10 Photosynthesis CAMPBELL BIOLOGY TENTH EDITION Reece Urry Cain Wasserman Minorsky Jackson Lecture Presentation by Nicole Tunbridge and Kathleen Fitzpatrick energy ECOSYSTEM CO 2 H 2 O Organic O 2 powers most cellular work Heat energy Concept 10.1: Photosynthesis converts light energy to the chemical energy of food energy ECOSYSTEM Anabolic pathways endergonic CO 2 H 2 O Photosynthesis in chloroplasts Organic O 2 Autotrophs = producers of the biosphere Photoautotrophs= energy users powers most cellular work Heat energy 1
10 μm 1 μm Figure 10.2 (a) Plants (d) Cyanobacteria 40 μm photosynthetic bacteria (b) Multicellular alga (e) Purple sulfur bacteria (c) Unicellular eukaryotes Figure 10.1a Heterotrophs = consumers of the biosphere Figure 10.4a Leaves are the major locations of photosynthesis interior tissue of the leaf Mesophyll Leaf cross section Chloroplasts Vein Stomata Chloroplast CO 2 O 2 Mesophyll cell 20 μm 2
Figure 10.4b photosynthetic bacteria (in membrane) Chloroplast 1) Outer membrane Stroma Granum space Intermembrane space 2) Inner membrane structural organization of chloroplast allows for the chemical reactions of photosynthesis 1 μm 3) s are connected sacs in the chloroplast which compose a third membrane system becomes oxidized becomes reduced Summary Equation: Cellular respiration During cellular respiration, the food (such as glucose) is oxidized, and O 2 is reduced O 2 H = H 2 O H 2 O CO 2 2) Calvin cycle (the synthesis part) -in the stroma Stroma Chloroplast LIGHT REACTIONS O 2 ADP P i CALVIN CYCLE [CH 2 O] (sugar) 1) forms sugar from CO 2, 2) using and 3) Calvin cycle begins with carbon fixation, incorporating CO 2 into organic 3
Figure 10.UN01 becomes reduced becomes oxidized Summary Equation: Photosynthesis redox process in which H 2 O is oxidized and CO 2 is reduced overall chemical change during photosynthesis is the reverse of the one that occurs during cellular respiration Figure 10.5 Reactants: Products: 6 CO 2 12 H 2 O C 6 H 12 O 6 6 H 2 O 6 O 2 split H 2 O into hydrogen and oxygen Photosynthesis as a Redox Process Photosynthesis reverses the direction of electron flow compared to respiration Photosynthesis is a redox process in which H 2 O is oxidized and CO 2 is reduced Photosynthesis is an endergonic process; the energy boost is provided by light 4
Figure 10.6-2 H 2 O 1) light reactions (the photo part) LIGHT REACTIONS Stroma ADP P i -in the thylakoids 1) Split H 2 O to Release O 2 2) Reduce the electron to 3) Generate from ADP by photophosphorylation Chloroplast O 2 High rate of photosynthesis = more O 2 H 2 O CO 2 2) Calvin cycle (the synthesis part) -in the stroma Stroma Chloroplast LIGHT REACTIONS O 2 ADP P i CALVIN CYCLE [CH 2 O] (sugar) 1) forms sugar from CO 2, 2) using and 3) Calvin cycle begins with carbon fixation, incorporating CO 2 into organic BioFlix: Photosynthesis 5
Concept 10.2: The light reactions convert solar energy to the chemical energy of and Chloroplasts are solar-powered chemical factories Their thylakoids transform light energy into the chemical energy of and Leaves appear green because chlorophyll reflects and transmits green light Figure 10.10c Aerobic bacteria Filament of alga O 2 High rate of photosynthesis = more O 2 An action spectrum profiles the relative effectiveness of different wavelengths of radiation in driving a process 400 500 600 700 (c) Engelmann s experiment 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 2 He used the growth of aerobic bacteria clustered along the alga as a measure of O 2 production Figure 10.11 CH 3 CH3 in chlorophyll a CHO 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 6
Energy of electron Excitation of Chlorophyll by 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 Figure 10.12 Excited state If illuminated, an isolated solution of chlorophyll will fluoresce, giving off light and heat Heat Photon Chlorophyll molecule Photon (fluorescence) Ground state (a) Excitation of isolated chlorophyll molecule (b) Fluorescence A Photosystem: A Reaction-Center Complex Associated with -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 bound to proteins) transfer the energy of photons to the reaction center 7
membrane Figure 10.13a Photosystem Photon harvesting complexes Reactioncenter complex STROMA electron Transfer of energy Special pair of chlorophyll a THYLAKOID SPACE (INTERIOR OF THYLAKOID) (a) How a photosystem harvests light Pigment A primary electron in the reaction center accepts excited electrons and is reduced as a result Solar-powered transfer of an electron from a chlorophyll a molecule to the primary electron is the first step of the light reactions There are two types of photosystems in the thylakoid membrane (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 8
