8 Photosynthesis

Leaf cross section Chloroplasts Vein Mesophyll Stomata CO2 O2 Figure 8.3 Leaf cross section Chloroplasts Vein Mesophyll Stomata CO2 O2 Chloroplast Mesophyll cell Figure 8.3 Zooming in on the location of photosynthesis in a plant Outer membrane Thylakoid Granum Thylakoid space Intermembrane space Stroma 20 m Inner membrane 1 m 2

Calvin Cycle Light Reactions [CH2O] (sugar) Figure 8.5-4 H2O CO2 Light NADP ADP  P i Calvin Cycle Light Reactions ATP Figure 8.5-4 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle (step 4) NADPH Chloroplast [CH2O] (sugar) O2 3

Wavelength of light (nm) (a) Absorption spectra Figure 8.9 Results Chloro- phyll a Chlorophyll b Absorption of light by chloroplast pigments Carotenoids 400 500 600 700 Wavelength of light (nm) (a) Absorption spectra (measured by O2 photosynthesis Rate of release) 400 500 600 700 Figure 8.9 Inquiry: which wavelengths of light are most effective in driving photosynthesis? (b) Action spectrum Aerobic bacteria Filament of alga 400 500 600 700 (c) Engelmann’s experiment 4

interacts with hydrophobic regions of proteins inside Figure 8.10 CH3 in chlorophyll a CH3 CHO in chlorophyll b Porphyrin ring: light-absorbing “head” of molecule; note magnesium atom at center Figure 8.10 Structure of chlorophyll molecules in chloroplasts of plants Hydrocarbon tail: interacts with hydrophobic regions of proteins inside thylakoid membranes of chloroplasts; H atoms not shown 5

(a) Excitation of isolated chlorophyll molecule (b) Fluorescence Figure 8.11 Excited state e− Heat Energy of electron Photon (fluorescence) Photon Figure 8.11 Excitation of isolated chlorophyll by light Ground state Chlorophyll molecule (a) Excitation of isolated chlorophyll molecule (b) Fluorescence 6

(b) Structure of a photosystem Figure 8.12b Chlorophyll STROMA Thylakoid membrane Protein subunits THYLAKOID SPACE Figure 8.12b The structure and function of a photosystem (part 2) (b) Structure of a photosystem 7

Photosystem I (PS I) Photosystem II (PS II) Figure 8.13-4 4 Electron transport chain Primary acceptor Primary acceptor Pq e− 2 2 H H2O e− Cytochrome complex   2 1 3 O2 Pc e− P700 e− P680 1 5 Light Light 6 ATP Figure 8.13-4 How linear electron flow during the light reactions generates ATP and NADPH (step 6) Pigment molecules Photosystem I (PS I) Photosystem II (PS II) 8

Mill makes ATP NADPH Photosystem II Photosystem I Photon Photon Figure 8.14 Mill makes ATP NADPH Photon Figure 8.14 A mechanical analogy for linear electron flow during the light reactions Photon Photosystem II Photosystem I 9

Electron transport chain ATP synthase Figure 8.15 MITOCHONDRION STRUCTURE CHLOROPLAST STRUCTURE Inter- membrane space H Diffusion Thylakoid space Electron transport chain Inner membrane Thylakoid membrane Figure 8.15 Comparison of chemiosmosis in mitochondria and chloroplasts ATP synthase Matrix Stroma Key ADP  P i ATP Higher [H] H Lower [H] 10

Cytochrome complex NADP reductase To Calvin Cycle ATP synthase Figure 8.16 Cytochrome complex NADP reductase Photosystem II Photosystem I Light 4 H 3 Light NADP  H Fd Pq NADPH e− Pc e− 2 H2O 1  2 1 O2 THYLAKOID SPACE (high H concentration) 2 H 4 H To Calvin Cycle Figure 8.16 The light reactions and chemiosmosis: the organization of the thylakoid membrane Thylakoid membrane ATP synthase STROMA (low H concentration) ADP  ATP P i H 11

