PHOTOSYNTHESIS Variations on a Theme

WHAT DO PLANTS NEED? Photosynthesis Light reactions Light H 2 O Calvin cycle CO 2 O O C  sun  ground  air What structures have plants evolved to supply these needs?

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

CONTROLLING WATER LOSS Hot or dry days Stomates close to conserve water Guard cells Gain H 2 O = stomates open Loss H 2 O = stomates close Adaptation to living on land, but … there are problems

WHEN STOMATES CLOSE … Closed stomates lead to … O 2 build up from light reactions CO 2 depletion in Calvin cycle Causes problems in Calvin cycle xylem (water) phloem (sugars) H2OH2O O2O2 CO 2 Houston, we have a problem … -

THE PROBLEM WITH RUBISCO: CO 2 VS O 2 RuBisCo in the Calvin cycle Carbon fixation molecule Normally bonds C to RuBP CO 2 is the optimal substrate Reduction of RuBP Builds sugars When O 2 concentration is high RuBisCo bonds O to RuBP O 2 is a competitive substrate Oxidation of RuBP Breakdown of sugars photosynthesis photorespiration

CALVIN CYCLE WHEN CO 2 IS ABUNDANT 6C unstable intermediate 1C CO 2 5C RuBP 3C PGA ADP ATP 3C NADP NADPH ADP ATP G3P to make glucose 3C G3P 5C RuBisCo C3 plants

CALVIN CYCLE WHEN O 2 IS HIGH 5C RuBP 3C 2C to mitochondria ––––––– lost as CO 2 without making ATP photorespiration O2O2 RuBisCo Hey Dude, are you high on oxygen! It’s so sad to see a good enzyme, go BAD!

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 produced ADP is not recycled No C 6 H 12 O 6 produced If photorespiration could be reduced, plant would become 50% more efficient Strong selective pressure to evolve alternative carbon fixation systems

REDUCING PHOTORESPIRATION Separate carbon fixation from Calvin cycle C4 plants PHYSICALLY separate carbon fixation from Calvin cycle Use different cells to fix carbon vs where the Calvin cycle occurs Store carbon in a C4 compound Different enzyme captures CO 2 (fix carbon) PEP carboxylase Different leaf structure CAM plants Separate carbon fixation from Calvin cycle by TIME OF DAY Fix carbon during the night Store carbon in a C4 compound Perform Calvin cycle during the day PEP = phosphoenolpyruvate

C4 PLANTS A better way to capture CO2 1 st 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

C4 LEAF ANATOMY PEP (3C) + CO 2  oxaloacetate (4C) CO 2 bundle sheath cell RuBisCo PEP carboxylase stomate C3 anatomy O 2 light reactions C4 anatomy PEP carboxylase enzyme Higher attraction for CO 2 than O 2 Better than RuBisCo Fixes CO 2 into 4C compounds Regenerates CO 2 in inner cells for RuBisCo Keeping O 2 away from RuBisCo

COMPARATIVE ANATOMY C3C4 Location, location,location! PHYSICALLY separate C fixation from Calvin cycle

CAM (CRASSULACEAN ACID METABOLISM) PLANTS Adaptation to hot, dry climates Separate carbon fixation from Calvin cycle by TIME OF DAY 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 to Calvin cycle Increases concentration of CO 2 in cells Succulents, some cacti, pineapple It’s all in the timing!

CAM PLANTS succulents cacti pineapple

C4 VS CAM SUMMARY Solves CO 2 /O 2 gas exchange vs H 2 O loss challenge 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

WHY THE C3 PROBLEM? Possibly evolutionary baggage 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 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 selective pressure to evolve a better way to fix carbon & minimize photorespiration We’ve all got baggage!

It’s not easy being green. Any questions?