10 hotosynthesis CAMBELL BIOLOGY TENTH EDITION Reece Urry Cain Wasserman Minorsky Jackson Outline 1. hotosynthesis overview 2. and pigments 3. -dependent rxn 4. -independent rxn () 5. C3, C4 and CAM Lecture resentation by Dr Burns NVC Biol 120 Copyright 2014 earson 2009 Education, earson Education, Inc. Inc. The rocess That Feeds the Biosphere Autotrophs hotosynthesis 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 and other inorganic molecules Almost all plants are photoautotrophs, using the energy of sunlight to make organic molecules 3 4 hotosynthetic Organisms Figure 10.2 hotosynthesis occurs in plants, algae, certain other protists, and some prokaryotes These organisms feed not only themselves but also most of the living world (a) lants (b) Multicellular alga (c) Unicellular protists 10 m (d) Cyanobacteria 40 m 5 (e) urple sulfur bacteria 1 m 6 1
Heterotrophs 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 2 BioFlix: hotosynthesis 7 8 hotosynthesis 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 hotosynthesis in lants Leaves are the major locations of photosynthesis Their green color is from chlorophyll, the green pigment within chloroplasts Chloroplasts are found mainly in cells of the mesophyll layer, the interior tissue of the leaf Each mesophyll cell contains 30 40 chloroplasts 9 10 Chloroplast Structure Figure 10.4a Leaf cross section Chloroplasts Vein enters and O 2 exits the leaf through microscopic pores called stomata 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 interior fluid Mesophyll Chloroplast Stomata O 2 Mesophyll cell 11 20 12 m 2
Figure 10.4b Chloroplast Stroma Thylakoid Granum Thylakoid space Outer membrane Intermembrane space Inner membrane 1 m 14 What is the stomata? Chlorophyll a is found in the: 1. The watery matrix of the chloroplast 2. ores that let in 3. A stack of thylakoids 33% 33% 33% 1. Thylakoid membrane 2. Thylakoid lumen 3. Stroma 33% 33% 33% The watery matrix of... ores that let CO2 in A stack of thylakoids Thylakoid membrane Thylakoid lumen Stroma Tracking Atoms Through hotosynthesis The Splitting of Water hotosynthesis is a complex series of reactions that can be summarized as the following overall equation: Chloroplasts split H 2 O into hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules and releasing oxygen as a by-product 6 + 12 H 2 O + energy C 6 H 12 O 6 + 6 O 2 + 6 H 2 O 17 18 3
Figure 10.5 hotosynthesis as a Redox rocess Reactants: roducts: 6 12 H 2 O C 6 H 12 O 6 6 H 2 O 6 O 2 hotosynthesis reverses the direction of electron flow compared to respiration hotosynthesis is a redox process in which H 2 O is oxidized and is reduced hotosynthesis is an endergonic process; the energy boost is provided by light 19 20 Figure 10.UN01 hotosynthesis two stages becomes reduced Energy 6 6 H 2 O C 6 H 12 O 6 6 O 2 becomes oxidized 1. dependent stage: Happens in the thylakoids energy is converted to chemical energy 2. independent reactions ( ): Happens in the stroma Carbon is fixed from to glucose 21 The Two Stages of hotosynthesis: An Overview The Two Stages of hotosynthesis: An Overview hotosynthesis consists of the light reactions (the photo part) and cycle (the synthesis part) The light reactions (in the thylakoids) Split H 2 O Release O 2 Reduce NAD + to NADH Generate from AD by photophosphorylation The cycle (in the stroma) forms sugar from, using and NADH The cycle begins with carbon fixation, incorporating into organic molecules 23 24 4
Figure 10.6-1 Figure 10.6-2 H 2 O H 2 O AD + i AD + i Reactions Reactions NADH Chloroplast Chloroplast 25 O 2 26 Figure 10.6-3 Figure 10.6-4 H 2 O H 2 O Reactions AD + i Reactions AD + i NADH NADH Chloroplast Chloroplast O 2 27 O 2 [CH 2 O] (sugar) 28 The light reactions convert solar energy to the chemical energy of and NADH Figure 10.1 Chloroplasts are solar-powered chemical factories Their thylakoids transform light energy into the chemical energy of and NADH 29 30 5