Phase 1: Carbon fixation Rubisco Figure 8.17-3 Input 3 as 3 CO2 Phase 1: Carbon fixation Rubisco 3 P P 3 P P 6 P RuBP 3-Phosphoglycerate 6 ATP 6 ADP 3 ADP Calvin Cycle 6 P P 3 ATP 1,3-Bisphosphoglycerate 6 NADPH Phase 3: Regeneration of RuBP 6 NADP 6 P i 5 P Figure 8.17-3 The Calvin cycle (step 3) G3P 6 P G3P Phase 2: Reduction Glucose and other organic compounds 1 P G3P Output 12

Calvin Cycle Calvin Cycle Figure 8.18 Sugarcane Pineapple 1 1 CO2 CO2 C4 CAM Mesophyll cell Organic acid Organic acid Night CO2 2 CO2 2 Figure 8.18 C4 and CAM photosynthesis compared Bundle- sheath cell Day Calvin Cycle Calvin Cycle Sugar Sugar (a) Spatial separation of steps (b) Temporal separation of steps 13

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 © 2014 Pearson Education, Inc. 14

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 © 2014 Pearson Education, Inc. 15

Figure 8.1 Figure 8.1 How can sunlight, seen here as a spectrum of colors in a rainbow, power the synthesis of organic substances? 16

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 © 2014 Pearson Education, Inc. 17

Photosynthesis occurs in plants, algae, certain other protists, and some prokaryotes These organisms feed not only themselves but also most of the living world © 2014 Pearson Education, Inc. 18

(c) Unicellular eukaryotes Figure 8.2 40 m (d) Cyanobacteria (a) Plants Figure 8.2 Photoautotrophs (b) Multicellular alga 1 m (e) Purple sulfur bacteria 10 m (c) Unicellular eukaryotes 19

Figure 8.2a Figure 8.2a Photoautotrophs (part 1: plants) (a) Plants 20

(b) Multicellular alga Figure 8.2b Figure 8.2b Photoautotrophs (part 2: multicellular alga) (b) Multicellular alga 21

(c) Unicellular eukaryotes Figure 8.2c 10 m Figure 8.2c Photoautotrophs (part 3: unicellular eukaryotes) (c) Unicellular eukaryotes 22

40 m (d) Cyanobacteria Figure 8.2d Figure 8.2d Photoautotrophs (part 4: cyanobacteria) (d) Cyanobacteria 23

1 m (e) Purple sulfur bacteria Figure 8.2e Figure 8.2e Photoautotrophs (part 5: purple sulfur bacteria) (e) Purple sulfur bacteria 24

Concept 8.1: Photosynthesis converts light energy to the chemical energy of food The structural organization of photosynthetic cells includes enzymes and other molecules grouped together in a membrane This organization allows for the chemical reactions of photosynthesis to proceed efficiently Chloroplasts are structurally similar to and likely evolved from photosynthetic bacteria © 2014 Pearson Education, Inc. 25

Leaf cross section Chloroplasts Vein Mesophyll Stomata CO2 O2 Figure 8.3a Leaf cross section Chloroplasts Vein Mesophyll Figure 8.3a Zooming in on the location of photosynthesis in a plant (part 1: leaf cross section) Stomata CO2 O2 26

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 Chloroplasts are found mainly in cells of the mesophyll, the interior tissue of the leaf Each mesophyll cell contains 30–40 chloroplasts © 2014 Pearson Education, Inc. 27

Chloroplast Mesophyll cell Outer Thylakoid membrane Granum Thylakoid Figure 8.3b Chloroplast Mesophyll cell Outer membrane Thylakoid Granum Thylakoid space Intermembrane space Stroma 20 m Inner membrane Figure 8.3b Zooming in on the location of photosynthesis in a plant (part 2: chloroplast) 1 m 28

