Photosynthesis 2: The Calvin Cycle & Control

1. Photosynthesis 2: The Calvin Cycle & Control

2. Big Questions • Why is the Calvin Cycle necessary? • How do the products of the light reactions contribute to the function of the Calvin cycle? • Why have some plants had to adapt photosynthesis to the constraints of their environment?

3. Whoops! Wrong Calvin… The Calvin Cycle 1950s | 1961

4. + water + energy → glucose + oxygen light energy carbon dioxide 6CO2 6H2O 6O2 C6H12O6 + + → + Remember what it means to be a plant… • Need to produce all organic molecules necessary for growth • carbohydrates, lipids, proteins, nucleic acids • Need to store chemical energy (ATP) produced from light reactions • in a more stable form • that can be moved around plant • saved for a rainy day

5. →→ build stuff !! photosynthesis ATP Light reactions • Convert solar energy to chemical energy • ATP • NADPH • What can we do now? → energy → reducing power

6. carbon fixation CO2 reduces CO2 C6H12O6 How is that helpful? • Want to make C6H12O6 • synthesis • How? From what? What raw materials are available? NADPH NADP

7. From CO2→ C6H12O6 • CO2 has very little chemical energy • fully oxidized • C6H12O6contains a lot of chemical energy • highly reduced • Synthesis = endergonic process • put in a lot of energy • Reduction of CO2→C6H12O6proceeds in many small uphill steps • each catalyzed by a specific enzyme • using energy stored in ATP & NADPH

8. stroma ATP thylakoid From Light reactions to Calvin cycle • Calvin cycle • chloroplast stroma • Need products of light reactions to drive synthesis reactions • ATP • NADPH

9. Calvin cycle

10. 2 2 2 4 NAD+ ADP ADP Photosynthesis glycolysis 2 4 ATP ATP glucose C-C-C-C-C-C Remember G3P? fructose-1,6bP P-C-C-C-C-C-C-P glyceraldehyde3-phosphate G3P C-C-C-P DHAP P-C-C-C pyruvate C-C-C

11. To G3Pand beyond! To G3P and Beyond! • Glyceraldehyde-3-P • end product of Calvin cycle • energy rich 3 carbon sugar • “C3 photosynthesis” • G3P is an important intermediate • G3P→→ glucose →→carbohydrates →→ lipids →→phospholipids,fats, waxes →→ amino acids →→proteins →→ nucleic acids →→DNA, RNA

12. I’m green with envy! It’s not easy being green! RuBisCo • Enzyme which fixes carbon from air • ribulosebisphosphate carboxylase • the most important enzyme in the world! • it makes life out of air! • definitely the most abundant enzyme AP Biology

13. Accounting • The accounting is complicated • 3 turns of Calvin cycle = 1G3P • 3 CO2→1G3P (3C) • 6 turnsof Calvin cycle = 1C6H12O6(6C) • 6 CO2→1C6H12O6(6C) • 18 ATP+ 12 NADPH→1C6H12O6 • anyATPleft over from light reactions will be used elsewhere by the cell

14. C C H H H 6 NADPH H H C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C H C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C C | | | | | | 3 ATP 6 ATP CO2 6C 3C = = used to makeglucose 5C 1C 3C 3C RuBisCo 5C starch,sucrose,cellulose& more ribulose bisphosphate carboxylase – – 6 NADP 3 ADP 6 ADP Calvin cycle 1. Carbon fixation 3. Regenerationof RuBP RuBP ribulose bisphosphate glyceraldehyde-3-P PGA G3P phosphoglycerate 2. Reduction

15. ATP light energy NADPH H2O O2 + → + + ATP NADPH sunlight Light Reactions H2O • produces ATP • produces NADPH • releases O2 as a waste product Energy Building Reactions O2

16. CO2 C6H12O6 + + → + + ATP ATP NADPH ADP NADP ADP NADPH NADP Calvin Cycle • builds sugars • uses ATP & NADPH • recycles ADP & NADP • back to make more ATP & NADPH CO2 SugarBuilding Reactions sugars

17. light energy ATP ADP NADP NADPH CO2 H2O C6H12O6 O2 + + → + sunlight Putting it all together H2O CO2 • Plants make both: • energy • ATP & NADPH • sugars SugarBuilding Reactions Energy Building Reactions sugars O2

18. Energy cycle

19. light energy 6CO2 6H2O C6H12O6 6O2 + + → + Summary of photosynthesis • Where did the CO2 come from? • Where did the CO2 go? • Where did the H2O come from? • Where did the H2O go? • Where did the energy come from? • What’s the energy used for? • What will the C6H12O6be used for? • Where did the O2 come from? • Where will the O2 go? • What else is involved…not listed in this equation?

