Chapter 10 Photosynthesis Wi ¥ PowerPoint? Lecture Presentations for Biology | \) Eighth Edition 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* Autotrophs sustain themselves without eating anything derived from other organisms + Autotrophs are the producers of the biosphere, producing organic molecules from CO, and other inorganic molecules ¢ Almost all plants are photoautotrophs, using the energy of sunlight to make organic molecules from H,O and CO,* 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) Cyanobacteria eatry wee (a) Plants rig wen(c) Unicellular protist — 10 um (d) Cyanobacteria -— 40 pmcig ee (e) Purple sulfur bacteria HK 1.5 ym + 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 O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 * CO, enters and O, exits the leaf through microscopic pores called stomataChloroplasts are found mainly in cells of the mesophyll, the interior tissue of the leaf A typical mesophyll cell has 30-40 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Outer membrane Intermembrane ‘space Inner membraneTracking Atoms Through Photosynthesis: Scientific Inquiry * Photosynthesis can be summarized as the following equation: 6 CO, + 12 H20 + Light energy > CgH120g + 6 O2 + 6 H2O The Splitting of Water * Chloroplasts split H,O into hydrogen and oxygen, incorporating the electrons of hydrogen into sugar moleculesReactants: 6CO, 12 H,0 Products: C5H120,5 6H,0 60, Photosynthesis as a Redox Process * Photosynthesis is a redox process in which H,0O is oxidized and CO, is reduced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 H,O — Release O, — Reduce NADP* to NADPH — Generate ATP from ADP by photophosphorylation * The Calvin cycle (in the stroma) forms sugar from CO,, using ATP and NADPH * The Calvin cycle begins with carbon fixation, incorporating CO, into organic moleculesChloroplast ri we ChloroplastChloroplast Chloroplast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* 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 * Light also behaves as though it consists of discrete particles, called photons Visible light i T T U y U i l Kr) rt | A Shorter wavelength —————————> Longer wavelength Higher energy ——————————> 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 PLAY Animation: LightandPigments Chloroplast+ Aspectrophotometer 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 ry wo White Refracting Chlorophyll Photoelectric light Prism Ce Slit moves to pass light of selected wavelength = solution Blue light tube Galvanometer The high transmittance (low absorption) reading indicates that chlorophyll absorbs very little green light. chlorophyll absorbs most blue light.- 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 - Anaction spectrum profiles the relative effectiveness of different wavelengths of radiation in driving a process {a) Absorption spectra fs00" 00 Wavelength of light (nm) Rate of photosynthesis (measured by 0, release) (b) Action spectrum oer fans of alga (6) Engemann’s experiment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 O, He used the growth of aerobic bacteria clustered along the alga as a measure of O production 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cig ww ‘ety in chlorophyll a HO in chlorophyll b 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 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 * If illuminated, an isolated solution of chlorophyll will fluoresce, giving off light and heatEnergy of electron Photon (fluorescence) oto Dan (a) Excitation of isolated chlorophyll molecule (b) Fluorescence A Photosystem: A Reaction-Center Complex Associated with Light-Harvesting Complexes « Aphotosystem 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+ Aprimary electron acceptor in the reaction center accepts an excited electron from chlorophyll a * Solar-powered transfer of an electron from a chlorophyll a molecule to the primary electron acceptor is the first step of the light reactions Photon Photosystem Light-harvesting Reaction-center /P electron complexes complex acceptor e é & 4 i = 3 = Ss > = =eThere are two types of photosystems in the thylakoid membrane Photosystem Il (PS Il) 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 Il is called P680 Photosystem | (PS I) is best at absorbing a wavelength of 700 nm The reaction-center chlorophyll a of PS | is called P700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 both photosystems and produces ATP and NADPH using light energy « Aphoton 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Primary | bos [Pigment molecules Photosystem (Psi) P680* (P680 that is missing an electron) is a very strong oxidizing agent H,0 is split by enzymes, and the electrons are transferred from the hydrogen atoms to P680*, thus reducing it to P680 O, is released as a by-product of this reactionPigment molecules Photosystem (Psi) + Each electron “falls” down an electron transport chain from the primary electron acceptor of PS Ilto PSI + Energy released by the fall drives the creation of a proton gradient