Chloroplasts also contain stroma, a dense interior fluid CO2 enters and O2 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 © 2014 Pearson Education, Inc. 29

Mesophyll cell 20 m Figure 8.3c Figure 8.3c Zooming in on the location of photosynthesis in a plant (part 3: mesophyll cell, LM) 20 m 30

Granum Stroma 1 m Figure 8.3d Figure 8.3d Zooming in on the location of photosynthesis in a plant (part 4: chloroplast, TEM) 1 m 31

Tracking Atoms Through Photosynthesis: Scientific Inquiry Photosynthesis is a complex series of reactions that can be summarized as the following equation 6 CO2  12 H2O  Light energy  C6H12O6  6 O2  6 H2O © 2014 Pearson Education, Inc. 32

The Splitting of Water Chloroplasts split H2O into hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules and releasing oxygen as a by-product © 2014 Pearson Education, Inc. 33

Reactants: 6 CO2 12 H2O Products: C6H12O6 6 H2O 6 O2 Figure 8.4 Figure 8.4 Tracking atoms through photosynthesis 34

Photosynthesis as a Redox Process Photosynthesis reverses the direction of electron flow compared to respiration Photosynthesis is a redox process in which H2O is oxidized and CO2 is reduced Photosynthesis is an endergonic process; the energy boost is provided by light © 2014 Pearson Education, Inc. 35

becomes reduced becomes oxidized Figure 8.UN01 Figure 8.UN01 In-text figure, photosynthesis equation, p. 158 36

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 the electron acceptor, NADP, to NADPH Generate ATP from ADP by adding a phosphate group, photophosphorylation © 2014 Pearson Education, Inc. 37

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 © 2014 Pearson Education, Inc. 38

Video: Photosynthesis

Calvin Cycle Light Reactions [CH2O] (sugar) Figure 8.5 H2O CO2 Light NADP ADP  P i Calvin Cycle Light Reactions ATP Figure 8.5 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle NADPH Chloroplast [CH2O] (sugar) O2 40

H2O Light NADP ADP  Light Reactions Chloroplast P i Figure 8.5-1 Figure 8.5-1 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle (step 1) Chloroplast 41

H2O Light NADP ADP  Light Reactions Chloroplast O2 P i ATP NADPH Figure 8.5-2 H2O Light NADP ADP  P i Light Reactions ATP Figure 8.5-2 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle (step 2) NADPH Chloroplast O2 42

Calvin Cycle Light Reactions Figure 8.5-3 H2O CO2 Light NADP ADP  P i Calvin Cycle Light Reactions ATP Figure 8.5-3 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle (step 3) NADPH Chloroplast O2 43

Concept 8.2: 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 © 2014 Pearson Education, Inc. 44

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 © 2014 Pearson Education, Inc. 45

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 © 2014 Pearson Education, Inc. 46

1 m (109 nm) 10−5 nm 10−3 nm 1 nm 103 nm 106 nm 103 m Micro- waves Figure 8.6 1 m (109 nm) 10−5 nm 10−3 nm 1 nm 103 nm 106 nm 103 m Micro- waves Gamma rays Radio waves X-rays UV Infrared Visible light Figure 8.6 The electromagnetic spectrum 380 450 500 550 600 650 700 750 nm Shorter wavelength Longer wavelength Higher energy Lower energy 47

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 © 2014 Pearson Education, Inc. 48

Animation: Light and Pigments Right click slide / Select play

Light Reflected light Chloroplast Absorbed Granum light Transmitted Figure 8.7 Light Reflected light Chloroplast Figure 8.7 Why leaves are green: interaction of light with chloroplasts Absorbed light Granum Transmitted light 50

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 © 2014 Pearson Education, Inc. 51

