SPhotosynthesis

SPhotosynthesis - Photosynthesis The Producers...

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Unformatted text preview: Photosynthesis The Producers Photosynthesis Ecological communities are often named for the dominant producers • Produce the food (photosynthesis) • Condition the environment • Create shelter and habitat Photosynthetic bacteria Cyanobacteria — “blue-green algae ” use chlorophyll Halophilic archaea “purple bacteria ” use bacteriorhodopsin • Not all producers are plants Photosynthetic protists Phytoplankton — earth ’s dominant producers! • diatoms Algae — Aquatic Plants • Three Divisions (Phyla) 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. Heyer 1 Photosynthesis Leaf: photosynthetic organ of Photosynthesis • Many tissues • Specialized structure for – Light-gathering surface – Gas exchange – Reduce desiccation – Vasculature terrestrial plants • Glucose for energy fuel, organic chemical monomers, structural polymers. • Oxygen for aerobic respiration. Plants Cells Chloroplast structure allows photosynthesis to work Lots of membrane and compartments Terrestrial Plant Cells Photosynthesis: 2 main parts v 1. Light reactions: ß in grana ß light energy Ë e-, ATP v 2. Light-independent reactions: ß in stroma ß e-, ATP Ë sugar Heyer 2 Photosynthesis Photosynthesis: 2 main parts v 1. Light reactions: light energy Ë e-, ATP • Photosystem 1 • Photosystem 2 reflected Photosystem 2 v Light excites e- in PS2 chlorophyll. absorbed Light reactions chloroplasts absorb some light. transmitted Pigments absorb light -- and power photosynthesis. Heyer 3 Photosynthesis Chlorophyll: location & structure Energy absorbed by chlorophyll is re-emitted thylakoid membrane Photosynthesis captures this energy v Light excites e- in PS2 chlorophyll. v Energy is passed to reaction center chlorophyll. Light reactions Photosystems concentrate energy w Reaction center chlorophyll loses electrons. Heyer Photosystems concentrate energy w Photons are absorbed by all the pigment molecules. w Energy is passed to the reaction center chlorophyll by resonance transfer. Photosystems concentrate energy v Photons are absorbed by all the chlorophylls. v Energy is passed to reaction center chlorophyll. 4 Photosynthesis v Light excites e- in PS2 chlorophyll. v Energy is passed to reaction center chlorophyll. Light reactions: noncyclic electron flow v High-energy e- are passed to an electron transport chain. Light reactions v Light excites e- in PS2 chlorophyll. v Energy is passed to reaction center chlorophyll. Light reactions: noncyclic electron flow v High-energy e- are passed to an electron transport chain. v H+ gradient used for ATP synthesis. Light reactions The light reactions Heyer Light reactions: noncyclic electron flow 5 Photosynthesis v Light excites e- in PS2 chlorophyll. v Energy is passed to reaction center chlorophyll. Light reactions: noncyclic electron flow v High-energy e- are passed to an electron transport chain. v H+ gradient used for ATP synthesis. v PS1 excites e- again; e- passed to NADP. Light reactions Noncyclic e- flow makes ATP, NADPH The light reactions Cyclic e- flow makes extra ATP Photosynthesis: 2 main parts v 1. Light reactions: light energy Ë e-, ATP v 2. Light-independent reactions: e-, ATP Ë 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 C5–2 P + (RuBP) Calvin Cycle Repeat } six times Net } Reaction Phase 1: Carbon fixation Phase 2: Glycerate reduction CO 2 Phase 1: Carbon fixation Phase 2: Glycerate reduction C5–2 P CO2 + RuBP [C6–2 P ] C 3– P ATP NADPH ADP NADP+ P C 3– P C 3– P 2 ATP (3P-glycerate) ATP 2 NADPH ADP NADPH C 3– P 6 ATP 6 ADP 2 NADP+ 2 C3-sugar [3PG] Calvin Cycle Repeat } six times (RuBP) 2 ADP 2P NADP+ P Phase 1: Carbon fixation Phase 2: Glycerate reduction Phase 3: RuBP 6 C 5–2 P regeneration Net } Reaction 6 CO2 + 6 RuBP 6 [C5– P] 4P 10 [3PG] Heyer 2 [3PG] C 3– P (3P-glycerate) ATP NADPH ADP NADP+ P C 3– P C 3– P (3P-glycerate) ATP NADPH 12 ATP X6= 12 ATP 12 ADP 12 NADP+ 12 ADP 12 NADPH ADP 12 NADP+ 12 P NADP+ P C 3– P (3P-glyceraldehyde) (3P-glyceraldehyde) [3PG] [3PG] 12 C3-sugar [3PG] Calvin Cycle Repeat } six times Phase 1: Carbon fixation Phase 2: Glycerate reduction Phase 3: RuBP regeneration Overall Net Reaction 6 CO2 18 ATP 12 NADPH (glucose ) 6 CO2 + 6 RuBP (intermediate) (3P-glyceraldehyde) (3P-glyceraldehyde) [3PG] [3PG] C6-sugar CO 2 [C6–2 P ] (intermediate) (3P-glycerate) + (RuBP) Net } Reaction 12 NADPH 18 ADP 12 NADP+ 12 P 12 P 12 C3-sugar [3PG] C6-sugar (glucose) 7 Photosynthesis Calvin Cycle Summary w Uses ATP, NADPH. Photosynthesis: Light rxns & “Light-independent” rxns w Reduces CO2 to make G3P. w Rubisco is the carbon-fixing enzyme. • (16% of chloroplast protein content) Calvin Cycle: not completely “light-independent” Calvin cycle does go faster in the light • Dependent upon ATP & NADPH production from light rxns • Light reactions › permeability of stromal membranes to cofactors (esp. Mg ++ ) required for Calvin cycle enzymes • Ferredoxin oxidized by light reactions reduces thioredoxin. Reduced thioredoxin coenzyme for Calvin cycle enzymes Comparing Photosynthesis & Respiration Photosynthesis & Respiration Electron transport chain & ATP synthase Similarities: v Both use ETC Ë proton gradient Ë ATP v Both have redox cycles v Both use electron carriers Heyer 8 Photosynthesis Photosynthesis & Respiration Plants must do both! Gas exchange in vascular plants • CO2 taken in and O2 given out by leaves for/from photosynthesis. • Dissolved O2 taken in with H2O from soil by roots for tissue respiration. Differences: v Source of energy, ev Where e- go • During daylight: O2 out > O2 in • In dark of night: O2 out < O2 in v Oxidize vs. reduce carbon Calvin Cycle — organic synthesis Phase 1: Carbon fixation Repeat Net } Reaction Phase 2: Glycerate reduction} six times Phase 3: RuBP regeneration Proteins Cellular Respiration Amino acids Sucrose (transport to other cells) Lipids Fatty acids C6-sugar Pyruvate Glycerol • Open: allow CO 2 in & O2 out for/from photosynthesis. • Closed: reduce water loss (transpiration). ADP NADPH NADP + P 2 [3PG] (glucose ) — adjustable openings for gas exchange on the undersides of leaves 6 CO2 + 6 RuBP ATP Starch (storage) Stomata — “little mouths” 12 C3-sugar [3PG] next: Photorespiration: When a good enzyme turns bad. O2 bubbles forming from stomata Requires: high [CO2] low [O2 ] P P Calvin Cycle Sugar Heyer Carbon fixation catalyzed by Rubisco CO2 P P ATP NADPH “C3 plants” 9 Photosynthesis Rubisco: ribulose-bisphosphate carboxylase/oxygenase • At typical conditions, carboxylase activity is much greater than oxygenase activity – Add C to Ru -bP • But at ØCO 2 / ↑O2, oxygenase activity becomes greater than carboxylase activity – Remove C ’s from Ru -bP v Stomata close when leaf gets dehydrated to retain water. v [O2] increases; [CO2] decreases. • Even under good conditions, ~20% of Ru -bP is oxygenated rather than carboxylated O2 P Photorespiration: Waste P Photorespiration P Photorespiration catalyzed by Rubisco if: low CO2 / high O2 Photorespiration: • RuBP is oxidized (destroyed) rather than recycled • CO2 is produced rather than consumed (fixed) C4 Photosynthesis v 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 — independent convergence – Hardy weeds: crab grass, summer annuals – Important drought-resistant crops: corn, sugar cane, sorghum Heyer 10 Photosynthesis C3-plant leaf structure C4 Photosynthesis • Photophosphorylation & C 4-carbon fixation in mesophyll cells – PEP carboxylase insensistive to O 2 – O2 from photophosphorylation diffuses back to air space C4-plant leaf structure • Mesophyll cells directly associated with bundle sheath cells • Oxaloacetate shuttles CO 2 to bundle sheath cells • Calvin cycle uses CO 2 for synthesis of organic macromolecules – ↑↑ CO 2 favors photosynthesis rather than photorespiration – Sugars secreted into vascular