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chapter 10 overview the process that feeds the biosphere · photosynthesis is the process that converts solar energy into chemical energy · directly or indirectly photosynthesis nourishes almost the entire living world photosynthesis powerpoint® lecture presentations for biology eighth edition neil campbell and jane reece lectures by chris romero updated by erin barley with contributions from joan sharp copyright © 2008 pearson education inc publishing as pearson benjamin cummings copyright © 2008 pearson education inc publishing as pearson benjamin cummings fig 10-1 · autotrophs sustain themselves without eating anything derived from other organisms · autotrophs are the producers of the biosphere producing organic molecules from co2 and other inorganic molecules · almost all plants are photoautotrophs using the energy of sunlight to make organic molecules from h2o and co2 copyright © 2008 pearson education inc publishing as pearson benjamin cummings fig 10-2 · photosynthesis occurs in plants algae certain other protists and some prokaryotes · these organisms feed not only themselves but also most of the living world a plants c unicellular protist 10 µm e purple sulfur bacteria 1.5 µm bioflix photosynthesis b multicellular alga copyright © 2008 pearson education inc publishing as pearson benjamin cummings d cyanobacteria 40 µm 1

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fig 10-2a fig 10-2b a plants b multicellular alga fig 10-2c fig 10-2d c unicellular protist d cyanobacteria 10 µm 40 µm fig 10-2e · heterotrophs obtain their organic material from other organisms · heterotrophs are the consumers of the biosphere · almost all heterotrophs including humans depend on photoautotrophs for food and o2 e purple sulfur bacteria 1.5 µm copyright © 2008 pearson education inc publishing as pearson benjamin cummings 2

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concept 10.1 photosynthesis converts light energy to the chemical energy of food · chloroplasts are structurally similar to and likely evolved from photosynthetic bacteria · the structural organization of these cells allows for the chemical reactions of photosynthesis chloroplasts the sites of photosynthesis in plants · leaves are the major locations of photosynthesis · their green color is from chlorophyll the green pigment within chloroplasts · light energy absorbed by chlorophyll drives the synthesis of organic molecules in the chloroplast · co2 enters and o2 exits the leaf through microscopic pores called stomata copyright © 2008 pearson education inc publishing as pearson benjamin cummings copyright © 2008 pearson education inc publishing as pearson benjamin cummings fig 10-3 leaf cross section vein mesophyll · chloroplasts are found mainly in cells of the mesophyll the interior tissue of the leaf · a typical mesophyll cell has 30­40 chloroplasts · the chlorophyll is in the membranes of thylakoids connected sacs in the chloroplast thylakoids may be stacked in columns called grana · chloroplasts also contain stroma a dense fluid 1 µm chloroplast stomata co2 o2 mesophyll cell outer membrane thylakoid stroma granum thylakoid space intermembrane space inner membrane 5 µm copyright © 2008 pearson education inc publishing as pearson benjamin cummings fig 10-3a leaf cross section vein fig 10-3b chloroplast mesophyll outer membrane stomata thylakoid co2 o2 stroma chloroplast granum thylakoid space intermembrane space inner membrane mesophyll cell 5 µm 1 µm 3

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tracking atoms through photosynthesis scientific inquiry · photosynthesis can be summarized as the following equation 6 co2 12 h2o light energy c6h12o6 6 o2 6 h2o the splitting of water · chloroplasts split h2o into hydrogen and oxygen incorporating the electrons of hydrogen into sugar molecules copyright © 2008 pearson education inc publishing as pearson benjamin cummings copyright © 2008 pearson education inc publishing as pearson benjamin cummings fig 10-4 photosynthesis as a redox process · photosynthesis is a redox process in which h2o is oxidized and co2 is reduced reactants 6 co2 12 h2o products c6h12o6 6 h 2o 6 o2 copyright © 2008 pearson education inc publishing as pearson benjamin cummings the two stages of photosynthesis a preview · photosynthesis consists of the light reactions the photo part and calvin cycle the synthesis part · the light reactions in the thylakoids ­ split h2o ­ release o2 ­ reduce nadp to nadph ­ generate atp from adp by photophosphorylation copyright © 2008 pearson education inc publishing as pearson benjamin cummings copyright © 2008 pearson education inc publishing as pearson benjamin cummings · the calvin cycle in the stroma forms sugar from co2 using atp and nadph · the calvin cycle begins with carbon fixation incorporating co2 into organic molecules 4