The high transmittance (low absorption) reading indicates Figure 8.8 Technique White light Refracting prism Chlorophyll solution Photoelectric tube Galvanometer 2 3 1 4 The high transmittance (low absorption) reading indicates that chlorophyll absorbs very little green light. Slit moves to pass light of selected wavelength. Green light Figure 8.8 Research method: determining an absorption spectrum The low transmittance (high absorption) reading indicates that chlorophyll absorbs most blue light. Blue light 52

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 Accessory pigments include chlorophyll b and a group of pigments called carotenoids An action spectrum profiles the relative effectiveness of different wavelengths of radiation in driving a process For the Cell Biology Video Space-Filling Model of Chlorophyll a, go to Animation and Video Files. © 2014 Pearson Education, Inc. 53

Chloro- phyll a Absorption of light by chloroplast pigments Figure 8.9a Chloro- phyll a Chlorophyll b Absorption of light by chloroplast pigments Carotenoids 400 500 600 700 Figure 8.9a Inquiry: which wavelengths of light are most effective in driving photosynthesis? (part 1: absorption spectra) Wavelength of light (nm) (a) Absorption spectra 54

(measured by O2 photosynthesis Rate of release) Figure 8.9b (measured by O2 photosynthesis Rate of release) 400 500 600 700 Figure 8.9b Inquiry: which wavelengths of light are most effective in driving photosynthesis? (part 2: action spectrum) (b) Action spectrum 55

(c) Engelmann’s experiment Figure 8.9c Aerobic bacteria Filament of alga 400 500 600 700 Figure 8.9c Inquiry: which wavelengths of light are most effective in driving photosynthesis? (part 3: Engelmann’s experiment) (c) Engelmann’s experiment 56

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 © 2014 Pearson Education, Inc. 57

Chlorophyll a is the main photosynthetic pigment Accessory pigments, such as chlorophyll b, broaden the spectrum used for photosynthesis A slight structural difference between chlorophyll a and chlorophyll b causes them to absorb slightly different wavelengths Accessory pigments called carotenoids absorb excessive light that would damage chlorophyll © 2014 Pearson Education, Inc. 58

Video: Chlorophyll Model

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 © 2014 Pearson Education, Inc. 60

(b) Fluorescence Figure 8.11a Figure 8.11a Excitation of isolated chlorophyll by light (photo: fluorescence) (b) Fluorescence 61

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) transfer the energy of photons to the reaction center © 2014 Pearson Education, Inc. 62

(INTERIOR OF THYLAKOID) Protein subunits THYLAKOID SPACE Figure 8.12 Photosystem STROMA Photon Light- harvesting complexes Reaction- center complex Primary electron acceptor e Chlorophyll STROMA Thylakoid membrane Thylakoid membrane Transfer of energy Figure 8.12 The structure and function of a photosystem Special pair of chlorophyll a molecules Pigment molecules THYLAKOID SPACE (INTERIOR OF THYLAKOID) Protein subunits THYLAKOID SPACE (a) How a photosystem harvests light (b) Structure of a photosystem 63

A primary electron acceptor 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 acceptor is the first step of the light reactions © 2014 Pearson Education, Inc. 64

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 © 2014 Pearson Education, Inc. 65

Photosystem I (PS I) is best at absorbing a wavelength of 700 nm The reaction-center chlorophyll a of PS I is called P700 © 2014 Pearson Education, Inc. 66

Linear Electron Flow Linear electron flow involves the flow of electrons through both photosystems to produce ATP and NADPH using light energy © 2014 Pearson Education, Inc. 67

Linear electron flow can be broken down into a series of steps 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 (we now call it P680) 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 © 2014 Pearson Education, Inc. 68

Calvin Cycle Light Reactions Figure 8.UN02 H2O CO2 Light NADP ADP Calvin Cycle Light Reactions ATP Figure 8.UN02 In-text figure, light reaction schematic, p. 164 NADPH O2 [CH2O] (sugar) 69