system Called “C4 ” because first step of carbon-fixation produces C4-oxaloacetate instead of C3-glycerate C3 vs. C4 plants rubisco chlorophyll Mesophyll cell Mesophyll cell Bundle sheath cell Vein Stoma cutin cutin C4 Leaf — cross-section (corn) C4 Leaf — cross-section (corn) air space C4 Carbon Fixation in C4 Plants CO2 CO2 Bundle Sheath ß PEP + CO 2 Æ oxaloacetate Calvin Cycle ß Oxaloacetate oxidized to malate ß Malate moves to bundle-sheath cell Mesophyll ß Malate decarboxylated to pyruvate + CO2 C4 Leaf vein Heyer 11 Photosynthesis Photorespiration: v C4 plants have carbon fixation & Calvin cycle in different cells. ATP CO2 C Fixation: night CAM Photosynthesis: Crassulacean Acid Metabolism CO2 v CAM plants do carbon fixation & Calvin cycle at different times. Calvin Cycle: day ATP NADPH Sugar Crassulacean A cid M etabolism • Ancient Romans noted certain succulents tasted bland in the afternoon, but sour in the morning. • 1940s: studying South African succulents (Family: Crassulacea) daytimefi↑starch /Ømalic acid nighttimefiØstarch / ↑malic acid “Crassulacean acid metabolism” (CAM) Crassula ovata (jade plant) • 1980: CAM model of alternative photosynthesis • Mostly in desert succulents and tropical epiphytes CAM variants ATP CO 2 C Fixation: night CO 2 Calvin Cycle: • At night—stomata open day ⋅ Starch Æ [hydrolysis/glycolysis] Æ PEP Sugar ⋅ C4 carbon fixation: PEP + CO2 Æ oxaloacetic acid Æ malic acid ⋅ Malic acid stored in vacuole • At day—stomata close tightly ⋅ Malic acid released from vacuole ⋅ Malic acid Æ pyruvate + CO2 ATP NADPH ⋅ Pyruvate Æ back to starch ⋅ CO2 increases within cell to 0.2–2.5% ⋅ fi photosynthesis with minimal photorespiration Photorespiration: • Facultative CAM photosynthesis ⋅ During heat of day or dry season: stomata close / CAM photosynthesis ⋅ During cooler, humid late day or wet season: stomata open / switch to C3 photosynthesis • “CAM idling ” ⋅ During dry season or extended drought, stomata remain closed day and night ⋅ Extreme recycling! CO 2 Æ CAM photosynthesis ↑ ¨ Respiration ¨ O ø 2 ⋅ Not much growth; but can survive for extended periods without any external source of CO 2 or H2O ⋅ Sometimes for years! Heyer CAM Photosynthesis v C4 & CAM plants avoid photorespiration by providing more CO2 for rubisco. v They use a separate carbon fixation step before the Calvin cycle. 12 Photosynthesis C4 & CAM Plants air space So why don’t most plants use the C4 pathway? It’s expensive! Most environments are moist enough that the energy expense is not worth the increased fixation efficiency. vein Photorespiration: v Photorespiration can be minimized by providing more CO2 for rubisco. v Many aquatic algae & cyanobacteria concentrate with CO2/HCO3– pumps. Pumps activated when CO2 drops to ~0.03%. q Can ↑CO 2 1000-fold inside cell. q Chemosynthesis: the other autotrophs • Some archea & eubacteria (& fungi?) can fix inorganic carbon into organic molecules without sunshine. • Powered by strong inorganic reducing agents (electron donors). – esp., H2S, NH3, H2 O2 + 4 X-H2 Æ Electron transport system ATP & reduced coenzymes + CO 2 Æ Carbon fixation Æ CH 2O + 3H 2O + 4 X (sugar) Note : reaction requires O2 & CO2 . Still dependent on inputs from photosynthesis & aerobic respiration. Chemoautotroph based food webs O2 + 4 S-H 2 Æ Electron transport system ATP & reduced coenzymes + CO 2 Æ • Carbon fixation Æ CH 2O + 3H 2O + 4 S (sugar) In a few regions with no sunlight, little organic carbon input, and a source of H 2S (e.g., oceanic deep hydrothermal vents), chemosynthetic autotrophs may provide the primary production •Mats of chemosynthetic bacteria cover the rocks. •Clouds of chemosynthetic bacteria fill the water column. •Mouthless tubeworms host symbiotic chemosynthetic bacteria. •Suspension feeders filter the chemosynthetic bacteria from the water. •Grazers scrape the chemosynthetic bacterial mats. Heyer 1 ...
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