The Nature of Sunlight 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 roperties of 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 also behaves as though it consists of discrete particles, called photons The shorter wavelengths have higher energy than longer wavelengths. 31 32 Figure 10.7 1 m 10 5 nm 10 3 nm 1 nm 10 3 nm 10 6 nm (10 9 nm) 10 3 m hotosynthetic igments: The Receptors Gamma rays X-rays UV Infrared Microwaves Radio waves igments are substances that absorb visible light Different pigments absorb different wavelengths Wavelengths that are not absorbed are reflected or transmitted Visible light Leaves appear green because chlorophyll reflects and green light 380 450 500 550 600 650 700 750 nm Shorter wavelength Longer wavelength Higher energy Lower energy 33 34 Figure 10.8 Chloroplast Reflected light Absorbed light Granum Animation: and igments Right-click slide / select lay 35 Transmitted light 36 6
Absorption of light by chloroplast pigments Rate of photosynthesis (measured by O 2 release) 4/19/2015 Spectrophotometers A spectrophotometer measures a pigment s ability to absorb various wavelengths Figure 10.9 TECHNIQUE White light Refracting prism Chlorophyll solution hotoelectric tube Galvanometer 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 High transmittance (low absorption): Chlorophyll absorbs very little green light. 37 Blue light Low transmittance (high absorption): Chlorophyll absorbs most blue light. 38 Absorption spectrum Figure 10.10 RESULTS Chlorophyll a Chlorophyll b 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 (a) Absorption spectra (b) Action spectrum Carotenoids 400 500 600 700 Wavelength of light (nm) 400 500 600 700 Aerobic bacteria Filament of alga 39 (c) Engelmann s experiment 40 400 500 600 700 hotosynthetic igments Chlorophyll a is the main photosynthetic pigment Figure 10.11 CH 3 CH 3 in chlorophyll a CHO in chlorophyll b orphyrin ring Accessory pigments, such as chlorophyll b, broaden the spectrum used for photosynthesis Accessory pigments called carotenoids absorb excessive light that would damage chlorophyll Hydrocarbon tail (H atoms not shown) 41 42 7
Thylakoid membrane Thylakoid membrane Energy of electron Thylakoid membrane Thylakoid membrane 4/19/2015 Excitation of Chlorophyll by Figure 10.12 When a pigment absorbs light, it goes from a ground state to an excited state, which is unstable Excited state When excited electrons fall back to the ground state, photons are given off, an afterglow called fluorescence Heat If illuminated, an isolated solution of chlorophyll will fluoresce, giving off light and heat hoton Chlorophyll molecule hoton (fluorescence) Ground state (a) Excitation of isolated chlorophyll molecule (b) Fluorescence 43 44 A hotosystem: A Reaction-Center Complex Associated with -Harvesting Complexes Figure 10.13 A photosystem consists of a reaction-center complex (a type of protein complex) surrounded by light-harvesting complexes hoton hotosystem harvestincenter Reaction- complexes complex STROMA electron Chlorophyll STROMA The light-harvesting complexes (pigment molecules bound to proteins) transfer the energy of photons to the reaction center Transfer of energy Special pair of igment chlorophyll a molecules molecules THYLAKOID SACE (INTERIOR OF THYLAKOID) rotein subunits THYLAKOID SACE (a) How a photosystem harvests light (b) Structure of photosystem II 45 46 Figure 10.13a hoton harvesting complexes hotosystem Reactioncenter complex STROMA electron Figure 10.13b Chlorophyll STROMA Transfer of energy Special pair of chlorophyll a molecules (a) How a photosystem harvests light igment molecules THYLAKOID SACE (INTERIOR OF THYLAKOID) 47 rotein subunits (b) Structure of photosystem II THYLAKOID SACE 48 8
hotosystems A primary electron in the reaction center accepts excited electrons from the reaction center 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 hotosystem II (S 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 S II is called 680 49 50 hotosystems Linear Electron Flow hotosystem I (S I) is best at absorbing a wavelength of 700 nm The reaction-center chlorophyll a of S I is called 700 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 NADH using light energy 51 52 Linear Electron Flow Figure 10.14-1 1. A photon hits a pigment and its energy is passed among pigment molecules until it excites 680 2 2. An excited electron from 680 is transferred to the primary electron (we now call it 680 + ) 1 680 igment molecules hotosystem II (S II) 53 54 9
