Chapter 10 Photosynthesis: Life from Light

1. Chapter 10Photosynthesis:LifefromLight

2. Energy needs of life • All life needs a constant input of energy • Heterotrophs • get their energy from “eating others” • consumers of other organisms • consume organic molecules • Autotrophs • get their energy from “self” • get their energy from sunlight • use light energy to synthesize organic molecules

3. + water + energy  glucose + oxygen carbon dioxide glucose + oxygen  carbon + water + energy  C6H12O6 + 6O2 6CO2 + 6H2O + ATP dioxide light energy  6CO2 + 6H2O + + 6O2 C6H12O6 How are they connected? Heterotrophs making energy & organic molecules from ingesting organic molecules Autotrophs making energy & organic molecules from light energy

4. sun CO2 H2O glucose O2 ATP Energy cycle Photosynthesis Cellular Respiration

5. What does it mean to be a plant • Need to… • collect light energy • transform it into chemical energy • store light energy • in a stable form to be moved around the plant & also saved for a rainy day • need to get building block atoms from the environment • C,H,O,N,P,S • produce all organic molecules needed for growth • carbohydrates, proteins, lipids, nucleic acids

6. Plant structure • Obtaining raw materials • sunlight • leaves = solar collectors • CO2 • stomates = gas exchange • H2O • uptake from roots • nutrients • uptake from roots

8. H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ Plant structure • Chloroplasts • double membrane • stroma • thylakoid sacs • grana stacks • Chlorophyll & ETC in thylakoid membrane • H+ gradient built up within thylakoid sac

9. Pigments of photosynthesis • chlorophyll & accessory pigments • “photosystem” • embedded in thylakoid membrane • structure  function Why does this structure make sense?

10. A Look at Light • The spectrum of color

11. Light: absorption spectra • Photosynthesis performs work only with absorbed wavelengths of light • chlorophyll a — the dominant pigment — absorbs best in red & blue wavelengths & least in green • other pigments with different structures have different absorption spectra

12. Chloroplasts • Chloroplasts are green because they absorb light wavelengths in red &blue and reflect green back out structure  function

13. Photosystems • Photosystems • collections of chlorophyll molecules • 2 photosystems in thylakoid membrane • act as light-gathering “antenna complex” • Photosystem II • chlorophyll a • P680 = absorbs 680nm wavelength red light • Photosystem I • chlorophyll b • P700 = absorbs 700nm wavelength red light

14. Photosynthesis overview • Light reactions • convert solar energy to chemical energy • ATP • Calvin cycle • uses chemical energy (NADPH & ATP) to reduce CO2 to buildC6H12O6 (sugars)

15. Light reactions • Similar to ETC in cellular respiration • membrane-bound proteins in organelle • electron acceptors • NADPH • proton (H+) gradient across inner membrane • Where’s the double membrane? • ATP synthase enzyme

16. The ATP that Jack built photosynthesis respiration sunlight breakdown of C6H12O6 • moves the electrons • runs the pump • pumps the protons • forms the gradient • releases the free energy • allows the Pi to attach to ADP • forms the ATP • … that evolution built

17. ETC of Respiration • Mitochondria transfer chemical energy from food molecules into chemical energy of ATP • use electron carrier NADH generate H2O

18. Chloroplasts transform light energy into chemical energy of ATP • use electron carrier NADPH ETC of Photosynthesis split H2O

19. ETC of Photosynthesis

20. ETC of Photosynthesis

21. ETC of Photosynthesis

22. ETC of Photosynthesis • ETC produces from light energy • ATP & NADPH • NADPH (stored energy)goes to Calvin cycle • PS II absorbs light • excited electron passes from chlorophyll to “primary electron acceptor” • need to replace electron in chlorophyll • enzyme extracts electrons from H2O & supplies them to chlorophyll • splits H2O • O combines with another O to form O2 • O2released to atmosphere • and we breathe easier!

23. Experiment 1 Experiment 2 light energy light energy   6CO2 6CO2 + + 6H2O 6H2O + + + + 6O2 6O2 C6H12O6 C6H12O6 Experimental evidence • Where did the O2 come from? • radioactive tracer = O18 Proved O2 came from H2O not CO2 = plants split H2O

24. 2 Photosystems • Light reactions elevate electrons in 2 steps (PS II & PS I) • PS II generates energy as ATP • PS I generates reducing power as NADPH

25. Cyclic photophosphorylation • If PS I can’t pass electron to NADP, it cycles back to PS II & makes more ATP, but noNADPH • coordinates light reactions to Calvin cycle • Calvin cycle uses more ATP than NADPH