20. Variations on the Theme

21. Leaf anatomy

22. Remember The Needs of Plants! Plants need to take in: water (from soil) nutrients (from soil) CO2 (from atmosphere) Plants need to release: water vapor (through leaves) O2 (through leaves)

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

24. The best laidschemes ofmice and men…and plants! When stomates close… • Closed stomates lead to… • O2 build up → from light reactions • CO2 is depleted → in Calvin cycle • causes problems in Calvin Cycle

25. photosynthesis photorespiration Inefficiency of RuBisCo: CO2vs O2 • RuBisCo in Calvin cycle • carbon fixation enzyme • normally bonds CtoRuBP • CO2 is the optimal substrate • reduction of RuBP • building sugars • when O2 concentration is high • RuBisCo bonds OtoRuBP • O2 is a competitive substrate • oxidation of RuBP • breakdown sugars

26. NADPH RuBP ATP unstable intermediate PGA ATP CO2 3C 6C 3C RuBisCo 1C 3C 5C G3P to make glucose C3 plants 5C NADP ADP ADP G3P Calvin cycle when CO2 is abundant

27. RuBP O2 It’s so sad to see agood enzyme,go BAD! 3C 2C RuBisCo photorespiration to mitochondria ––––––– lost as CO2 withoutmaking ATP 5C Hey Dude, are you highon oxygen! Calvin cycle when O2 is high

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

29. Reducing photorespiration • Separate carbon fixation from Calvin cycle • C4 plants • PHYSICALLY separate carbon fixation from Calvin cycle • different cells fix carbon vs. where Calvin cycle occurs (different leaf structure) • PEP carboxylase • CAM plants • TEMPORALLY separate carbon fixation from Calvin cycle • fix carbon during night, Calvin cycle during day

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

31. PHYSICALLY separate C fixation from Calvin cycle Location,location,location! Comparative anatomy C3 C4 AP Biology

32. C4 Leaf Biochemistry Up Close:Photosynthesis across 2 different cells.

33. It’s all inthe timing! 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 carbonin 4C “storage” compounds • in day: release CO2 from 4C acids to Calvin cycle • increases concentration of CO2 in cells • succulents, some cacti, pineapple

34. CAM plants cacti succulents pineapple

35. CAM Plant Biochemistry:Photosynthesis at 2 times of day

36. 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 C4 vs CAM Summary solves CO2 / O2 gas exchangevs. H2O losschallenge

37. 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 • 21% O2 vs. 0.03% CO2 • 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

38. Supporting a biosphere • On global scale, photosynthesis is the most important process for the continuation of life on Earth • each year photosynthesis… • captures 121 billion tons of CO2 • synthesizes 160 billion tons of carbohydrate • heterotrophs are dependent on plants as food source for fuel & raw materials

39. Then all the plants, cats, dogs, elephants & people …are really particles of air woven together by strands of sunlight! air sun The poetic perspective… • All the solid material of every plantwas built by sunlight out of thin air • All the solid material of every animal was built from plant material

40. If plants can do it…You can learn it! Ask Questions!!

41. Review Questions

42. The final product of the Calvin Cycle is • Carbon dioxide • Fructose • Glucose • G3P • Oxygen

43. 2. Which of the following is true of the Calvin Cycle • It is controlled by enzymes in the stroma • It takes place in the thylakoid disks of the inner chloroplast membrane • Carbon dioxide is a product • It is an ATP-independent process • One cycle consumes four molecules of PGAL

44. If a toxin was administered to a plant that prevented the action of ribulosebisphosphate carboxylase, which of the following steps of the Calvin cycle would be most directly affected? • Regeneration of RUBP • Donation of phosphates from ATP to Calvin cycle intermediary compounds • The initial fixation of carbon dioxide • Oxidation of NADPH • Production of Glucose.

45. 0 4. In an experiment studying photosynthesis performed during the day, you provide a plant with radioactive carbon (14C) dioxide as a metabolic tracer. The 14C is incorporated first into oxaloacetic acid. The plant is best characterized as a • C4 plant. • C3 plant. • CAM plant. • heterotroph. • chemoautotroph.

46. The following questions refer to the following choices: • C3 plants • C4 plants C. CAM Plants • All plants • Use a temporal separation to reduce photorespiration • Do not have any adaptations to reduce photorespiration • Carry out carbon fixation by rubisco • Use a spatial separation to reduce photorespiration • Carry out aerobic cellular respiration

47. Keeping It Straight! Compare aerobic cellular respiration to photosynthesis (you will have 10 minutes). Write down as many similarities and differences as you can think of. The person with the most wins a prize!