across the thylakoid membrane * Diffusion of H* (protons) across the membrane drives ATP synthesisPigment molecules Photosystem (Psi) + In PS | (like PS Il), transferred light energy excites P700, which loses an electron to an electron acceptor + P700* (P700 that is missing an electron) accepts an electron passed down from PS II via the electron transport chainrig wena 3 cot aa bead ocak ton Photosystem | Psy Photosystem (Psi) * Each electron “falls” down an electron transport chain from the primary electron acceptor of PS | to the protein ferredoxin (Fd) * 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| ZNUZNIZS WANCyclic Electron Flow + Cyclic electron flow uses only photosystem | and produces ATP, but not NADPH * Cyclic electron flow generates surplus ATP, satisfying the higher demand in the Calvin cycle NADP: NADP+ +H reductase elmo ® 2) Cytochrome complex ‘Photosystem I* Some organisms such as purple sulfur bacteria have PS | 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 A Comparison of Chemiosmosis in Chloroplasts and Mitochondria * 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* 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 rig we Mitochondrion Chloroplast MITOCHONDRION CHLOROPLAST STRUCTURE STRUCTURE Intermembrane WS) Thylakoia space space Inner Thylakoid membrane hail membrane Matrix Key Ml Higher[H*]+ 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 H,O to NADPH STROMA (low H* concentration)Concept 10.3: The Calvin cycle uses ATP and NADPH to convert CO, 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 CO, 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 CO, * The Calvin cycle has three phases: Carbon fixation (catalyzed by rubisco) — Reduction — Regeneration of the CO, acceptor (RuBP)cig ie Input 3@(Enering one con ane) Shorted. intermediate 38-00-0008 50008 Albulose bisphosphate 2 Phosphoglyesrate (uBP) rig woe Input 2@ (Entering one co, meine) “Phase 1: Carbon fixation ‘Short ivod intermediate 32-09-9008 SOOO Aun Sposa ahephoaycera ahs hoatenate «ie ane Cabin rete : seme (RAPA s waves $8 soooe Iyeeraldohydo 3 phosphate (63). Phase 1: Carbon fixation Bosco ea Sho Inoo indie 318-000-008 (9000 Abuse Sosphoe .Phesphonerate “rush mag aE _ orste 5600-08 1.3.8isphosphoglycerete Concept 10.4: Alternative mechanisms of carbon fixation have evolved in hot, arid climates * 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 HO but also limits photosynthesis * The closing of stomata reduces access to CO, and causes O, to build up * These conditions favor a seemingly wasteful process called photorespirationPhotorespiration: An Evolutionary Relic? * In most plants (C3 plants), initial fixation of CO,, via rubisco, forms a three-carbon compound * In photorespiration, rubisco adds O, instead of CO, in the Calvin cycle * Photorespiration consumes O, and organic fuel and releases CO, 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 CO, ¢ 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C, Plants + C, plants minimize the cost of photorespiration by incorporating CO, into four-carbon compounds in mesophyll cells * This step requires the enzyme PEP carboxylase « PEP carboxylase has a higher affinity for CO, than rubisco does; it can fix CO, even when CO, concentrations are low * These four-carbon compounds are exported to bundle-sheath cells, where they release CO, that is then used in the Calvin cycle Ca leat anatomy The C, pathway ophyll plant leaf sheath cell Mesophyt cell Photosynthetic een cells of Cs BundleC, leaf anatomy Mesophyll cell Photosynthetic cells of C, Bundle- plant leaf sheath cell Vein (vascular tissue) rig. worseCAM Plants + Some plants, including succulents, use crassulacean acid metabolism (CAM) to fix carbon « CAM plants open their stomata at night, incorporating CO, into organic acids * Stomata close during the day, and CO, is released from organic acids and used in the Calvin cycle rig wen ‘Sugarcane c Pineapple co, Mesophyll @ CO; incorporated Night coll Organic acid: into four-carbon organic ads (carbon fixation) unde. Day ‘sheath Z - © organic acids felease CO, to Cavin eyele sugar sugar (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 * In addition to food production, photosynthesis produces the O, in our atmosphere ChloroplastPrimary “te, Cytochrome! e complex ¢ se. fs @ ake Photosystem | Photosystem II rig une 30, Carbon fixation 3x5C 6x3C Regeneration of CO, acceptor Reduction 1 G3P (3C)3x | (=3) 3x6(l8) 3 x5 (=I5) 6x3 (=18) 6x3(=18) 5x3 (=I1s)High HY concentration Low Ht concentration ADP + ®; 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. Trace the movement of electrons in linear electron flow 4. Trace the movement of electrons in cyclic electron flow. Describe the similarities and differences between oxidative phosphorylation in mitochondria and photophosphorylation in chloroplasts . Describe the role of ATP and NADPH in the Calvin cycle . Describe the major consequences of photorespiration . Describe two important photosynthetic adaptations that minimize photorespiration