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2 Photosynthesis Overview Energy for all life on Earth ultimately comes from photosynthesis 6CO H 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

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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5 Stages Light-dependent reactions –Require light 1.Capture energy from sunlight 2.Make ATP and reduce NADP + to NADPH Carbon fixation reactions or light- independent reactions –Does not require light 3.Use ATP and NADPH to synthesize organic molecules from CO 2

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7 Pigments Molecules that absorb light energy in the visible range Light is a form of energy Photon – particle of light –Acts as a discrete bundle of energy –Energy content of a photon is inversely proportional to the wavelength of the light

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Organisms have evolved a variety of different pigments Only two general types are used in green plant photosynthesis –Chlorophylls –Carotenoids In some organisms, other molecules also absorb light energy 9

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Chlorophylls Chlorophyll a –Main pigment in plants and cyanobacteria –Only pigment that can act directly to convert light energy to chemical energy –Absorbs violet-blue and red light Chlorophyll b –Accessory pigment or secondary pigment absorbing light wavelengths that chlorophyll a does not absorb 11

Carotenoids –Carbon rings linked to chains with alternating single and double bonds –Can absorb photons with a wide range of energies –Also scavenge free radicals – antioxidant Protective role Phycobiloproteins –Important in low-light ocean areas 12

13 Photosystem Organization Antenna complex –Hundreds of accessory pigment molecules –Gather photons and feed the captured light energy to the reaction center Reaction center –1 or more chlorophyll a molecules –Passes excited electrons out of the photosystem

14 In sulfur bacteria, only one photosystem is used Generates ATP via electron transport Anoxygenic photosynthesis Excited electron passed to electron transport chain Generates a proton gradient for ATP synthesis Cyclic photophosphorylation

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16 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 Z diagram

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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 21

22 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

23 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

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26 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

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28 C 4 plants Corn, sugarcane, sorghum, and a number of other grasses Initially fix carbon in mesophyll cells transported to bundle-sheath cells Carbon fixation then by rubisco and the Calvin cycle

29 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 during the night and store in vacuole