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fig 10-5-1 fig 10-5-2 h2o h2o light nadp adp p i light nadp adp p i light reactions r ti light reactions r ti atp nadph chloroplast chloroplast o2 fig 10-5-3 fig 10-5-4 h2o co2 h2o co2 light nadp adp p i light nadp adp p light reactions r ti atp nadph calvin cycle i light reactions r ti atp nadph calvin cycle chloroplast o2 chloroplast o2 [ch2o sugar concept 10.2 the light reactions convert solar energy to the chemical energy of atp and nadph · chloroplasts are solar-powered chemical factories · their thylakoids transform light energy into the chemical energy of atp and nadph the nature of sunlight · light is a form of electromagnetic energy also called electromagnetic radiation · like other electromagnetic energy light travels in rhythmic waves · wavelength is the distance between crests of waves · wavelength determines the type of electromagnetic energy copyright © 2008 pearson education inc publishing as pearson benjamin cummings copyright © 2008 pearson education inc publishing as pearson benjamin cummings 5

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fig 10-6 10­5 nm 10­3 nm 1 nm uv 103 nm 106 nm 1m 109 nm 103 m · the electromagnetic spectrum is the entire range of electromagnetic energy or radiation · visible light consists of wavelengths including those that drive photosynthesis that produce colors we can see gamma x-rays rays infrared microwaves radio waves visible light · light also behaves as though it consists of discrete particles called photons 380 450 500 550 600 650 700 750 nm shorter wavelength higher energy copyright © 2008 pearson education inc publishing as pearson benjamin cummings longer wavelength lower energy photosynthetic pigments the light receptors · pigments are substances that absorb visible light · different pigments absorb different wavelengths · wavelengths that are not absorbed are reflected or transmitted · leaves appear green because chlorophyll reflects and transmits green light animation light and pigments copyright © 2008 pearson education inc publishing as pearson benjamin cummings fig 10-7 light reflected light chloroplast absorbed light granum transmitted light fig 10-8 technique white light refracting chlorophyll photoelectric prism solution tube galvanometer 2 1 3 4 · a spectrophotometer measures a pigment s ability to absorb various wavelengths · this machine sends light through pigments and measures the fraction of light transmitted at each wavelength slit moves to pass light of selected wavelength green light the high transmittance low absorption reading indicates that hl h ll b b chlorophyll absorbs very little green light blue light the low transmittance high absorption reading indicates that chlorophyll absorbs most blue light copyright © 2008 pearson education inc publishing as pearson benjamin cummings 6

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fig 10-9 results chlorophyll a chlorophyll b carotenoids · an absorption spectrum is a graph plotting a pigment s light absorption versus wavelength · the absorption spectrum of chlorophyll a suggests that violet-blue and red light work best for photosynthesis · an action spectrum profiles the relative effectiveness of different wavelengths of radiation in driving a process a absorption spectra 400 500 600 700 wavelength of light nm b action spectrum aerobic bacteria filament of alga c engelmann s experiment copyright © 2008 pearson education inc publishing as pearson benjamin cummings 400 500 600 700 · the action spectrum of photosynthesis was first demonstrated in 1883 by theodor w engelmann · in his experiment he exposed different segments of a filamentous alga to different wavelengths · areas receiving wavelengths favorable to photosynthesis produced excess o2 · he used the growth of aerobic bacteria clustered along the alga as a measure of o2 production copyright © 2008 pearson education inc publishing as pearson benjamin cummings · chlorophyll a is the main photosynthetic pigment · accessory pigments such as chlorophyll b broaden the spectrum used for photosynthesis · accessory pigments called carotenoids absorb excessive light that would damage chlorophyll copyright © 2008 pearson education inc publishing as pearson benjamin cummings fig 10-10 ch3 cho in chlorophyll a in chlorophyll b excitation of chlorophyll by light · when a pigment absorbs light it goes from a ground state to an excited state which is unstable · when excited electrons fall back to the ground state photons are given off an afterglow called fluorescence porphyrin ring light-absorbing head of molecule note magnesium atom at center hydrocarbon tail interacts with hydrophobic regions of proteins inside thylakoid membranes of chloroplasts h atoms not shown · if illuminated an isolated solution of chlorophyll will fluoresce giving off light and heat copyright © 2008 pearson education inc publishing as pearson benjamin cummings 7

