM pathways differ for photosynthesis