Photosynthesis
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Photosynthesis The Producers Photosynthetic bacteria • Produce the food (photosynthesis) • Condition the environment • Create shelter Cyanobacteria — “blue-green algae” and habitat use chlorophyll Halophilic archaea “purple bacteria” use bacteriorhodopsin Algae — Aquatic Plants Photosynthetic protists • Three Divisions (Phyla) Phytoplankton — earth’s dominant producers! • diatoms Chlorophyta “Green Algae” Rhodophyta “Red Algae” Phaeophyta “Brown Algae” { including the “kelp” family } • Not directly related to each other, nor to terrestrial vascular plants. • Accessory pigments allow greater light sensitivity at depth. Historical perspectives Photosynthesis • 1600s – JB van Helmont: willow sapling grown in closed container with just watering for 5 years. Tree gained mass without loss of soil mass → gain mass from air. • 1770s – J Priestly: Discovered oxygen. Discovered that animals require oxygen. • Glucose for energy fuel, organic chemical Discovered that plants produce oxygen ∴ plants improve the air. monomers, structural polymers. • 1790s – J. Senebier & J Ingenhouz: “Carbon fixation” Plant mass “fixed” from CO in air. Dependent on light. • Oxygen for aerobic respiration. 2 • 1890s – TW Engelmann: Oxygen specifically produced by chloroplasts. Related to absorption of red & blue light. • – J von Sachs: Chlorophyll required. Chloroplasts also produce starch. (Derived the equation above) Heyer 1 Photosynthesis Leaf: photosynthetic organ of Historical perspectives • Many tissues terrestrial plants • Specialized structure for – Light-gathering surface – Gas exchange • 1925 – O Warburg: separate – Reduce desiccation “light” and “dark” reactions. – Vasculature • 1930s – R Hill: Oxygen comes from water; not CO2. • 1940s [advent of radioisotope studies] – 18 18 – SM Ruben, et al: H2 O → O2 in light-dependent reactions. 18 – M. Calvin: CO2 → sugar in light-independent (“dark”) reactions. • University of Aberdeen Learning Technology Unit — Photosynthesis – http://www.abdn.ac.uk/~clt011/flash/samples/photosyn.swf Plants Cells Chloroplast structure allows photosynthesis to work Lots of membrane and compartments Molecular Protein targeting to the thylakoids machines • ~95% of plastid proteins coded by nuclear DNA galore! • Cytosolic proteins targeted across plastid envelope to stroma leading signal sequence peptide – leading signal sequence peptide second signal sequence peptide of a newly synthesized polypeptide as it emerges from the ribosome – via protein-conducting channels, translocon (Toc & Tic) – signal peptide cleaved off in stroma • Target to thylakoid membrane or >335 different proteins associated with the thylakoid lumen by second signal peptide – via Tat – 89 in lumen • Stromal DNA-coded proteins may – 116 thylakoid membrane integral proteins also be targeted to thylakoid – 62 peripheral on stromal side – via signal recognition particles – 68 peripheral on lumenal side (SRP & SecA) • most involved in photosynthesis • Active transport • but many for targeting, folding, & processing other proteins – powered by ATP, GTP, or H+-gradient cotransport Heyer 2 Photosynthesis Photosynthesis: 2 main parts 1. Light reactions: . in grana “LIGHT- INDEPENDENT” . light energy e-, ATP REACTIONS 2. Light-independent reactions: . in stroma . e-, ATP sugar Photosynthesis: Photosystem 2 2 main parts Light excites e- in PS2 chlorophyll. 1. Light reactions: light energy e-, ATP • Photosystem 1 • Photosystem 2 Light reactions reflected chloroplasts absorb some light. absorbed transmitted Heyer 3 Photosynthesis Chlorophyll: location & structure Pigments absorb light -- thylakoid and power membrane photosynthesis. Energy absorbed by chlorophyll is re-emitted Light excites e- in PS2 chlorophyll. Energy is passed to reaction center chlorophyll. Light reactions Photosynthesis captures this energy Photosystems concentrate energy Photosystems concentrate energy Photons are absorbed by all the pigment molecules. Energy is passed to Reaction center the reaction center chlorophyll loses chlorophyll by electrons. resonance transfer. Heyer 4 Photosynthesis Photosystems concentrate energy Light excites e- in PS2 chlorophyll. Energy is passed to reaction center Photons are chlorophyll. absorbed by all the High-energy e- are passed to an chlorophylls. electron transport chain. Energy is H+ gradient used for ATP synthesis. passed to reaction center chlorophyll. Light reactions Light reactions: The light reactions noncyclic electron flow Light reactions: Light excites e- in PS2 chlorophyll. noncyclic electron flow Energy is passed to reaction center chlorophyll. High-energy e- are passed to an electron transport chain. H+ gradient used for ATP synthesis. PS1 excites e- again; e- passed to NADP. Light