Photosynthesis Chapter 8
Photosynthesis Overview Energy for all life on Earth ultimately comes from photosynthesis 6CO 2 + 12H 2 O C 6 H 12 O 6 + 6H 2 O + 6O 2 Oxygenic photosynthesis is carried out by Cyanobacteria 7 groups of algae All land plants chloroplasts 2
Chloroplast Thylakoid membrane internal membrane Contains chlorophyll and other photosynthetic pigments Pigments clustered into photosystems Grana stacks of flattened sacs of thylakoid membrane Stroma lamella connect grana Stroma semiliquid surrounding thylakoid membranes 3
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Stages Light-dependent reactions Require light 1.Capture energy from sunlight 2.Make ATP and reduce NADP + to NADPH Carbon fixation reactions or lightindependent reactions Does not require light 3.Use ATP and NADPH to synthesize organic molecules from CO 2 5
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Discovery of Photosynthesis Jan Baptista van Helmont (1580 1644) Demonstrated that the substance of the plant was not produced only from the soil Joseph Priestly (1733 1804) Living vegetation adds something to the air Jan Ingen-Housz (1730 1799) Proposed plants carry out a process that uses sunlight to split carbon dioxide into carbon and oxygen (O 2 gas) 7
F.F. Blackman (1866 1947) Came to the startling conclusion that photosynthesis is in fact a multistage process, only one portion of which uses light directly Light versus dark reactions Enzymes involved 8
C. B. van Niel (1897 1985) Found purple sulfur bacteria do not release O 2 but accumulate sulfur Proposed general formula for photosynthesis CO 2 + 2 H 2 A + light energy (CH 2 O) + H 2 O + 2 A Later researchers found O 2 produced comes from water Robin Hill (1899 1991) Demonstrated Niel was right that light energy could be harvested and used in a reduction reaction 9
Antenna complex Also called light-harvesting complex Captures photons from sunlight and channels them to the reaction center chlorophylls In chloroplasts, light-harvesting complexes consist of a web of chlorophyll molecules linked together and held tightly in the thylakoid membrane by a matrix of proteins 10
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Reaction center Transmembrane protein pigment complex When a chlorophyll in the reaction center absorbs a photon of light, an electron is excited to a higher energy level Light-energized electron can be transferred to the primary electron acceptor, reducing it Oxidized chlorophyll then fills its electron hole by oxidizing a donor molecule 12
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Capture of light energy Light-Dependent Reactions 1. Primary photoevent Photon of light is captured by a pigment molecule 2. Charge separation Energy is transferred to the reaction center; an excited electron is transferred to an acceptor molecule 3. Electron transport Electrons move through carriers to reduce NADP + 4. Chemiosmosis Produces ATP 14
Cyclic photophosphorylation In sulfur bacteria, only one photosystem is used Generates ATP via electron transport Excited electron passed to electron transport chain Generates a proton gradient for ATP synthesis 15
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Chloroplasts have two connected photosystems Oxygenic photosynthesis Photosystem I (P 700 ) Functions like sulfur bacteria Photosystem II (P 680 ) Can generate an oxidation potential high enough to oxidize water Working together, the two photosystems carry out a noncyclic transfer of electrons that is used to generate both ATP and NADPH 17
Photosystem I transfers electrons ultimately to NADP +, producing NADPH Electrons lost from photosystem I are replaced by electrons from photosystem II Photosystem II oxidizes water to replace the electrons transferred to photosystem I 2 photosystems connected by cytochrome/ b 6 -f complex 18
Noncyclic photophosphorylation Plants use photosystems II and I in series to produce both ATP and NADPH Path of electrons not a circle Photosystems replenished with electrons obtained by splitting water 19
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Photosystem II Core of 10 transmembrane protein subunits with electron transfer components and two P 680 chlorophyll molecules Essential for the oxidation of water b 6 -f complex Proton pump embedded in thylakoid membrane 21