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fig 10-11 a photosystem a reaction-center complex associated with light-harvesting complexes e­ excited state heat ener of electron rgy · a photosystem consists of a reaction-center complex a type of protein complex surrounded by light-harvesting complexes · the light-harvesting complexes pigment molecules bound to proteins funnel the energy of photons to the reaction center photon fluorescence photon chlorophyll molecule ground state a excitation of isolated chlorophyll molecule b fluorescence copyright © 2008 pearson education inc publishing as pearson benjamin cummings fig 10-12 photon photosystem light-harvesting reaction-center complex complexes stroma primary electron acceptor · solar-powered transfer of an electron from a chlorophyll a molecule to the primary electron acceptor is the first step of the light reactions thylakoid membrane m · a primary electron acceptor in the reaction center accepts an excited electron from chlorophyll a e­ transfer of energy special pair of chlorophyll a molecules pigment molecules thylakoid space interior of thylakoid copyright © 2008 pearson education inc publishing as pearson benjamin cummings · there are two types of photosystems in the thylakoid membrane · photosystem ii ps ii functions first the numbers reflect order of discovery and is best at absorbing a wavelength of 680 nm · the reaction-center chlorophyll a of ps ii is called p680 · photosystem i ps i is best at absorbing a wavelength of 700 nm · the reaction-center chlorophyll a of ps i is called p700 copyright © 2008 pearson education inc publishing as pearson benjamin cummings copyright © 2008 pearson education inc publishing as pearson benjamin cummings 8

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linear electron flow · during the light reactions there are two possible routes for electron flow cyclic and linear · linear electron flow the primary pathway involves b h photosystems and produces atp i l both h d d and nadph using light energy · a photon hits a pigment and its energy is passed among pigment molecules until it excites p680 · an excited electron from p680 is transferred to the primary electron acceptor copyright © 2008 pearson education inc publishing as pearson benjamin cummings copyright © 2008 pearson education inc publishing as pearson benjamin cummings fig 10-13-1 primary acceptor 2 e­ · p680 p680 that is missing an electron is a very strong oxidizing agent · h2o is split by enzymes and the electrons are transferred from the hydrogen atoms to p680 thus reducing it to p680 · o2 is released as a by-product of this reaction pigment molecules p680 1 light photosystem ii ps ii copyright © 2008 pearson education inc publishing as pearson benjamin cummings fig 10-13-2 primary acceptor 2 2 h 1 o 2 2 h2o 3 e­ e­ p680 1 light e­ · each electron falls down an electron transport chain from the primary electron acceptor of ps ii to ps i · energy released by the fall drives the creation of a proton gradient across the thylakoid membrane pigment molecules photosystem ii ps ii · diffusion of h protons across the membrane drives atp synthesis copyright © 2008 pearson education inc publishing as pearson benjamin cummings 9

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fig 10-13-3 primary acceptor 2 2 h o2 2 h2o 3 e­ e­ p680 1 light atp 5 e­ 4 pq cytochrome complex · in ps i like ps ii transferred light energy excites p700 which loses an electron to an electron acceptor pc 1 · p700 p700 that is missing an electron accepts an electron passed down from ps ii via the electron transport chain pigment molecules photosystem ii ps ii copyright © 2008 pearson education inc publishing as pearson benjamin cummings fig 10-13-4 primary acceptor 2 2 h o2 2 h2o 3 e­ e­ p680 1 light atp 5 e­ 4 primary acceptor e­ pq cytochrome complex · each electron falls down an electron transport chain from the primary electron acceptor of ps i to the protein ferredoxin fd light 6 1 pc p700 · the electrons are then transferred to nadp and reduce it to nadph · the electrons of nadph are available for the reactions of the calvin cycle pigment molecules photosystem ii ps ii photosystem i ps i copyright © 2008 pearson education inc publishing as pearson benjamin cummings fig 10-13-5 fig 10-14 atp primary acceptor 2 2 h o2 h2o 3 e­ e­ p680 1 light atp 5 6 e­ 4 e­ primary acceptor e­ 7 pq cytochrome complex fd e­ ­ e 8 nadp reductase nadp h nadph e­ e­ 1 2 pc p700 light e­ e­ nadph e­ mill makes atp e­ pigment molecules photosystem ii ps ii photosystem i ps i photosystem ii photosystem i 10

