10 photosynthesis


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  • Figure 10.1 How can sunlight, seen here as a spectrum of colors in a rainbow, power the synthesis of organic substances?
  • Figure 10.2 Photoautotrophs.
  • Figure 10.4 Zooming in on the location of photosynthesis in a plant.
  • Figure 10.5 Tracking atoms through photosynthesis.
  • Figure 10.UN01 In-text figure, p. 188
  • Figure 10.6 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle.
  • Figure 10.7 The electromagnetic spectrum.
  • Figure 10.8 Why leaves are green: interaction of light with chloroplasts.
  • Figure 10.9 Research Method: Determining an Absorption Spectrum
  • For the Cell Biology Video Space-Filling Model of Chlorophyll a, go to Animation and Video Files.
  • Figure 10.10 Inquiry: Which wavelengths of light are most effective in driving photosynthesis?
  • Figure 10.11 Structure of chlorophyll molecules in chloroplasts of plants.
  • Figure 10.12 Excitation of isolated chlorophyll by light.
  • Figure 10.13 The structure and function of a photosystem.
  • Figure 10.14 How linear electron flow during the light reactions generates ATP and NADPH.
  • Figure 10.14 How linear electron flow during the light reactions generates ATP and NADPH.
  • Figure 10.14 How linear electron flow during the light reactions generates ATP and NADPH.
  • Figure 10.14 How linear electron flow during the light reactions generates ATP and NADPH.
  • Figure 10.14 How linear electron flow during the light reactions generates ATP and NADPH.
  • Figure 10.15 A mechanical analogy for linear electron flow during the light reactions.
  • Figure 10.16 Cyclic electron flow.
  • Figure 10.17 Comparison of chemiosmosis in mitochondria and chloroplasts.
  • Figure 10.18 The light reactions and chemiosmosis: the organization of the thylakoid membrane.
  • Figure 10.19 The Calvin cycle.
  • Figure 10.20 C 4 leaf anatomy and the C 4 pathway.
  • Figure 10.21 C 4 and CAM photosynthesis compared.
  • 10 photosynthesis

    1. 1. LECTURE PRESENTATIONS For CAMPBELL BIOLOGY, NINTH EDITION Jane B. Reece, Lisa A. Urry, Michael L. Cain, Steven A. Wasserman, Peter V. Minorsky, Robert B. JacksonChapter 10Photosynthesis Lectures by Erin Barley Kathleen Fitzpatrick© 2011 Pearson Education, Inc.
    2. 2. Overview: The Process That Feeds theBiosphere • Photosynthesis is the process that converts solar energy into chemical energy • Directly or indirectly, photosynthesis nourishes almost the entire living world© 2011 Pearson Education, Inc.
    3. 3. • 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© 2011 Pearson Education, Inc.
    4. 4. Figure 10.1
    5. 5. • Photosynthesis occurs in plants, algae, certain other protists, and some prokaryotes • These organisms feed not only themselves but also most of the living world© 2011 Pearson Education, Inc.
    6. 6. Figure 10.2 (b) Multicellular (a) Plants alga (d) Cyanobacteria 40 µm (c) Unicellular 10 µm protists (e) Purple sulfur 1 µm bacteria
    7. 7. • 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© 2011 Pearson Education, Inc.
    8. 8. 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© 2011 Pearson Education, Inc.
    9. 9. Chloroplasts: The Sites of Photosynthesisin 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© 2011 Pearson Education, Inc.
    10. 10. • 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© 2011 Pearson Education, Inc.
    11. 11. Figure 10.4 Leaf cross section Chloroplasts Vein Mesophyll Stomata CO2 O2 Chloroplast Mesophyll cell Outer membrane Thylakoid Intermembrane Stroma Granum Thylakoid space 20 µm space Inner membrane 1 µm
    12. 12. 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© 2011 Pearson Education, Inc.
    13. 13. 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© 2011 Pearson Education, Inc.
    14. 14. Figure 10.5 Reactants: 6 CO2 12 H2O Products: C6H12O6 6 H2O 6 O2
    15. 15. 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 enery boost is provided by light© 2011 Pearson Education, Inc.
    16. 16. Figure 10.UN01 becomes reduced Energy + 6 CO2 + 6 H2O C6 H12 O6 + 6 O2 becomes oxidized
    17. 17. 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© 2011 Pearson Education, Inc.
    18. 18. • 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© 2011 Pearson Education, Inc.
    19. 19. Figure 10.6-4 H2O CO2 Light NADP+ ADP +Pi Calvin Light Cycle Reactions ATP NADPH Chloroplast O2 [CH2O] (sugar)
    20. 20. 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© 2011 Pearson Education, Inc.
    21. 21. 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© 2011 Pearson Education, Inc.
    22. 22. • 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© 2011 Pearson Education, Inc.
    23. 23. Figure 10.7 1m 10−5 nm 10−3 nm 1 nm 10 nm 3 106 nm (109 nm) 103 m Gamma Micro- Radio X-rays UV Infrared waves waves rays Visible light 380 450 500 550 600 650 700 750 nm Shorter wavelength Longer wavelength Higher energy Lower energy
    24. 24. Photosynthetic Pigments: The LightReceptors • 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© 2011 Pearson Education, Inc.