Photosystem I Reaction center consists of a core transmembrane complex consisting of 12 to 14 protein subunits with two bound P 700 chlorophyll molecules Photosystem I accepts an electron from plastocyanin into the hole created by the exit of a light-energized electron Passes electrons to NADP + to form NADPH 22
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Chemiosmosis Electrochemical gradient can be used to synthesize ATP Chloroplast has ATP synthase enzymes in the thylakoid membrane Allows protons back into stroma Stroma also contains enzymes that catalyze the reactions of carbon fixation the Calvin cycle reactions 24
Production of additional ATP Noncyclic photophosphorylation generates NADPH ATP Building organic molecules takes more energy than that alone Cyclic photophosphorylation used to produce additional ATP Short-circuit photosystem I to make a larger proton gradient to make more ATP 25
Carbon Fixation Calvin Cycle To build carbohydrates cells use Energy ATP from light-dependent reactions Cyclic and noncyclic photophosphorylation Drives endergonic reaction Reduction potential NADPH from photosystem I Source of protons and energetic electrons 26
Calvin cycle Named after Melvin Calvin (1911 1997) Also called C 3 photosynthesis Key step is attachment of CO 2 to RuBP to form PGA Uses enzyme ribulose bisphosphate carboxylase/oxygenase or rubisco 27
3 phases 1. Carbon fixation RuBP + CO 2 PGA 2. Reduction PGA is reduced to G3P 3. Regeneration of RuBP PGA is used to regenerate RuBP 3 turns incorporate enough carbon to produce a new G3P 6 turns incorporate enough carbon for 1 glucose 28
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Output of Calvin cycle Glucose is not a direct product of the Calvin cycle G3P is a 3 carbon sugar Used to form sucrose Major transport sugar in plants Disaccharide made of fructose and glucose Used to make starch Insoluble glucose polymer Stored for later use 30
Energy cycle Photosynthesis uses the products of respiration as starting substrates Respiration uses the products of photosynthesis as starting substrates Production of glucose from G3P even uses part of the ancient glycolytic pathway, run in reverse Principal proteins involved in electron transport and ATP production in plants are evolutionarily related to those in mitochondria 31
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Photorespiration Rubisco has 2 enzymatic activities Carboxylation Addition of CO 2 to RuBP Favored under normal conditions Photorespiration Oxidation of RuBP by the addition of O 2 Favored when stoma are closed in hot conditions Creates low-co 2 and high-o 2 CO 2 and O 2 compete for the active site on RuBP 33
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Types of photosynthesis C 3 Plants that fix carbon using only C 3 photosynthesis (the Calvin cycle) C 4 and CAM Add CO 2 to PEP to form 4 carbon molecule Use PEP carboxylase Greater affinity for CO 2, no oxidase activity C 4 spatial solution CAM temporal solution 35
C 4 plants Corn, sugarcane, sorghum, and a number of other grasses Initially fix carbon using PEP carboxylase in mesophyll cells Produces oxaloacetate, converted to malate, transported to bundle-sheath cells Within the bundle-sheath cells, malate is decarboxylated to produce pyruvate and CO 2 Carbon fixation then by rubisco and the Calvin cycle 36
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C 4 pathway, although it overcomes the problems of photorespiration, does have a cost To produce a single glucose requires 12 additional ATP compared with the Calvin cycle alone C 4 photosynthesis is advantageous in hot dry climates where photorespiration would remove more than half of the carbon fixed by the usual C 3 pathway alone 38
CAM plants Many succulent (water-storing) plants, such as cacti, pineapples, and some members of about two dozen other plant groups Stomata open during the night and close during the day Reverse of that in most plants Fix CO 2 using PEP carboxylase during the night and store in vacuole 39
When stomata closed during the day, organic acids are decarboxylated to yield high levels of CO 2 High levels of CO 2 drive the Calvin cycle and minimize photorespiration 40
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Compare C 4 and CAM Both use both C 3 and C 4 pathways C 4 two pathways occur in different cells CAM C 4 pathway at night and the C 3 pathway during the day 42