2 P680 1 Light Pigment molecules Photosystem II (PS II) Primary Figure 8.13-1 Primary acceptor 2 e− P680 1 Light Figure 8.13-1 How linear electron flow during the light reactions generates ATP and NADPH (steps 1-2) Pigment molecules Photosystem II (PS II) 70

2 2 H  3  P680 1 Light Pigment molecules Photosystem II (PS II) 1 2 Figure 8.13-2 Primary acceptor 2 2 H H2O e−   2 1 3 O2 e− e− P680 1 Light Figure 8.13-2 How linear electron flow during the light reactions generates ATP and NADPH (step 3) Pigment molecules Photosystem II (PS II) 71

4 Electron transport chain 2 2 H  3  P680 1 5 Light Pigment Figure 8.13-3 4 Electron transport chain Primary acceptor Pq 2 2 H H2O e− Cytochrome complex   2 1 3 O2 Pc e− P680 1 e− 5 Light ATP Figure 8.13-3 How linear electron flow during the light reactions generates ATP and NADPH (steps 4-5) Pigment molecules Photosystem II (PS II) 72

Photosystem I (PS I) Photosystem II (PS II) Figure 8.13-5 4 Electron transport chain 7 Electron transport chain Primary acceptor Primary acceptor Fd Pq e− 8 2 NADP e− 2 H H2O e− e− Cytochrome complex H   NADP reductase  2 1 3 O2 NADPH Pc e− P700 P680 1 e− 5 Light Light 6 ATP Figure 8.13-5 How linear electron flow during the light reactions generates ATP and NADPH (steps 7-8) Pigment molecules Photosystem I (PS I) Photosystem II (PS II) 73

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 © 2014 Pearson Education, Inc. 74

In PS I (like PS II), transferred light energy excites P700, causing it to lose an electron to an electron acceptor (we now call it P700) P700 accepts an electron passed down from PS II via the electron transport chain © 2014 Pearson Education, Inc. 75

Excited electrons “fall” down an electron transport chain from the primary electron acceptor of PS I to the protein ferredoxin (Fd) The electrons are transferred to NADP, reducing it to NADPH, and become available for the reactions of the Calvin cycle This process also removes an H from the stroma © 2014 Pearson Education, Inc. 76

The energy changes of electrons during linear flow can be represented in a mechanical analogy © 2014 Pearson Education, Inc. 77

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 © 2014 Pearson Education, Inc. 78

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 © 2014 Pearson Education, Inc. 79

Electron transport chain ATP synthase Figure 8.15 MITOCHONDRION STRUCTURE CHLOROPLAST STRUCTURE Inter- membrane space H Diffusion Thylakoid space Electron transport chain Inner membrane Thylakoid membrane Figure 8.15 Comparison of chemiosmosis in mitochondria and chloroplasts ATP synthase Matrix Stroma Key ADP  P i ATP Higher [H] H Lower [H] 80

Electron transport chain Figure 8.15a MITOCHONDRION STRUCTURE CHLOROPLAST STRUCTURE Inter- membrane space H Diffusion Thylakoid space Electron transport chain Inner membrane Thylakoid membrane ATP synthase Figure 8.15a Comparison of chemiosmosis in mitochondria and chloroplasts (detail) Matrix Stroma Key ADP  P i ATP Higher [H] H Lower [H] 81

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 © 2014 Pearson Education, Inc. 82

Calvin Cycle Light Reactions Figure 8.UN02 H2O CO2 Light NADP ADP Calvin Cycle Light Reactions ATP Figure 8.UN02 In-text figure, light reaction schematic, p. 164 NADPH O2 [CH2O] (sugar) 83

Cytochrome complex NADP reductase To Calvin Cycle ATP synthase Figure 8.16 Cytochrome complex NADP reductase Photosystem II Photosystem I Light 4 H 3 Light NADP  H Fd Pq NADPH e− Pc e− 2 H2O 1  2 1 O2 THYLAKOID SPACE (high H concentration) 2 H 4 H To Calvin Cycle Figure 8.16 The light reactions and chemiosmosis: the organization of the thylakoid membrane Thylakoid membrane ATP synthase STROMA (low H concentration) ADP  ATP P i H 84