Linear Electron Flow Figure 10.14-2 680 + is a very strong oxidizing agent 3. H 2 O is split by enzymes, and the electrons are transferred from the hydrogen atoms to 680 +, thus reducing it to 680 H 2 H 2O + 1 / O 3 2 2 1 2 680 O 2 is released as a by-product of this reaction igment molecules hotosystem II (S II) 55 56 Linear Electron Flow Figure 10.14-3 4. Each electron falls down an electron transport chain from the primary electron of S II to S I 5. Energy released by the fall drives the creation of a proton gradient across the thylakoid membrane 2 H e 2 H 2O + 1 / 2 O 3 2 680 1 q 4 Cytochrome complex 5 c Diffusion of H + (protons) across the membrane drives synthesis via synthase hotosystem II (S II) igment molecules 57 58 Linear Electron Flow Figure 10.14-4 6. In S I (like S II), transferred light energy excites 700, which loses an electron to an electron 2 H + 1 / 2 O 2 3 H 2O 2 q 4 Cytochrome complex c 700 + (700 that is missing an electron) accepts an electron passed down from S II via the electron transport chain 1 680 5 700 6 hotosystem II (S II) igment molecules hotosystem I (S I) 59 60 10
Linear Electron Flow Figure 10.14-5 7. Each electron falls down an electron transport chain from the primary electron of S I to the protein ferredoxin (Fd) 8. The electrons are then transferred to NAD + and reduce it to NADH 2 H + 1 / 2 O 2 2 H e 2O 3 680 q 4 Cytochrome complex 5 c 700 7 Fd 8 reductase + H NADH The electrons of NADH are available for the reactions of the cycle 1 6 This process also removes an H + from the stroma hotosystem II (S II) igment molecules hotosystem I (S I) 61 62 Figure 10.15 A Comparison of Chemiosmosis in Chloroplasts and Mitochondria hotosystem II Mill makes hotosystem I NADH 63 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 64 A Comparison of Chemiosmosis in Chloroplasts and Mitochondria Figure 10.17 Mitochondrion Chloroplast In mitochondria, protons are pumped to the intermembrane space and drive synthesis as they diffuse back into the mitochondrial lumen In chloroplasts, protons are pumped into the thylakoid space and drive synthesis as they diffuse back into the stroma MITOCHONDRION STRUCTURE Intermembrane space Inner membrane Matrix Electron transport chain H synthase Diffusion CHLOROLAST STRUCTURE Thylakoid space Thylakoid membrane Stroma 65 Key Higher [H ] Lower [H ] AD i H 66 11
Figure 10.18 and NADH are produced on the side facing the stroma, where the cycle takes place In summary, light reactions generate and increase the potential energy of electrons by moving them from H 2 O to NADH STROMA (low H concentration) H 2O 1 1 / 2 O 2 THYLAKOID SACE +2 H + (high H concentration) STROMA (low H concentration) hotosystem II 4 H + Thylakoid membrane Cytochrome complex hotosystem I q 2 4 H + c synthase AD + i H + reductase 3 + H Fd NADH To 67 68 Figure 10.UN08 Cyclic Electron Flow Cyclic electron flow uses only photosystem I and produces, but not NADH No oxygen is released Cyclic electron flow generates surplus, satisfying the higher demand in the cycle Occurs when there is a build up of NADH 69 70 Figure 10.16 Cyclic Electron Flow q Fd Cytochrome complex Fd reductase + H NADH Some organisms such as purple sulfur bacteria have S I but not S II Cyclic electron flow is thought to have evolved before linear electron flow hotosystem II c hotosystem I Cyclic electron flow may protect cells from light-induced damage 71 72 12