reactions Heyer 5 Photosynthesis Light reactions: Light reactions: noncyclic electron flow noncyclic electron flow Noncyclic e- flow makes ATP, NADPH The light reactions Cyclic e- flow makes extra ATP Photosynthesis: Fd: Ferredoxin 2 main parts 1. Light reactions: light energy e-, ATP 2. Light-independent reactions: e-, ATP sugar • Calvin cycle Heyer 6 Photosynthesis Calvin Cycle Calvin Cycle • Melvin Calvin, UC Berkeley, 1937–1980 • Nobel prize, Chemistry, 1961 • at Lawrence Berkeley Radiation Lab, used 14C-labeled compounds to map out complete photosynthetic carbon pathway Calvin Cycle Calvin Cycle Phase 1: Carbon fixation Net Phase 1: Carbon fixation Repeat Net Phase 2: Glycerate reduction }Reaction Phase 2: Glycerate reduction} six times }Reaction C5–2 P C5–2 P + CO2 + CO2 (RuBP) CO2 (RuBP) 6 CO2 + + [C6–2 P ] RuBP [C6–2 P ] 6 RuBP (intermediate) (intermediate) C – P C – P C – P C – P 3 3 2 ATP 2 ADP 3 3 12 ATP 12 ADP (3P-glycerate) (3P-glycerate) (3P-glycerate) (3P-glycerate) X6= ATP ADP ATP ADP 2 NADPH 2 NADP+ ATP ADP ATP ADP 12 NADPH 12 NADP+ 2 P 12 P NADPH NADP+ NADPH NADP+ NADPH NADP+ NADPH NADP+ P P P P C3– P C3– P C3– P C3– P (3P-glyceraldehyde) (3P-glyceraldehyde) (3P-glyceraldehyde) (3P-glyceraldehyde) 2 C3-sugar 12 C3-sugar [3PG] [3PG] [3PG] [3PG] [3PG] [3PG] Calvin Cycle Calvin Cycle Phase 1: Carbon fixation Repeat Net Phase 1: Carbon fixation Repeat Phase 2: Glycerate reduction} six times } Reaction Phase 2: Glycerate reduction} six times Phase 3: RuBP Phase 3: RuBP 6 C –2 P regeneration 5 6 CO regeneration (RuBP) 2 Overall Net + 6 CO2 Reaction 6 ATP 6 ADP 6 RuBP 6 [C5– P ] 18 ATP 18 ADP 4 P 12 ATP 12 ADP 12 NADPH 12 NADP+ 12 NADPH 12 NADP+ 12 P 12 P 10 [3PG] C6-sugar C6-sugar 2 [3PG] 12 C -sugar (glucose) 3 (glucose) [3PG] Heyer 7 Photosynthesis Photosynthesis: Calvin Cycle Summary Light rxns & “Light-independent” rxns Uses ATP, NADPH. Reduces CO2 to make G3P. Rubisco is the carbon-fixing enzyme. • (16% of chloroplast protein content) Calvin Cycle: not completely “light-independent” Comparing Calvin cycle does go faster in the light • Dependent upon ATP & NADPH production from light rxns Photosynthesis & • Light reactions ⇑ permeability of stromal membranes to cofactors (esp. Mg++) required for Calvin cycle enzymes Respiration • Ferredoxin oxidized by light reactions reduces thioredoxin. Reduced thioredoxin coenzyme for Calvin cycle enzymes Photosynthesis & Respiration Similarities: Electron transport Both use ETC chain & proton gradient ATP ATP synthase Both have redox cycles Both use electron carriers Heyer 8 Photosynthesis Photosynthesis & Plants must do both! Respiration Gas exchange in vascular plants Differences: • CO2 taken in and O2 given out by leaves for/from photosynthesis. - Source of energy, e • Dissolved O2 taken in with H2O from soil by roots for tissue Where e- go respiration. • During daylight: O2 out > O2 in Oxidize vs. reduce • In dark of night: O2 out < O2 in carbon Calvin Cycle — organic synthesis Stomata — “little mouths” — adjustable openings for gas exchange on the undersides of leaves Phase 1: Carbon fixation Repeat Net • Open: allow CO in & O out for/from photosynthesis. Phase 2: Glycerate reduction} six times } Reaction 2 2 • Closed: reduce water loss (transpiration). Phase 3: RuBP Proteins Lipids regeneration 6 CO2 Cellular + Respiration Fatty 6 RuBP Amino acids Sucrose acids (transport to other cells) ATP ADP Pyruvate Starch Glycerol NADPH NADP+ (storage) P C6-sugar 2 [3PG] 12 C -sugar (glucose or 3 fructose) [3PG] O2 bubbles forming from stomata next: Requires: Carbon high [CO ] CO fixation Photorespiration: 2 2 low [O2 ] catalyzed by Rubisco When a good P P Calvin P enzyme turns bad. Cycle P ATP Sugar NADPH “C3 plants” Heyer 9 Photosynthesis Rubisco: ribulose-bisphosphate carboxylase/oxygenase Photorespiration: • At typical conditions, carboxylase activity is much greater than oxygenase activity – Add C to Ru-bP • But at ↓CO2 /↑O2, oxygenase activity Stomata close when becomes greater than carboxylase activity – Remove C’s from Ru-bP leaf gets dehydrated to retain water. [O2] increases; [CO2] decreases. • Even under good conditions, ~20% of Ru-bP is oxygenated rather than carboxylated O 2 Waste Photorespiration P P P PCCC Photorespiration catalyzed by Rubisco if: • RuBP is oxidized (destroyed) low CO / high O rather than recycled 2 2 • CO2 is produced rather than consumed (fixed) Minimizing C4 Photosynthesis photorespiration: C4 plants have carbon fixation & Calvin cycle in different cells. • 1960’s — Australian sugarcane researchers trying to replicate Calvin’s experiments • Mostly in tropical grasses – Hardy weeds: crab grass, summer annuals – Important drought-resistant crops: corn, sugar cane, sorghum • Independent convergence — Developed independently 45 times! – >10,000