Cytochrome complex ATP synthase Figure 8.16a Cytochrome complex Photosystem II Photosystem I Light 4 H Light Fd Pq Pc e− 2 e− H2O 1  2 1 O2 THYLAKOID SPACE (high H concentration) 2 H 4 H Figure 8.16a The light reactions and chemiosmosis: the organization of the thylakoid membrane (part 1) Thylakoid membrane ATP synthase STROMA (low H concentration) ADP  ATP P i H 85

Cytochrome complex NADP reductase To Calvin Cycle ATP synthase Figure 8.16b Cytochrome complex NADP reductase Photosystem I Light 3 NADP  H Fd NADPH Pc 2 THYLAKOID SPACE (high H concentration) 4 H To Calvin Cycle Figure 8.16b The light reactions and chemiosmosis: the organization of the thylakoid membrane (part 2) ATP synthase STROMA (low H concentration) ADP  ATP P i H 86

Concept 8.3: The Calvin cycle uses the chemical energy of ATP and NADPH to reduce CO2 to sugar The Calvin cycle, like the citric acid cycle, regenerates its starting material after molecules enter and leave the cycle Unlike the citric acid cycle, the Calvin cycle is anabolic It builds sugar from smaller molecules by using ATP and the reducing power of electrons carried by NADPH © 2014 Pearson Education, Inc. 87

The Calvin cycle has three phases Carbon enters the cycle as CO2 and leaves as a sugar named glyceraldehyde 3-phospate (G3P) For net synthesis of one G3P, the cycle must take place three times, fixing three molecules of CO2 The Calvin cycle has three phases Carbon fixation Reduction Regeneration of the CO2 acceptor © 2014 Pearson Education, Inc. 88

Phase 1, carbon fixation, involves the incorporation of the CO2 molecules into ribulose bisphosphate (RuBP) using the enzyme rubisco © 2014 Pearson Education, Inc. 89

Calvin Cycle Light Reactions Figure 8.UN03 H2O CO2 Light NADP ADP Calvin Cycle Light Reactions ATP Figure 8.UN03 In-text figure, Calvin cycle schematic, p. 168 NADPH O2 [CH2O] (sugar) 90

Phase 1: Carbon fixation Rubisco Figure 8.17-1 Input 3 as 3 CO2 Phase 1: Carbon fixation Rubisco 3 P P 3 P P 6 P RuBP 3-Phosphoglycerate Calvin Cycle Figure 8.17-1 The Calvin cycle (step 1) 91

Phase 1: Carbon fixation Rubisco Figure 8.17-2 Input 3 as 3 CO2 Phase 1: Carbon fixation Rubisco 3 P P 3 P P 6 P RuBP 3-Phosphoglycerate 6 ATP 6 ADP Calvin Cycle 6 P P 1,3-Bisphosphoglycerate 6 NADPH 6 NADP 6 P i Figure 8.17-2 The Calvin cycle (step 2) 6 P G3P Phase 2: Reduction Glucose and other organic compounds 1 P G3P Output 92

Phase 1: Carbon fixation Rubisco Figure 8.17-3 Input 3 as 3 CO2 Phase 1: Carbon fixation Rubisco 3 P P 3 P P 6 P RuBP 3-Phosphoglycerate 6 ATP 6 ADP 3 ADP Calvin Cycle 6 P P 3 ATP 1,3-Bisphosphoglycerate 6 NADPH Phase 3: Regeneration of RuBP 6 NADP 6 P i 5 P Figure 8.17-3 The Calvin cycle (step 3) G3P 6 P G3P Phase 2: Reduction Glucose and other organic compounds 1 P G3P Output 93