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cyclic electron flow · cyclic electron flow uses only photosystem i and produces atp but not nadph · cyclic electron flow generates surplus atp satisfying the higher demand in the calvin cycle fig 10-15 primary acceptor pq fd primary acceptor fd nadp reductase nadp h nadph cytochrome p complex pc photosystem i photosystem ii atp copyright © 2008 pearson education inc publishing as pearson benjamin cummings a comparison of chemiosmosis in chloroplasts and mitochondria · some organisms such as purple sulfur bacteria have ps i but not ps ii · cyclic electron flow is thought to have evolved before linear electron flow · cyclic electron flow may protect cells from light-induced damage · chloroplasts and mitochondria generate atp by chemiosmosis but use different sources of energy · mitochondria transfer chemical energy from food to atp chloroplasts transform light energy into the chemical energy of atp · spatial organization of chemiosmosis differs between chloroplasts and mitochondria but also shows similarities copyright © 2008 pearson education inc publishing as pearson benjamin cummings copyright © 2008 pearson education inc publishing as pearson benjamin cummings fig 10-16 mitochondrion chloroplast · in mitochondria protons are pumped to the intermembrane space and drive atp synthesis as they diffuse back into the mitochondrial matrix · in chloroplasts protons are pumped into the thylakoid space and drive atp synthesis as they diffuse back into the stroma mitochondrion structure intermembrane space inner membrane electron transport chain atp synthase key chloroplast structure h diffusion thylakoid space thylakoid membrane matrix adp p i h stroma atp higher [h lower [h copyright © 2008 pearson education inc publishing as pearson benjamin cummings 11

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fig 10-17 · atp and nadph are produced on the side facing the stroma where the calvin cycle takes place · in summary light reactions generate atp and increase the potential energy of electrons by moving them from h2o to nadph stroma low h concentration photosystem ii light 4 h cytochrome photosystem i complex light fd nadp reductase 3 nadp h nadph pq e­ 1 2 h2o thylakoid space high h concentration e­ 1 2 o2 4 h pc +2 h to calvin cycle thylakoid membrane stroma low h concentration atp synthase adp pi atp h copyright © 2008 pearson education inc publishing as pearson benjamin cummings concept 10.3 the calvin cycle uses atp and nadph to convert co2 to sugar · the calvin cycle like the citric acid cycle regenerates its starting material after molecules enter and leave the cycle · the cycle builds sugar from smaller molecules by using atp and the reducing power of electrons carried by nadph · carbon enters the cycle as co2 and leaves as a sugar named glyceraldehyde-3-phospate g3p · for net synthesis of 1 g3p the cycle must take place three times fixing 3 molecules of co2 · the calvin cycle has three phases ­ carbon fixation catalyzed by rubisco ­ reduction ­ regeneration of the co2 acceptor rubp copyright © 2008 pearson education inc publishing as pearson benjamin cummings copyright © 2008 pearson education inc publishing as pearson benjamin cummings fig 10-18-1 input 3 co2 entering one at a time fig 10-18-2 input 3 co2 entering one at a time phase 1 carbon fixation rubisco rubisco phase 1 carbon fixation 3 p short-lived intermediate p 3p ribulose bisphosphate rubp p 3 p short-lived intermediate 3 p p ribulose bisphosphate rubp p 6 p 3-phosphoglycerate 6 p 3-phosphoglycerate 6 6 adp atp calvin cycle 6 p p 1,3-bisphosphoglycerate 6 nadph 6 nadp 6 pi 6 p glyceraldehyde-3-phosphate g3p phase 2 reduction 1 output g3p a sugar p glucose and other organic compounds 12

