Photosynthesis - Chapter 7 PDF

Chapter 7 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 Cumming...

Chapter 7 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 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings  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  Photosynthesis occurs in plants, algae, certain other protists, and some prokaryotes These organisms feed not only themselves but also most of the living world Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 10-2 (a) Plants (c) Unicellular protist 10 µm (e) Purple sulfur 1.5 µm bacteria (b) Multicellular alga (d) Cyanobacteria 40 µm  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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings 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  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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 10-3 Leaf cross section Vein Mesophyll Stomata CO2 O2 Chloroplast Mesophyll cell Outer membrane Thylakoid Intermembrane 5 µm Stroma Thylakoid space Granum space Inner membrane 1 µm Fig. 10-3a Leaf cross section Vein Mesophyll Stomata CO2 O2 Chloroplast Mesophyll cell 5 µm Fig. 10-3b Chloroplast Outer membrane Thylakoid Intermembrane Stroma Granum Thylakoid space space Inner membrane 1 µm 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings 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 Fig. 10-4 Reactants: 6 CO2 12 H2O Products: C6H12O6 6 H2 O 6 O2 Photosynthesis as a Redox Process Photosynthesis is a redox process in which H2O is oxidized and CO2 is reduced 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  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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 10-5-4 H2O CO2 Light NADP+ ADP + P i Calvin Light Cycle Reactions ATP NADPH Chloroplast O2 [CH2O] (sugar) 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings 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 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 10-7 Light Reflected light Chloroplast Absorbed Granum light Transmitted light  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 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 10-11 Excited e– state Energy of electron Heat Photon (fluorescence) Photon Ground Chlorophyll state molecule (a) Excitation of isolated chlorophyll molecule (b) Fluorescence A Photosystem: A Reaction-Center Complex Associated with Light-Harvesting Complexes 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings  A primary 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 10-12 Photosystem STROMA Photon Light-harvesting Reaction-center Primary complexes complex electron acceptor Thylakoid membrane e– Transfer Special pair of Pigment of energy chlorophyll a molecules molecules THYLAKOID SPACE (INTERIOR OF THYLAKOID)  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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings  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 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings  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 Fig. 10-13-1 Primary acceptor 2 e– P680 1 Light Pigment molecules Photosystem II (PS II)  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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 10-13-2 Primary acceptor 2 H2O e– 2 H+ + 1/ O 3 2 2 e– e– P680 1 Light Pigment molecules Photosystem II (PS II)  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 Diffusion of H+ (protons) across the membrane drives ATP synthesis Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 10-13-3 Primary 4 acceptor Pq 2 H2O e– Cytochrome 2 H+ complex + 1/ O2 3 2 Pc e– e– P680 5 1 Light ATP Pigment molecules Photosystem II (PS II)  In PS I (like PS II), 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 10-13-4 Primary Primary 4 acceptor acceptor Pq e– 2 H2O e– Cytochrome 2 H+ complex + 1/ O2 3 2 Pc e– e– P700 P680 5 Light 1 Light 6 ATP Pigment molecules Photosystem I (PS I) Photosystem II (PS II)  Each electron “falls” down an electron transport chain from the primary electron acceptor of PS I 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 10-13-5 Primary Primary 4 acceptor 7 acceptor Fd Pq e– 2 e– 8 H2O e– e– NADP+ Cytochrome 2 H+ NADP+ + H+ complex + reductase 1/ O 3 NADPH 2 2 Pc e– e– P700 P680 5 Light 1 Light 6 ATP Pigment molecules Photosystem I (PS I) Photosystem II (PS II) Fig. 10-14 e– ATP e– e– NADPH e– e– e– Mill makes ATP e– Photosystem II Photosystem I 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 10-15 Primary Primary acceptor Fd acceptor Fd NADP+ Pq NADP+ + H+ reductase Cytochrome NADPH complex Pc Photosystem I Photosystem II ATP  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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings  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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 10-16 Mitochondrion Chloroplast MITOCHONDRION CHLOROPLAST STRUCTURE STRUCTURE H+ Diffusion Intermembrane Thylakoid space space Electron Inner Thylakoid transport membrane chain membrane ATP synthase Matrix Stroma Key ADP + P i ATP Higher [H+] H+ Lower [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 H2O to NADPH Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 10-17 STROMA (low H+ concentration) Cytochrome Photosystem II Photosystem I complex 4 H+ Light NADP+ Light reductase Fd 3 NADP+ + H+ Pq NADPH e– Pc e– 2 H2O THYLAKOID SPACE 1 1/ 2 O2 (high H+ concentration) +2 H+ 4 H+ To Calvin Cycle Thylakoid membrane ATP synthase STROMA ADP (low H+ concentration) + ATP Pi H+ 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings  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 Fig. 10-18-1 Input 3 (Entering one at a time) CO2 Phase 1: Carbon fixation Rubisco 3 P P Short-lived intermediate 3P P 6 P Ribulose bisphosphate 3-Phosphoglycerate (RuBP) Fig. 10-18-2 Input 3 (Entering one at a time) CO2 Phase 1: Carbon fixation Rubisco 3 P P Short-lived intermediate 3P P 6 P Ribulose bisphosphate 3-Phosphoglycerate (RuBP) 6 ATP 6 ADP Calvin Cycle 6 P P 1,3-Bisphosphoglycerate 6 NADPH 6 NADP+ 6 Pi 6 P Glyceraldehyde-3-phosphate Phase 2: (G3P) Reduction 1 P Glucose and Output G3P other organic (a sugar) compounds Fig. 10-18-3 Input 3 (Entering one at a time) CO2 Phase 1: Carbon fixation Rubisco 3 P P Short-lived intermediate 3P P 6 P Ribulose bisphosphate 3-Phosphoglycerate (RuBP) 6 ATP 6 ADP 3 ADP Calvin Cycle 6 P P 3 ATP 1,3-Bisphosphoglycerate 6 NADPH Phase 3: Regeneration of 6 NADP+ the CO2 acceptor 6 Pi (RuBP) 5 P G3P 6 P Glyceraldehyde-3-phosphate Phase 2: (G3P) Reduction 1 P Glucose and Output G3P other organic (a sugar) compounds 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 H2O but also limits photosynthesis 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 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings  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 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 The C4 pathway Mesophyll Mesophyll cell cell CO2 Photosynthetic PEP carboxylase cells of C4 Bundle- plant leaf sheath cell Oxaloacetate (4C) PEP (3C) Vein ADP (vascular tissue) Malate (4C) ATP Pyruvate (3C) Bundle- Stoma sheath CO2 cell Calvin Cycle Sugar 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 Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 10-20 Sugarcane Pineapple C4 CAM CO2 CO2 Mesophyll 1 CO2 incorporated Night cell Organic acid into four-carbon Organic acid organic acids (carbon fixation) Bundle- CO2 CO2 Day sheath cell 2 Organic acids Calvin release CO2 to Calvin Cycle Calvin cycle Cycle Sugar Sugar (a) Spatial separation of steps (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 O2 in our atmosphere Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 10-21 H2O CO2 Light NADP+ ADP + P i Light RuBP Reactions: 3-Phosphoglycerate Photosystem II Calvin Electron transport chain Cycle Photosystem I Electron transport chain ATP G3P Starch NADPH (storage) Chloroplast O2 Sucrose (export)