1. Photosynthesis Chapter 10

2. n Objectives F Compare the overall reaction of photosynthesis with the overall reaction for respiration F Describe where the processes of photosynthesis occur F 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. F Explain how pigments capture light and excite electrons F Describe the structural and functional differences between photosystem I and photosystem II, and cyclic and noncyclic photophosphorylation F Outline the steps in the cyclic fixation of carbon in the Calvin cycle and where these occur F Contrast the C 4 and CAM photosynthetic systems with the simpler C 3 system, and how they are adaptions to hot, dry climates

4. Introduction n Overall equation is reversal of cellular respiration u 6CO 2 +12H 2 O+energy--->C 6 H 12 O 6 +6O 2 +6H 2 O n Increasingly probing studies provided knowledge about how photosynthesis works u van Helmont-developed early ideas about where plants obtain materials for growth F showed that soil not sufficient F concluded that water important

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

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

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

8. Underlying Processes n Oxygen produced by splitting water u demonstrated using 18 O-labeled reactants F plant given C 18 O 2 does not release 18 O 2 F plant given H 2 18 O does give off 18 O 2 n Photosynthesis is redox process u H 2 O oxidized---> 1 / 2 O 2 +2H + +2e - u CO 2 reduced to glucose by addition of e - ’s and H + ’s u compare with respiration where glucose oxidized and O 2 reduced

9. n In photosynthesis, electrons travel “uphill” from water to glucose, adding light energy captured by chlorophyll n In respiration, electrons travel “downhill” from glucose to water, releasing energy to ATP

10. Overview n Photosynthesis is a two-stage process u light-dependant reactions F convert light energy to chemical energy, releases O 2 as waste product F occurs in thylakoid membranes and produces energy shuttles ATP and NADPH u Calvin cycle F cyclic series of steps that assemble organic molecules from CO 2

11. F occur in stroma and use energy and electrons from ATP and NADPH in carbon fixation F light not required but usually run during day as require shuttles from light-dependant reactions

12. The Light Reactions n Driven by visible light u light is electromagnetic radiation u only small fraction of em radiation perceived by organisms F different wavelengths=different colors

13. u leaf absorbs some wavelengths (red-orange and blue-violet) and reflects others (green)

14. u in plants light absorbed by chlorophyll a, chlorophyll b and carotenoids

15. n only chlorophyll a directly involved in light reactions; other pigments act as “antenna” molecules to broaden range of energy absorbed

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

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

18. n Two photosystems (antenna assembly+primary electron acceptor) identified u absorb at different wavelengths F photosystem I-absorbs maximally at 700nm (P700) F photosystem II-absorbs maximally at 680nm (P680) u function together to carryout non-cyclic electron transport F also known as non-cyclic photophosphorylation

19. u photosystem I can also carryout cyclic electron transport (cyclic photophosphorylation) F thought to be the earliest form of photosynthesis present in many primitive photosynthetic bacteria F synthesizes only ATP

20. Chemical Energy Generation n Electron transport chains generate ATP, NADPH and O 2 u kinetic energy of light absorbed and excites electrons u excited electrons passed along electron transport chain-series of redox reactions u released energy used to generate ATP, NADPH and O 2

21. u production of NADPH requires 2 electrons F supplied to PS I by PS II F replaced in PS II by splitting water F H 2 O ---> 1 / 2 O 2 + 2H + + 2e -

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

23. Carbon Fixation n ATP and NADPH from light-dependant reactions power Calvin cycle u net result of Calvin cycle is 3C molecules from CO 2 using energy and electrons in ATP and NADPH from light-dependant reactions u CO 2 added to 5C intermediate ribulose-1,5- bisphosphate (RuBP) F catalyzed by RuBP carboxylase/oxygenase (rubisco)

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

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

26. n Plants that use only Calvin cycle to fix carbon called C 3 plants u first identifiable product of carbon fixation is 3C molecule

27. Carbon-fixing Variations n C 3 plants conserve water by closing stomata u allows buildup of O 2 in leaves u Rubisco fixes O 2 rather than CO 2 u called photorespiration F uses ATP and NADPH but makes no sugars

28. u C 4 plants adapted to conserve water and prevent photorespiration F CO 2 incorporated into 4C molecule in mesophyll cells F diffuses into bundle sheath cells and released F enters Calvin cycle in bundle sheath chloroplasts

29. n CAM (crassulacean acid metabolism) plants incorporate carbon during night u stomata open at night, closed during day u CO 2 incorporated in 4C molecule and stored in vacuole at night u during day, 4C molecules exported into cytoplasm and CO 2 released u CO 2 enters Calvin cycle

30. n C 4 separate carbon incorporation and fixation spatially n CAM plants separate carbon incorporation and carbon fixation temporally