Phase 2, reduction, involves the reduction and phosphorylation of 3-phosphoglycerate to G3P © 2014 Pearson Education, Inc. 94

Phase 3, regeneration, involves the rearrangement of G3P to regenerate the initial CO2 receptor, RuBP © 2014 Pearson Education, Inc. 95

Evolution of Alternative Mechanisms of Carbon Fixation in Hot, Arid Climates Adaptation to dehydration is a problem for land 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 an apparently wasteful process called photorespiration © 2014 Pearson Education, Inc. 96

In most plants (C3 plants), initial fixation of CO2, via rubisco, forms a three-carbon compound (3-phosphoglycerate) In photorespiration, rubisco adds O2 instead of CO2 in the Calvin cycle, producing a two-carbon compound Photorespiration decreases photosynthetic output by consuming ATP, O2, and organic fuel and releasing CO2 without producing any ATP or sugar © 2014 Pearson Education, Inc. 97

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 © 2014 Pearson Education, Inc. 98

C4 Plants C4 plants minimize the cost of photorespiration by incorporating CO2 into a four-carbon compound An enzyme in the mesophyll cells has a high affinity for CO2 and can fix carbon 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 © 2014 Pearson Education, Inc. 99

Calvin Cycle Calvin Cycle Figure 8.18 Sugarcane Pineapple 1 1 CO2 CO2 C4 CAM Mesophyll cell Organic acid Organic acid Night CO2 2 CO2 2 Figure 8.18 C4 and CAM photosynthesis compared Bundle- sheath cell Day Calvin Cycle Calvin Cycle Sugar Sugar (a) Spatial separation of steps (b) Temporal separation of steps 100

Figure 8.18a Figure 8.18a C4 and CAM photosynthesis compared (part 1: C4, sugarcane) Sugarcane 101

Figure 8.18b Figure 8.18b C4 and CAM photosynthesis compared (part 2: CAM, pineapple) Pineapple 102

Calvin Cycle Calvin Cycle Figure 8.18c 1 1 CO2 CO2 C4 CAM Mesophyll cell Organic acid Organic acid Night CO2 2 CO2 2 Bundle- sheath cell Day Calvin Cycle Calvin Cycle Figure 8.18c C4 and CAM photosynthesis compared (part 3: detail) Sugar Sugar (a) Spatial separation of steps (b) Temporal separation of steps 103

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 © 2014 Pearson Education, Inc. 104

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 the chloroplasts and in structures such as roots, tubers, seeds, and fruits In addition to food production, photosynthesis produces the O2 in our atmosphere © 2014 Pearson Education, Inc. 105

Electron transport chain Electron transport chain Figure 8.19 H2O CO2 Light NADP ADP  P i Light Reactions: RuBP 3-Phosphpglycerate Photosystem II Electron transport chain Calvin Cycle Photosystem I Electron transport chain ATP G3P Figure 8.19 A review of photosynthesis Starch (storage) NADPH Chloroplast O2 Sucrose (export) 106

Figure 8.UN04 Figure 8.UN04 Skills exercise: making scatter plots with regression lines 107

Primary acceptor Electron transport chain Primary acceptor Figure 8.UN05 Primary acceptor Electron transport chain Primary acceptor Electron transport chain Fd NADP H2O Pq NADP reductase  H O2 NADPH Cytochrome complex Pc Figure 8.UN05 Summary of key concepts: the light reactions Photosystem I ATP Photosystem II 108

Calvin Cycle Regeneration of CO2 acceptor Figure 8.UN06 3 CO2 Carbon fixation 3  5C 6  3C Calvin Cycle Regeneration of CO2 acceptor 5  3C Figure 8.UN06 Summary of key concepts: the Calvin cycle Reduction 1 G3P (3C) 109

Figure 8.UN07 pH 4 pH 7 pH 4 pH 8 Figure 8.UN07 Test your understanding, question 9 (thylakoid experiment) 110