Photosystem I (PS I) is best at absorbing a wavelength of 700 nm The reaction-center chlorophyll a of PS I 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 and using light energy Figure 10.UN02 H 2 S H 2 O CO 2 LIGHT REACTIONS ADP CALVIN CYCLE S O 2 [CH 2 O] (sugar) 9
Figure 10.14-1 2 1 P680 (PS II) Pigment There are 8 steps in linear electron flow: 1. A photon hits a pigment and its energy is passed among pigment until it excites P680 2. An excited electron from P680 is transferred to the primary electron (we now call it P680 ) From which molecule or photosystem II needs replacement electrons and take it from water 2 H H 2O O 3 ½ 2 1 2 P680 (PS II) Pigment 3. H 2 O is split by enzymes, and the electrons are transferred from the hydrogen atoms to P680 O 2 is released as a byproduct of this reaction at PSII 10
2 H H 2O O 3 ½ 2 1 2 P680 4 Electron transport chain Pq Cytochrome complex 5 Pc 4. Each electron falls down an electron transport chain from the primary electron of PS II to PS I 5. Energy released by the fall drives the creation of a proton gradient across the thylakoid membrane (PS II) Pigment Diffusion of H (protons) across the membrane drives synthesis Figure 10.18 4 H Cytochrome complex Photosystem I Fd reductase 3 H Electron flow Pq e e 2 H 2O THYLAKOID SPACE 1 ½ O 2 2 H 4 H (high H concentration) Pc To Calvin Cycle STROMA (low H concentration) membrane synthase ADP P i H Figure 10.14-4 2 H H 2O O 3 ½ 2 1 2 P680 4 (PS II) Electron transport chain Pq Cytochrome complex 5 Pigment Pc P700 Photosystem I (PS I) 6 6. In PS I (like PS II), transferred light energy excites P700, which loses an electron to an electron 11
Figure 10.14-5 2 H H 2O O 3 ½ 2 1 2 P680 4 (PS II) Electron transport chain Pq Cytochrome complex 5 Pigment Pc P700 Photosystem I (PS I) 7 Fd Electron transport chain 8 reductase H 7. Each electron falls down an 6 electron transport chain 8. The electrons are then transferred to and reduce it to 24) Which of the following are directly associated with photosystem I? A) receiving electrons from the thylakoid membrane electron transport chain B) generation of molecular oxygen C) extraction of hydrogen electrons from the splitting of water D) passing electrons to the cytochrome complex 24) Which of the following are directly associated with photosystem I? A) receiving electrons from the thylakoid membrane electron transport chain B) generation of molecular oxygen C) extraction of hydrogen electrons from the splitting of water D) passing electrons to the cytochrome complex 12
21) The final electron associated with photosystem I is. A) oxygen B) water C) NADP D) 21) The final electron associated with photosystem I is. A) oxygen B) water C) NADP D) Figure 10.15 Mill makes Photosystem I 13
Figure 10.14-5 2 H H 2O O 3 ½ 2 1 2 P680 4 (PS II) Electron transport chain Pq Cytochrome complex 5 Pigment Pc P700 Photosystem I (PS I) 7 Fd Electron transport chain 8 reductase H 7. Each electron falls down an 6 electron transport chain 8. The electrons are then transferred to and reduce it to Figure 10.16 Cyclic Electron Flow O 2 Pq Fd Cytochrome complex Fd reductase H Pc Photosystem I made But no and no O 2 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 14
A Comparison of Chemiosmosis in Chloroplasts and Mitochondria Chloroplasts and mitochondria generate by chemiosmosis, but use different sources of energy Mitochondria transfer chemical energy from food to ; chloroplasts transform light energy into the chemical energy of Spatial organization of chemiosmosis differs between chloroplasts and mitochondria but also shows similarities In mitochondria, protons are pumped to the intermembrane space and drive synthesis as they diffuse back into the mitochondrial matrix In chloroplasts, protons are pumped into the thylakoid space and drive synthesis as they diffuse back into the stroma Figure 10.17 MITOCHONDRION STRUCTURE Intermembrane space Inner membrane Electron transport chain H Diffusion CHLOROPLAST STRUCTURE space membrane Key Matrix Higher [H ] Lower [H ] synthase ADP P i H Stroma 15
and are produced on the side facing the stroma, where the Calvin cycle takes place In summary, light reactions generate and increase the potential energy of electrons by moving them from H 2 O to Figure 10.18 4 H Cytochrome complex Photosystem I Fd reductase 3 H Pq e e 2 H 2O THYLAKOID SPACE 1 ½ O 2 2 H 4 H (high H concentration) Pc To Calvin Cycle STROMA (low H concentration) membrane synthase ADP P i H 16