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Photosynthesis

Photosynthesis

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Photosynthesis

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  1. Photosynthesis • Heterotrophs: depend on external C sources (e.g. animals) • Autotrophs: can survive on CO2 as sole C source (e.g. plants, etc) • Requires large E input • Chemoautotrophs: use E in inorganic chemical compounds (e.g. NH3, etc) • Photoautotrophs: use E in light E = hc /  • Shorter wavelength = higher energy photon • Evolution of modern metabolic pathways • Initial CO2 atmosphere 1. Heterotrophs (anaerobic) 2. Chemoautotrophs (anaerobic) 3. Photoautotrophs: CO2 + H2O --> CH2O + O2 • O2 atmosphere, aerobic metabolism evolved • Symbiotic bacterium --> --> --> modern mitochondrion • Symbiotic cyanobacterium --> --> --> modern chloroplast

  2. Photosynthesis • The photosynthetic redox reaction: 6H2O + 6CO2 --> C6H12O6 + 6O2 weak reducer + weak oxidizer --> strong reducer + strong oxidizer • E = hc /  @  = 680nm, E = -42 kcal/mol photons ( ~ 6 ATP)

  3. Photosynthesis • Chloroplast structure and function • Membranes • Outer: permeable to many things • Porins, large central pore • Inner: highly impermeable • Specific channels for certain molecules

  4. Chloroplast structure and function • Membranes • Thylakoid membrane system • Contained within the inner membrane system • Arranged in stacks: Grana • Enzymes for light capture are embedded within this membrane • Photosystem II (PSII) • Cytochrome b6f (like ETC Comp. III) (move protons) • Photosystem I (PSI) • ATP synthase

  5. Chloroplast structure and function • Enclosed spaces • Intermembrane space: between outer and inner membranes • Stroma: space enclosed by inner mem. • Contains the thylakoids • Contains the Calvin cycle enzymes for CO2 fixation into sugar • Contains DNA, ribosomes • Lumen: Space enclosed by thylakoids • Accumulates high [H+] for ATP synthesis by ATP synthase Stroma stroma lumen

  6. Photosynthesis • The photosynthetic reaction H2O + CO2 --> CH2O + O2 • For a long time, the O2 released was thought to come from CO2 (wrong) • Studies on sulfur bacteria showed: H2S + CO2 --> CH2O + 2S • So van Niel postulated a generic scheme: H2X + CO2 --> CH2O + 2X • And it was later shown that indeed the O2 comes from H2O 6H2O + 6CO2 --> C6H12O6 + 6O2 weak reducer + weak oxidizer --> strong reducer + strong oxidizer

  7. Light dependent reactions • Capture E of light into ATP and NADPH • Produce O2 from H2O • Light independent reactions • Use ATP and NADPH to capture and reduce CO2 into sugar • Plants also use aerobic respiration (mitochondria)

  8. Absorption of light • Photon is absorbed by a molecule • ‘pushes’ an electron from an inner (lower E) to an outer (higher E) orbital e- + photon --> e* (excited state) • # orbitals is finite and E levels are specific • Different molecules can only absorb photons of certain E (wavelength)

  9. Photosynthetic pigments • Chlorophyll • Beta-carotene • Conjugated systems • Alternating single and double bonds • Delocalized electron cloud • Can absorb more varied wavelengths • Strong absorbers of visible light

  10. Photosynthetic units • 100s of chlorophyll molecules • Noncovalent link to thylakoid membrane (Light Harvesting Complexes) • Group acts as an antenna for light • Photon is passed around • each pass reduces E (wavelength longer) • Only one is the reaction-center • P680, PSII • P700, PSI • Transfers e* to a carrier

  11. H2O + NADP+ --> 1/2O2 + NADPH + H+ Eo’ = 1.14V In cell, need ~ 2V Cell uses 2 photons, in 2 steps • Photosynthetic units • Photosystem II (PSII) • Boost e* halfway to NADP+ • Photosystem I (PSI) • Boost e* above NADP+

  12. Photosystem II • 20 subunits, embedded in thylakoid membrane • Associated with Light Harvesting Complex II (LHCII) • Antenna pigments (chlorophyll) + protein subunits • Light absorbed into D1/2 complex, e* transfer to Pheophytin P680* + Pheo --> P680+ + Pheo-(charge separation)

  13. P680* + Pheo --> P680+ + Pheo-(charge separation) • P680+= strong oxidizing agent (most powerful in biology) • Will accept e- from H2O and yield O2 in process (photolysis) • Pheo-= strong reducing agent • Will pass e- to Plastoquinone (PQ) --> PQH2

  14. Cytochrome b6f (structure-function similarity to Complex III) • Accepts 2e- from PQH2 • Translocates 4H+ per pair of e- • Transfers e- to Plastocyanin protein (PC) • PC carries e- to PSI

  15. Photosystem I (PSI) • LHCI • Contains light antenna • P700 rxn center P700* + A0 --> P700+ + A0- • P700+ receives e- from PC • A0- txfr e- to ferredoxin (Fd) • Fd donates e- to: NADP+ + H:- --> NADPH Fd NADP+ reductase (FNR)

  16. Light reactions summary 2H2O + 2NADP+ + 2H+ + 8photons --> O2 + 2NADPH Also, 18H+ difference generated across thylakoid membrane • Acidic inside lumen ATP synthase can generate ~ 5 ATP

  17. Noncyclic versus cyclic photophosphorylation • Noncyclic: passage of e- from H2O to NADP+ yielding H2O and NADPH plus, the proton gradient for ATP synthase • Cyclic: Fd passes e- to cytochrome b6f instead of Fd NADP+ reductase creates proton gradient, but no NADPH • ATP synthesis can be uncoupled from NADPH synthesis

  18. The light reactions: structures

  19. Light independent rxns: Calvin cycle • Key step: Ribulose bisphosphate carboxylase (RuBisCo) • 5C + CO2 --> 2x 3C (3-phosphoglycerate from glycolysis) • Plants that fix CO2 this way are called C3 plants because of the 3C intermediate 6CO2 + 18ATP + 12NADPH --> Fructose + 18ADP + 12NADP+ + 18Pi • Calvin cycle enzymes are in the stroma

  20. Light independent rxns: Calvin cycle 6CO2 + 18ATP + 12NADPH --> Fructose + 18ADP + 12NADP+ + 18Pi

  21. Light independent rxns: Calvin cycle • Many steps are actually light sensitive • Redox control: in absence of light, enzymes become inactivated • Transfer e- from Fd to Thioredoxin • Reduce disulfide bridges in proteins for light-dependent regulation of activity: R-S-S-R(inactive) + 2e- + 2H+ --> R-SH(active) + R-SH(active)

  22. Photorespiration • RuBisCo is at the mercy of the [CO2]/[O2] ratio • Rise in global [CO2] likely linked to increased crop yields • 270ppm(1870) --> 380ppm(now) • Only modest preference of enzyme for CO2 glycolate CO2 release

  23. Coordination of cellular organelles in photorespiration

  24. Hot dry climates are hard on C3 plants • Must shut stomata to prevent H2O loss during day • Also keeps CO2 out (and O2 builds up inside = photorespiration problem) • C4 plants use PEP carboxylase enzyme • PEP (3C) + CO2 --> OA (4C) PEP carboxylase works at much lower [CO2], open stomata less often!