Chapter 15 - Photosynthesis

1. Chapter 15 - Photosynthesis • Photosynthesis: a process that converts atmospheric CO2 and H2O to carbohydrates • Light reactions: • Solar energy is converted into chemical energy ATP and NADPH • Dark reactions • ATP and NADPH are used to convert CO2 to hexosephosphates Both processes can occur simultaneously Modern euk absolutely depend on O2 by photo (and as food source)

2. Chloroplasts: specialized organelles in algae and plants where photosynthesis occurs Thylakoid membrane: highly folded continuous membrane network, site of the light-dependent reactions that produce NADPH and ATP Lumen: aqueous space within the thylakoid membrane Stroma: aqueous matrix of the chloroplast which surrounds the thylakoid membrane Pump protons into lumen--produce ATP/NADPH in stroma for biosyn (dark rxns)

3. Photosynthesis in purple bacteria (a PSII system).

4. Photo system complex in membrane chlorophylls and accessory pigments Absorb light energy and pump H+ produce ATP/ NADPH

5. Chlorophyll and bacteriochlorophyll Absorb in red range (reflect green) Absorb light: excite/transfer e- • Chlorophylls - usually most abundant and most important pigments inlight harvesting • Contain tetrapyrrole ring (chlorin) similar to heme, but contains Mg2+ • Chlorophyllsa (Chl a) and b(Chl b) in plants • Bacteriochlorophyllsa (BChla) and b (BChlb) are major pigments in bacteria

6. Absorption maxima depends on structure and micro-environment within the pigment protein complex

7. Photo system I in bacteria Light gathering part of complex (proteins, pigments and cofactors) 96 chlorophylls and 22 accessory pigments (carotenoids) One special pair (P700)

8. Reaction centers of the photosystems • PSI and PSII each contain a reaction center (site of the photochemical reaction) • Special pair: two chlorophylls in each reaction center that are energized by light (2 e- lost) • In PSI special pair is: P700 (absorb light maximally at 700nm) • In PSII the special pair is: P680 (absorb light maximally at 680nm) • (Notice PSII in purple bacteria it is P870) A reaction center Remaining chlo act as: antenna molecules Capture light energy and transfer to special pair Resonance energy transfer Special pair: only two chlo mol that give up e- to begin e- transfer chain

9. Different states of Chlorophyll (special pair) Special pair--identified as pigments that absorb at specific wavelength Two special chlo mol that actually give up e- e- from low-energy level promoted to higher-energy molecular orbital Excited state reduced oxidized

10. Photo system I in bacteria 96 chlorophylls and 22 accessory pigments (carotenoids) One special pair (P700)

11. Accessory pigments Absorb light/transfer energy to adjacent chl Conjugated double bonds allow light absorption Absorb in blue: appear red/yellow Fall colors Broaden the range of system Stabilize adjacent chl Prevent loss of e-

12. Evolution of photosystems PSII Bacteria: simple system Photosystem II: purple bacteria Photosystem I: green sulfur bacteria Cyanobacteria: more complex (coupled II/I) Most abundant class of photosyn bacteria Plants and algae: coupled II/I

13. Photosynthesis in purple bacteria (a PSII system). (Antenna pigment mol not shown) Light energy excites P870 release of e- e- transferred one at a time only down the right branch Tightly bound quinone transfer via Fe mol to Loosely bound Q on left Diffuse in membrane Two photons of light: 2 e- transferred 2H+ from cyto to QH2

14. Photosynthesis in purple bacteria (a PSII system). 2 photons: 4 H+ released Generate ATP Cyclic process No outside source of e- needed

15. Photosynthesis in green sulfur bacteria (PS I)

16. Photosynthesis in green sulfur bacteria (PS I) Pigment centers are chlorophylls Both branches are active Three Fe-S clusters Terminal e- acceptor: ferredoxin Primarily Leads to NADPH formation Non cyclic e- transfer process

17. Photosynthesis in green sulfur bacteria (PS I) Usually non-cyclic NADPH formation Cyto c reduced by sulfer compounds Allows reduction of P700+ cyclic If need ATP Can pass e- to quinone

18. Evolution of photosystems Bacteria: simple system Photosystem II: purple bacteria Photosystem I: green sulfur bacteria Cyanobacteria: more complex (coupled) Generated O2 environment Most abundant class of photosyn bacteria poison Plants and algae: coupled II/I

19. Photosynthesis in cyanobacteria (PS I - PSII) Allow light energy to form ATP and NADPH Mobile quinone: plastoquinone (PQ) Oxygen evolving complex splits water And reduces P680+ Transfer e- to Cbf complex (Most important biochem event in history of life) Terminal e- acceptor (plastocyanin) passes on to PSI feulle = leaf (french)

20. Cyclic electron flow pathway H+ (Not to NADP+ but back to the PQ pool via a specialized cytochrome.) • For each CO2 reduced to (CH2O) in carbohydrate synthesis (dark cycle), 2 NADPH and 3 ATP are required • Cyclicelectrontransport yields ATP but not NADPH, thus balancing the need for 3 ATP for every 2 NADPH, • (Cyclic flow increases the protonmotive force without NADPH synthesis.)