The wavelength of light that the chlorophyll a pigment found at the reaction center in photosystem I absorbs is: The electron donor for photosystem II is: 1. 660 nm 2. 700 nm 3. 680 nm 4. 800 nm 25% 25% 25% 25% 1. NADH 2. Water 3. NADH 4. Oxygen 25% 25% 25% 25% 660 nm 700 nm 680 nm 800 nm NADH Water NADH Oxygen The final electron from photosystem I is: The cycle uses the chemical energy of and NADH to reduce to sugar 1. NAD+ 2. Water 3. NAD+ 4. Oxygen 25% 25% 25% 25% The 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 and the reducing power of electrons carried by NADH NAD+ Water NAD+ Oxygen 76 Takes place in the stroma of the chloroplasts Needs: 18, 12 NADH, 6 Carbon dioxide ( ) to make 1 glucose 6 molecules of 6 molecules of ribulose bisphosphate (RuB) 1 uptake 6 AD phase 6 molecules of ribulose CALVIN phosphate (R) CYCLE 3 2 RuB Carbon regeneration reduction phase phase 10 molecules of G3 Glucose and other carbohydrate synthesis 12 molecules of glyceraldehyde-3- phosphate (G3) Through a series of 2 molecules reactions G3 is of glyceraldehyde-3- rearranged into new phosphate (G3) RuB molecules or another sugar molecules are captured by RuB, resulting in an unstable intermediate that is immediately broken apart into 2 GA 12 molecules of phosphoglycerate (GA) 12 12 AD 12 NADH 12 NAD + GA is phosphorylated by and reduced by NADH. Removal of a phosphate results in formation of G3. 13
The The Carbon enters the cycle as and leaves as a sugar named glyceraldehyde 3-phospate (G3) For net synthesis of 1 G3, the cycle must take place three times, fixing 3 molecules of For the synthesis of 1 glucose, the cycle must take place six times, Fixing 6 molecules of The cycle has three phases Carbon fixation (catalyzed by rubisco) Reduction Regeneration of the (RuB) 79 80 Figure 10.19-1 Input Figure 10.19-2 Input 3 (Entering one at a time) 3 (Entering one at a time) hase 1: Carbon fixation hase 1: Carbon fixation Rubisco Rubisco 3 Short-lived intermediate 3 Short-lived intermediate 3 Ribulose bisphosphate (RuB) 6 3-hosphoglycerate 3 Ribulose bisphosphate (RuB) 6 3-hosphoglycerate 6 6 AD 6 1,3-Bisphosphoglycerate 6 NADH 6 6 i 6 Glyceraldehyde 3-phosphate (G3) hase 2: Reduction 81 1 G3 (a sugar) Output Glucose and other organic compounds 82 Figure 10.19-3 Input 3 (Entering one at a time) 1 st step: Carbon uptake = carbon fixation 3 Ribulose bisphosphate (RuB) 3 AD 3 hase 3: Regeneration of the (RuB) 5 G3 hase 1: Carbon fixation Rubisco 3 Short-lived intermediate 6 3-hosphoglycerate 6 6 AD 6 1,3-Bisphosphoglycerate 6 NADH 6 6 i 6 Glyceraldehyde 3-phosphate hase 2: (G3) Reduction and a phosphorylated five carbon molecule, ribulose bisphosphate (RuB) combines with the help of an enzyme, rubisco, to form a six carbon molecule that spontaneously splits into two 3 carbon molecules, phosphoglycerate (GA) rubisco + RuB 2 GA 1 G3 (a sugar) Output Glucose and other organic compounds 83 14
2 cd Step: Carbon Reduction hosphoglycerate (GA) is converted to glyceraldehyde-3-phosphate (G3). is used to phosphorylate GA and NADH is used to reduce GA 3 molecules, 9 and 6 NADH are needed for the cycle to produce 1 glyceraldehyde-3-phosphate (G3) This needs to happen 2 times to make a glucose molecule So 6 molecules, 18 and 12 NADH are needed for the cycle to produce 1 glucose During the light dependent reactions... 1. Carbon is fixed 2. H2O is split to produce oxygen 3. NADH and are produced 4. 2 and 3 25% 25% 25% 25% http://www.youtube.com/watch?v=q_1mx ZdF2TY http://www.youtube.com/watch?v=oysd1j OD1dQ&feature=related Carbon is fixed H2O is split to produ.. NADH and are... 2 and 3 15
lants have mitochondria lants carry out cellular respiration 1. True 2. False 50% 50% 1. True 2. False 50% 50% True False True False 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 H 2 O but also limits photosynthesis The closing of stomata reduces access to and causes O 2 to build up These conditions favor an apparently wasteful process called photorespiration hotorespiration: An Evolutionary Relic? In most plants, C 3 plants, initial fixation of, via rubisco, forms a three-carbon compound (3- phosphoglycerate) In photorespiration, rubisco adds O 2 instead of in the cycle, producing a two-carbon compound hotorespiration consumes O 2 and organic fuel and releases without producing or sugar 93 94 hotorespiration C 4 lants hotorespiration may be an evolutionary relic because rubisco first evolved at a time when the atmosphere had far less O 2 and more hotorespiration limits damaging products of light reactions that build up in the absence of the 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 cycle C 4 plants minimize the cost of photorespiration by incorporating into four-carbon compounds in mesophyll cells This step requires the enzyme E carboxylase E carboxylase has a higher affinity for than rubisco does; it can fix even when concentrations are low These four-carbon compounds are exported to bundle-sheath cells, where they release that is then used in the cycle 95 96 16