26. Photophosphorylation cyclic photophosphorylation noncyclic photophosphorylation

27. Photosynthesis summary Where did the energy come from? Where did the H2O come from? Where did the electrons come from? Where did the O2 come from? Where did the H+ come from? Where did the ATP come from? Where did the O2 go? What will the ATP be used for? What will the NADPH be used for? …stay tuned for the Calvin cycle

28. Chapter 10.Photosynthesis: The Calvin Cycle LifefromAir

29. 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 • in stable form • can be moved around plant • saved for a rainy day

30. + water + energy  glucose + oxygen carbon dioxide light energy  6CO2 + 6H2O + + 6O2 C6H12O6 Autotrophs • Making energy & organic molecules from light energy • photosynthesis

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

32. From CO2 C6H12O6 • CO2 has very little chemical energy • fully oxidized • C6H12O6contains a lot of chemical energy • reduced • endergonic • Reduction of CO2C6H12O6proceeds in many small uphill steps • each catalyzed byspecific enzyme • using energy stored in ATP & NADPH

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

34. 1C 3C CO2 RuBP 5C 6C unstable intermediate 3 ATP PGAL to make glucose 3 ADP 3C PGA PGAL 2x x2 3C 6 ATP 6 NADPH 2x 6 NADP 6 ADP Calvin cycle (don’t count the carbons!) ribulose bisphosphate 1. Carbon fixation 3. Regeneration Rubisco ribulose bisphosphatecarboxylase sucrose cellulose etc. 2. Reduction

35. Calvin cycle • PGAL • end product of Calvin cycle • energy rich sugar • 3 carbon compound • “C3 photosynthesis” • PGAL  important intermediate PGAL   glucose   carbohydrates   lipids   amino acids   nucleic acids

36. PGA PGA PGA RuBP RuBP RuBP PGAL PGAL

37. Rubisco • Enzyme which fixescarbon from atmosphere • ribulose bisphosphate carboxylase • the most important enzyme in the world! • it makes life out of air! • definitely the most abundant enzyme

38. Accounting • The accounting is complicated • 3 turns of Calvin cycle = 1 PGAL • 3 CO21 PGAL (3C) • 6 turnsof Calvin cycle = 1 C6H12O6(6C) • 6 CO21 C6H12O6(6C) • 18 ATP + 12 NADPH  1 C6H12O6 • 6 ATP = left over from light reactions for cell to use elsewhere

39. Photosynthesis summary • Light reactions • produced ATP • produced NADPH • consumed H2O • produced O2 as byproduct • Calvin cycle • consumed CO2 • produced PGAL • regenerated ADP • regenerated NADP

40. light energy  6CO2 + 6H2O + + 6O2 C6H12O6 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 that is not listed in this equation?

41. Chapter 10.Photosynthesis:Variations on the Theme

42. Remember what plants need… • Photosynthesis • light reactions • Calvin cycle • light sun • H2O ground • CO2 air What structures have plants evolved to supply these needs?

43. CO2 O2 O2 CO2 H2O A second look at stomates… • Gas exchange • CO2 in for Calvin cycle • O2 out from light reactions • H2O out for light reactions photosynthesis xylem (water) phloem (sugars) gas exchangewater loss

44. 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!

45. Closed stomates • closed stomates lead to… • O2 builds up (from light reactions) • CO2 is depleted (in Calvin cycle) • causes problems in Calvin Cycle

46. Inefficiency of Rubisco: CO2 vs O2 • Rubisco in Calvin cycle • carbon fixation enzyme • normally bonds C to RuBP • reduction ofRuBP • building sugars • when O2 concentration is high • Rubisco bonds O to RuBP • O2 is alternative substrate • oxidation of RuBP • breakdown sugars photosynthesis photorespiration

47. 1C 3C CO2 RuBP 5C 6C unstable intermediate ATP PGAL to make glucose ADP 3C ATP PGA PGAL 2x ADP x2 3C NADPH 2x NADP Calvin cycle review Rubisco

48. O2 RuBP 5C 3C 2C to mitochondria ----------- lost as CO2 without making ATP Calvin cycle with O2 Rubisco photorespiration

49. 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 • decreases photosynthetic output by siphoning off carbons • no ATP (energy) produced • no C6H12O6 (food) produced • if photorespiration could be reduced, plant would become 50% more efficient • strong selection pressure

50. Reducing photorespiration • Separate carbon fixation from Calvin cycle • C4 plants • physically separate carbon fixation from Calvin cycle • different enzyme to capture CO2 • PEP carboxylase stores carbon in 4C compounds • different leaf structure • CAM plants • separate carbon fixation from Calvin cycle by time of day • fix carbon (capture CO2) during night • store carbon in organic acids • perform Calvin cycle during day