21. The Z-scheme • Z-scheme:path of electron flow and reduction potentials of the components in photosynthesis • Absorption of light energy converts P680 and P700 (poor reducing agents) toexcited molecules (good reducing agents) • Light energy drives the electron flow uphill • NADP+ is ultimately reduced to NADPH

22. Cyanobacteria internal structure Chloroplasts Evolved into

23. Thylakoid membrane: site of the light-dependent reactions Lumen: aqueous space within the thylakoid membrane Stroma: aqueous matrix of the chloroplast which surrounds the thylakoid membrane Pump protons into lumen (from stroma)-- produce ATP/NADPH in stroma for biosyn

24. photosynthesis plant membrane systems Interior Inner/ Outer memb • Light is captured by antenna complexes • Light energy drives the transport of electrons from PSII through cytochrome bf complex to PSI and ferridoxin and then to NADPH • The proton gradient generated is used to drive ATP production • For 2 H2O oxidized to O2, 2 NADP+ are reduced to 2 NADPH

25. The Dark Reactions • Reductive conversion of CO2 into carbohydrates • Process is powered by ATP and NADPH (formed during the light reactions of photosynthesis) Occurs in light (inhibited in dark): need ATP/NADPH Produces Starch (provide energy during the night) and sucrose (mobile form of carbo) The CO2 fixation pathway has several names: The Calvin cycle. CO2 enters the plant through pores on the leaf surface called stomata,

26. The Calvin Cycle 3 stages: (1) Carboxylation :catalyzed by Rubisco (2) Reduction: 3-phosphoglycerate converted to glyceraldehyde 3-phosphate (G3P) (3) Regeneration: most of the G3P is converted to ribulose 1,5-bisphosphate Glc 3CO2 + 9ATP + 6NADPH + 5H2O 9ADP + 8 Pi + 6NADP+ + G3P ribulose 1,5-bisphosphate

27. main product is G3P Glc 3CO2 + 9ATP + 6NADPH + 5H2O 9ADP + 8 Pi + 6NADP+ + G3P ribulose 1,5-bisphosphate 3CO2 needed before one C3 unit (G3P) can be removed without diminishing metabolic pools

28. The Calvin Cycle. Pentose phosphate pathway 2 1 3

29. The Calvin Cycle Sucrose Starch Gluconeogenesis Carboxylation Reduction 3ADP 3ATP Pentose Phosphate Regeneration Cycle

30. Carboxylation: Ribulose 1,5-Bisphosphate Carboxylase-Oxygenase (Rubisco) • Gaseous CO2 and the 5-carbon sugar ribulose 1,5-bisphosphate form two molecules of 3-phosphoglycerate • Reaction is metabolically irreversible • Rubisco makes up about 50% of the soluble protein in plant leaves, and is one of the most abundant enzymes in nature Not very efficient so a lot needed

31. The Rubisco Reaction

32. Regulation of the Rubisco Reaction • Rubisco cycles between an active form (in the light) and an inactive form (in the dark) • Activation requires light, CO2, Mg2+ and correct stromal pH (H+ gradient) • At night 2-carboxyarabinitol 1-phosphate (synthesized in plants) inhibits Rubisco Why shut down Rubisco at night? Need photosyn for ATP/NADPH Need to turn off calvin cycle Prevent inefficient accumulation of 3PG and oxygenation rxn

33. Oxygenation of Ribulose 1,5-Bisphosphate CO2 3PG ATP and NADPH Photorespiration Calvin C (3PG) O2 and CO2 compete for same active site Limits crop yields No CO2 fixation (O2 utilization). consumes ATP and NADPH without hexose production. But…..no Rubisco mutants found without this process (i.e. with only CO2 fixation and no oxygenation).

34. Oxygenation of Ribulose 1,5-Bisphosphate CO2 3PG ATP and NADPH Photorespiration Calvin C (3PG) Normally carboxylation 3-4 fold higher than oxygenation rate High temp and light conditions: inc oxygenation rate and also lose water Other plants get around problem: increase local CO2 concentration by using a secondary pathway to fix carbon

35. The C4 Pathway Avoid photorespiration loss Rubisco No Rubisco (A CO2 shuttle, present in corn, sorghum, sugar cane, many weeds) Bundle sheath cells are impermeable to gases (both CO2 and O2). Concentrates CO2 for Rubisco Allow growth at high temp/light High temp favors oxygenation of Rubisco

37. Crassulacean acid metabolism (CAM) Occurs in succulent plants. H2O can be lost during CO2 fixation due to open stomata, succulent plants reduce this by fixing CO2 at night (when it is cooler) and in the day the stomata remain closed. Malate is stored in large vacuole and released during the day decarboxylated and CO2 utilized Closed stomata in day: keeps H2O in O2 out and allows high CO2 buildup Temporally separation : PEP carboxylase inhibited in day (malate) No competition between PEP carb and Rubisco for CO2