Photosynthesis

1. Photosynthesis Chapter 10

2. Objectives • Compare the overall reaction of photosynthesis with the overall reaction for respiration • Describe where the processes of photosynthesis occur • Describe the basic processes involved in photosynthesis: water splitting to obtain electrons, redox reactions of the electron transport chains, electron and energy shuttling by means of ATP and NADPH, and the coupling of the light-dependent reactions and the Calvin cycle

3. Explain how pigments capture light and excite electrons • Describe the structural and functional differences between photosystem I and photosystem II, and cyclic and noncyclic photophosphorylation • Outline the steps in the cyclic fixation of carbon in the Calvin cycle and where these occur • Contrast the C4 and CAM photosynthetic systems with the simpler C3 system, and how they are adaptions to hot, dry climates

4. Photosynthesis • The process that converts solar energy into chemical energy • Plants and other autotrophs are the producers for the biosphere • Autotroph means self –feeding • Means they make their own food without eating, decomposing, or absorbing other organisms or organic molecules

5. Introduction • Overall equation is reversal of cellular respiration • 6CO2+12H2O+energy--->C6H12O6+6O2+6H2O • Increasingly probing studies provided knowledge about how photosynthesis works • van Helmont-developed early ideas about where plants obtain materials for growth • showed that soil not sufficient • concluded that water important

6. Priestly-showed that plants restore “bad” air • Ingenhousz-plants only restore air when exposed to light

7. Autotrophs are Producers • Autotroph-means self-feeding • applies to any organism that makes own food without eating, decomposing or absorbing other organisms or organic molecules • Photosynthetic autotrophs include plants, algae and photosynthetic bacteria

8. Site of Photosynthesis • Photosynthesis occurs in chloroplasts in all photosynthetic organisms except monerans • Leaves (specifically, mesophyll cells) are primary site of photosynthesis • Light-absorbing pigment is chlorophyll • located in protein complexes in internal membranes of chloroplasts • Sugars assembled in stroma

11. Yea Stable Isotopes!Underlying Processes • Oxygen produced by splitting water • demonstrated using 18O-labeled reactants • plant given C18O2 does not release 18O2 • plant given H218O does give off 18O2

13. Photosynthesis is redox process • H2O oxidized--->1/2O2+2H++2e- • CO2 reduced to glucose by addition of e-’s and H+’s • compare with respiration where glucose oxidized and O2 reduced

14. In photosynthesis, electrons travel “uphill” from water to glucose, adding light energy captured by chlorophyll • REMEMBER: In respiration, electrons travel “downhill” from glucose to water, releasing energy to ATP

15. Overview • Photosynthesis is a two-stage process • light-dependant reactions • convert light energy to chemical energy, releases O2 as waste product • occurs in thylakoid membranes and produces energy shuttles ATP and NADPH • Calvin cycle • cyclic series of steps that assemble organic molecules from CO2

17. The Light Reactions • Driven by visible light • light is electromagnetic radiation • only small fraction of em radiation perceived by organisms • different wavelengths=different colors

18. leaf absorbs some wavelengths (red-orange and blue-violet) and reflects others (green) • in plants light captured and absorbed by either chlorophyll a, chlorophyll b and carotenoids

19. Light is absorbed by chlorophyll a, b and carotenoids • only chlorophyll a directly involved in light reactions; other pigments act as “antenna” molecules to broaden range of energy absorbed

21. In the photossynthetic light reaction where does oxygen come from???? • Carbon dioxide • Surrounding air • water

22. The Photosystems • Light behaves like particles-photons • When pigment absorbs photon, energy level of one electron is raised to excited, unstable state • if pigment is isolated from molecular environment, excited electron loses energy as heat or light and returns to normal level • chlorophyll fluoresces red

24. In chloroplasts, 200-300 chlorophyll molecules grouped with proteins to form antenna assembly around two chlorophyll a molecules-reaction center chlorophylls • excited electrons passed from antenna chlorophylls to reaction center chlorophylls then to primary electron acceptor • series of redox reactions • final is oxidation of reaction center chlorophyll and reduction of primary electron acceptor

26. Two photosystems (antenna assembly+primary electron acceptor) identified • absorb at different wavelengths • photosystem I-absorbs maximally at 700nm (P700) • photosystem II-absorbs maximally at 680nm (P680) • function together to carryout non-cyclic electron transport

28. Chemical Energy Generation • Electron transport chains generate ATP, NADPH and O2 • kinetic energy of light absorbed and excites electrons • excited electrons passed along electron transport chain-series of redox reactions • released energy used to generate ATP, NADPH and O2

29. production of NADPH requires 2 electrons • supplied to PS I by PS II • replaced in PS II by splitting water • H2O ---> 1/2O2 + 2H+ + 2e-

30. Chemiosmosis • Powers ATP synthesis • H+ ions from splitting water and those pumped across thylakoid membrane by electron transport chain form gradient across thylakoid membrane (inside to outside) • ATP synthase provides port for H+ to diffuse back into stroma • releases energy and phosphorylates ADP to ATP • similar process to ATP generation in mitochondria • known as photophosphorylation

33. Carbon Fixation • ATP and NADPH from light-dependant reactions power Calvin cycle • net result of Calvin cycle is 3C molecules from CO2 using energy and electrons in ATP and NADPH from light-dependant reactions • CO2 added to 5C intermediate ribulose-1,5-bisphosphate (RuBP) • catalyzed by RuBP carboxylase/oxygenase (rubisco)

35. Number of rearrangements occur in many steps, using energy in ATP and oxidation of NADPH • last step in cycle regenerates RuBP • all steps occur simultaneously but ultimately regenerate starting reactants, hence cycle

36. Three RuBP enter cycle for each 3C molecule released from chloroplast • Calvin cycle occurs in chloroplast stroma • 3C molecules exported to cytoplasm • used to synthesize glucose and other organic molecules

37. Plants that use only Calvin cycle to fix carbon called C3 plants • first identifiable product of carbon fixation is 3C molecule

38. Is oxygen released in the Calvin cycle?

39. Carbon-fixing Variations • C3 plants conserve water by closing stomata • allows buildup of O2 in leaves • Rubisco fixes O2 rather than CO2 • called photorespiration • uses ATP and NADPH but makes no sugars

40. C4 plants adapted to conserve water and prevent photorespiration • CO2 incorporated into 4C molecule in mesophyll cells • diffuses into bundle sheath cells and released • enters Calvin cycle in bundle sheath chloroplasts • Rubisco not used

41. CAM (crassulacean acid metabolism) plants incorporate carbon during night • stomata open at night, closed during day • CO2 incorporated in 4C molecule and stored in vacuole at night • during day, 4C molecules exported into cytoplasm and CO2 released • CO2 enters Calvin cycle • Rubisco not used

42. C4 separate carbon incorporation and fixation spatially • CAM plants separate carbon incorporation and carbon fixation temporally

44. Checklist • What is photosynthesis? • What does autotroph mean? • What is the primary site of photosynthesis? • How is the oxygen released by photosynthesis produced? • What are the 2 stages of photosynthesis? What are the reactants and products of each? • How do light reactions work? • What are the 3 phases of the Calvin cycle? • What is the difference between C3, C4 and CAM plants