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fig 10-18-3 input 3 co2 entering one at a time concept 10.4 alternative mechanisms of carbon fixation have evolved in hot arid climates phase 1 carbon fixation p rubisco 3 p short-lived intermediate 3p p ribulose bisphosphate rubp 6 p 3-phosphoglycerate 6 6 adp atp · dehydration is a problem for plants sometimes requiring trade-offs with other metabolic processes especially photosynthesis · on hot dry days plants close stomata which conserves h2o but also li i photosynthesis b l limits h h i · the closing of stomata reduces access to co2 and causes o2 to build up · these conditions favor a seemingly wasteful process called photorespiration copyright © 2008 pearson education inc publishing as pearson benjamin cummings 3 adp 3 atp calvin cycle 6 p p 1,3-bisphosphoglycerate 6 nadph phase 3 regeneration of the co2 acceptor rubp 6 nadp 6 pi 5 g3p p 6 p glyceraldehyde-3-phosphate g3p phase 2 reduction 1 output g3p a sugar p glucose and other organic compounds photorespiration an evolutionary relic · in most plants c3 plants initial fixation of co2 via rubisco forms a three-carbon compound · in photorespiration rubisco adds o2 instead of co2 in the calvin cycle · photorespiration consumes o2 and organic fuel and releases co2 without producing atp or sugar · photorespiration may be an evolutionary relic because rubisco first evolved at a time when the atmosphere had far less o2 and more co2 · photorespiration limits damaging products of light reactions that build up in the absence of the calvin cycle · in many plants photorespiration is a problem because on a hot dry day it can drain as much as 50 of the carbon fixed by the calvin cycle copyright © 2008 pearson education inc publishing as pearson benjamin cummings copyright © 2008 pearson education inc publishing as pearson benjamin cummings c4 plants · c4 plants minimize the cost of photorespiration by incorporating co2 into four-carbon compounds in mesophyll cells · this step requires the enzyme pep carboxylase · pep carboxylase has a higher affinity for co2 than rubisco does it can fix co2 even when co2 concentrations are low · these four-carbon compounds are exported to bundle-sheath cells where they release co2 that is then used in the calvin cycle copyright © 2008 pearson education inc publishing as pearson benjamin cummings fig 10-19 c4 leaf anatomy mesophyll cell photosynthetic cells of c4 bundleplant leaf sheath cell vein vascular tissue the c4 pathway mesophyll cell co2 pep carboxylase oxaloacetate 4c malate 4c pep 3c adp atp stoma bundle bundlesheath cell pyruvate 3c co2 calvin cycle sugar vascular tissue 13

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fig 10-19a fig 10-19b the c4 pathway c4 leaf anatomy mesophyll cell photosynthetic cells of c4 bundleplant leaf sheath cell vein vascular tissue mesophyll cell pep carboxylase co2 oxaloacetate 4c malate 4c pep 3c adp atp bundlesheath cell pyruvate 3c co2 calvin cycle sugar stoma vascular tissue cam plants · some plants including succulents use crassulacean acid metabolism cam to fix carbon · cam plants open their stomata at night incorporating co2 into organic acids · stomata close during the day and co2 is released from organic acids and used in the calvin cycle fig 10-20 sugarcane c4 co2 pineapple cam co2 night mesophyll cell organic acid 1 co2 incorporated into four-carbon organic acid organic acids carbon fixation co2 2 organic acids release co2 to calvin cycle calvin cycle sugar bundlesheath cell co2 calvin cycle sugar day a spatial separation of steps copyright © 2008 pearson education inc publishing as pearson benjamin cummings b temporal separation of steps the importance of photosynthesis a review · the energy entering chloroplasts as sunlight gets stored as chemical energy in organic compounds · sugar made in the chloroplasts supplies chemical energy and carbon skeletons to synthesize the organic molecules of cells · plants store excess sugar as starch in structures such as roots tubers seeds and fruits fig 10-21 h2o co2 light nadp adp p i light reactions photosystem ii electron transport chain photosystem i electron transport chain atp nadph chloroplast rubp 3-phosphoglycerate calvin cycle g3p starch storage · in addition to food production photosynthesis produces the o2 in our atmosphere copyright © 2008 pearson education inc publishing as pearson benjamin cummings o2 sucrose export 14

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fig 10-un1 fig 10-un2 h2o co2 3 co2 carbon fixation primary acceptor h2o o2 pq cytochrome complex l pc primary acceptor fd nadp reductase nadp h nadph 3 × 5c calvin cycle regeneration of co2 acceptor 5 × 3c 6 × 3c atp photosystem ii o2 photosystem i reduction 1 g3p 3c fig 10-un3 fig 10-un4 ph 7 ph 4 ph 4 ph 8 atp fig 10-un5 you should now be able to 1 describe the structure of a chloroplast 2 describe the relationship between an action spectrum and an absorption spectrum 3 3 trace the movement of electrons in linear electron flow 4 trace the movement of electrons in cyclic electron flow copyright © 2008 pearson education inc publishing as pearson benjamin cummings 15

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