Figure 10.20 Figure 10.20a hotosynthetic cells of C 4 plant leaf Mesophyll cell Bundlesheath cell Vein (vascular tissue) C 4 leaf anatomy Stoma The C 4 pathway Mesophyll cell E carboxylase Oxaloacetate (4C) E (3C) AD Bundlesheath cell Malate (4C) yruvate (3C) hotosynthetic cells of C 4 plant leaf Mesophyll cell Bundlesheath cell Vein (vascular tissue) C 4 leaf anatomy Sugar Vascular tissue Stoma 97 98 Figure 10.20b The C 4 pathway Mesophyll cell E carboxylase Oxaloacetate (4C) Bundlesheath cell Malate (4C) E (3C) AD yruvate (3C) Sugar CAM lants Some plants, including succulents, use crassulacean acid metabolism (CAM) to fix carbon CAM plants open their stomata at night, incorporating into organic acids Stomata close during the day, and is released from organic acids and used in the cycle Vascular tissue 99 100 Figure 10.21 The Importance of hotosynthesis: A Review The energy entering chloroplasts as sunlight gets stored as chemical energy in organic compounds Sugarcane Mesophyll cell Bundlesheath cell C 4 Organic acid CO2 1 ineapple incorporated (carbon fixation) 2 released to the cycle CAM Organic acid Night Day Sugar made in the chloroplasts supplies chemical energy and carbon skeletons to synthesize the organic molecules of cells lants store excess sugar as starch in structures such as roots, tubers, seeds, and fruits In addition to food production, photosynthesis produces the O 2 in our atmosphere Sugar (a) Spatial separation of steps Sugar 101 (b) Temporal separation of steps 102 17
Figure 10.22 Figure 10.UN02 H 2 O Reactions: hotosystem II Electron transport chain hotosystem I Electron transport chain Chloroplast AD + i RuB NADH 3-hosphoglycerate G3 Starch (storage) O 2 H 2 O hotosystem II q Cytochrome complex c hotosystem I Fd reductase + H NADH O 2 Sucrose (export) 103 104 Figure 10.UN03 3 Important Concepts Carbon fixation Know the vocabulary in this lecture Regeneration of 3 5C 6 3C 5 3C Reduction Know the overall reaction of photosynthesis, what is being reduced and oxidized Understand the light spectrum, what wavelengths and colors are absorbed by chlorohyll, know the pigments involved in photosynthesis, the general structure of chlorophyll, know which wavelengths have higher energy (longer or shorter) 1 G3 (3C) 105 Important Concepts Know the leaf structure and the functions of the components including the Stomata, veins, Chloroplasts, mesophyll layer Know the chloroplast structure Where do the two stages of photosynthesis (light dependent and light independent) take place Know what happens during the light dependent stage, including what is needed, what is produced, what molecules are donating and accepting electrons, who do they give them to or get them from? Important Concepts Understand the role of the reaction centers, photosystem I, photosystem II, the electron transport chains in the light dependent reaction, know the differences between photosystem I and II Understand how is produced during the light dependent stage, and how NADH is produced. Know the difference between production in the mitochondria and in the chloroplast Understand the cyclic electron flow, when does this happen and what are the benefits? 18
Important Concepts Important Concepts Know what happens during the light independent () stage, know what is needed and what is produced, including: 2 glyceraldehyde-3- phosphate (G3) which are made into one glucose. Know the three stages of the cycle and what goes into and out of each stage and what is the purpose of the stage Know that you need 18, 12 NADH, and 6CO2 to make one glucose molecule in the calvin cycle Know the role of the enzymes rubisco and E carboxylase Know what C3 plants are and how they fix carbon and know the C4 and CAM adaptations. What photorespiration? 19