Photosynthesis Chapter 10 Part 2

1. PhotosynthesisChapter 10Part 2

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

3. The Light Reactions • Leaf absorbs some wavelengths (red-orange and blue-violet) and reflects others (green) • In plants, light absorbed by chlorophyll a, chlorophyll b and carotenoids

4. The Light Reactions • Chlorophyll a main photosynthetic pigment • Chlorophyll b + other accessory pigments act as “antenna” molecules to broaden range of energy absorbed • Carotenoids absorb excessive light that would damage chlorophyll

5. The Photosystems • Light behaves like particles (photons) • When pigment absorbs photon, energy level of one electron is raised to excited, unstable state • In chloroplasts, 200-300 chlorophyll molecules grouped with proteins to form antenna assembly around two chlorophyll amolecules • A photosystem consists of a reaction-center complex (a type of protein complex) surrounded by light-harvesting complexes • The light-harvesting complexes (pigment molecules bound to proteins) transfer the energy of photons to the reaction center

6. The Photosystems • 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 Photosystem STROMA Photon Light-harvestingcomplexes Reaction-centercomplex Primaryelectronacceptor Thylakoid membrane e Pigmentmolecules Special pair ofchlorophyll amolecules Transferof energy THYLAKOID SPACE(INTERIOR OF THYLAKOID) (a) How a photosystem harvests light

7. The Photosystems • Two photosystems (antenna assembly + primary electron acceptor) identified • Absorb at different wavelengths • photosystem II-absorbs maximally at 680nm (P680) • photosystem I-absorbs maximally at 700nm (P700) • Function together to carryout linear electron flow • Produces ATP and NADPH using light energy • Photosystem I can also carryout cyclic electron flow • Thought to be the earliest form of photosynthesis • present in many primitive photosynthetic bacteria • Synthesizes only ATP

8. The Photosystems

9. 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 • released energy used to generate ATP, NADPH and O2 • 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-

10. 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

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

14. Number of rearrangements occur, using energy in ATP and oxidation of NADPH • Last step in cycle regenerates RuBP • Carbon enters the cycle as CO2 and leaves as a sugar named glyceraldehyde 3-phospate (G3P) • 3C molecules exported to cytoplasm • used to synthesize glucose and other organic molecules For net synthesis of 1 G3P, the cycle must take place three times, fixing 3 molecules of CO2

15. Alternative Mechanisms • Dehydration is a problem for plants • Water conservation involves trade-offs with other metabolic processes, especially photosynthesis • On hot, dry days, plants close stomata, which conserves H2O but also limits photosynthesis • The closing of stomata reduces access to CO2 and causes O2 to build up • These conditions favor photorespiration

16. C3plants • Plants that use only Calvin cycle to fix carbon called C3 plants • first identifiable product of carbon fixation is 3C molecule • Photorespiration • C3plants conserve water by closing stomata • Allows buildup of O2 in leaves • Rubisco fixes O2 rather than CO2 • Uses ATP and NADPH but makes no sugars • Photorespiration limits damaging products of light reactions that build up in the absence of the Calvin cycle

17. C4 plants • C4 plants adapted to conserve water and prevent photorespiration • CO2 incorporated into 4C molecule in mesophyll cells • diffuses into bundle sheath cells, releases CO2 • Enters Calvin cycle in bundle sheath chloroplasts

18. CAM plants • CAM (crassulacean acid metabolism) plants incorporate carbon during night • Common in succulent plants • Stomata open at night, closed during day • CO2 incorporated in 4C molecule (organic acid) and stored in vacuole at night • During day, 4C molecules exported into cytoplasm and CO2 released • CO2 enters Calvin cycle

19. CAM plants separate carbon incorporation and carbon fixation temporally C4 separate carbon incorporation and fixation spatially

20. Summary of Photosynthesis • The energy entering chloroplasts as sunlight gets stored as chemical energy in organic compounds • Sugar made in the chloroplasts supplies chemical energy and carbon skeletons to synthesize the organic molecules of cells • Plants store excess sugar as starch in structures such as roots, tubers, seeds, and fruits • In addition to food production, photosynthesis produces the O2 in our atmosphere