    25. 25. Figure 10.8 Light Reflected light Chloroplast Absorbed Granum light Transmitted light
    26. 26. • 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© 2011 Pearson Education, Inc.
    27. 27. Figure 10.9 TECHNIQUE Refracting Chlorophyll Photoelectric White prism solution tube light Galvanometer High transmittance Slit moves to Green (low absorption): pass light light Chlorophyll absorbs of selected very little green light. wavelength. Low transmittance Blue (high absorption): light Chlorophyll absorbs most blue light.
    28. 28. • 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 • An action spectrum profiles the relative effectiveness of different wavelengths of radiation in driving a process© 2011 Pearson Education, Inc.
    29. 29. Figure 10.10 RESULTS Absorption of light by chloroplast pigments Chloro- phyll a Chlorophyll b Carotenoids (a) Absorption spectra 400 500 600 700 Wavelength of light (nm) (measured by O2 release) Rate of photosynthesis (b) Action spectrum 400 500 600 700 Aerobic bacteria Filament of alga (c) Engelmann’s experiment 400 500 600 700
    30. 30. • 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© 2011 Pearson Education, Inc.
    31. 31. • 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© 2011 Pearson Education, Inc.
    32. 32. Figure 10.11 CH3 in chlorophyll a CH3 CHO in chlorophyll b Porphyrin ring Hydrocarbon tail (H atoms not shown)
    33. 33. 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© 2011 Pearson Education, Inc.
    34. 34. Figure 10.12 Excited e− state Energy of electron Heat Photon (fluorescence) Photon Ground Chlorophyll state molecule (a) Excitation of isolated chlorophyll molecule (b) Fluorescence
    35. 35. A Photosystem: A Reaction-Center ComplexAssociated with Light-HarvestingComplexes • 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© 2011 Pearson Education, Inc.
    36. 36. Figure 10.13 Photosystem STROMA Photon Light- Reaction- Primary harvesting center electron complexes complex acceptor Thylakoid membrane Chlorophyll STROMA Thylakoid membrane e − Transfer Special pair of Pigment of energy chlorophyll a molecules molecules Protein THYLAKOID SPACE subunits THYLAKOID (INTERIOR OF THYLAKOID) SPACE (a) How a photosystem harvests light (b) Structure of photosystem II
    37. 37. • A primary electron acceptor in the reaction center accepts an excited electron 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© 2011 Pearson Education, Inc.
    38. 38. • 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© 2011 Pearson Education, Inc.
    39. 39. • Photosystem I (PS I) is best at absorbing a wavelength of 700 nm • The reaction-center chlorophyll a of PS I is called P700© 2011 Pearson Education, Inc.
    40. 40. 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© 2011 Pearson Education, Inc.
    41. 41. • 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+)© 2011 Pearson Education, Inc.
    42. 42. Figure 10.14-1 Primary acceptor 2 e− P680 1 Light Pigment molecules Photosystem II (PS II)
    43. 43. • P680+ 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© 2011 Pearson Education, Inc.
    44. 44. Figure 10.14-2 Primary acceptor 2 H2O e− 2 H+ +1 /2 O2 3 e− e− P680 1 Light Pigment molecules Photosystem II (PS II)
    45. 45. • 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© 2011 Pearson Education, Inc.
    46. 46. Figure 10.14-3 Ele Primary c tron 4 acceptor tran s por Pq t ch 2 ai n H2O e− 2 H+ Cytochrome + complex1 /2 O2 3 e− Pc e− P680 5 1 Light ATP Pigment molecules Photosystem II (PS II)
    47. 47. • 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© 2011 Pearson Education, Inc.
    48. 48. Figure 10.14-4 Primary Ele acceptor Primary c tron 4 acceptor tran s por Pq t ch e− 2 ai n H2O e− 2 H+ Cytochrome + complex1 /2 O2 3 e− Pc e− P700 P680 5 Light 1 Light 6 ATP Pigment molecules Photosystem I (PS I) Photosystem II (PS II)
    49. 49. • 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 • This process also removes an H+ from the stroma© 2011 Pearson Education, Inc.
    50. 50. Figure 10.14-5 E tra lect ch ns ron ai po Primary n rt Ele acceptor Primary c tron 4 7 acceptor tran s por Fd Pq t ch e− 2 ai n e− 8 e− e− NADP+ H2O Cytochrome 2H + NADP+ + H+ + complex 3 reductase1 /2 O2 NADPH e− Pc e− P700 P680 5 Light 1 Light 6 ATP Pigment molecules Photosystem I (PS I) Photosystem II (PS II)
    51. 51. Figure 10.15 e− e− e− Mill makes ATP NADPH e − e− e− n Photo e− ATP n Photo Photosystem II Photosystem I
    52. 52. Cyclic Electron Flow • Cyclic electron flow uses only photosystem I and produces ATP, but not NADPH • No oxygen is released • Cyclic electron flow generates surplus ATP, satisfying the higher demand in the Calvin cycle© 2011 Pearson Education, Inc.
    53. 53. Figure 10.16 Primary Primary acceptor Fd acceptor Fd Pq NADP+ NADP+ + H+ reductase Cytochrome NADPH complex Pc Photosystem I Photosystem II ATP
    54. 54. A Comparison of Chemiosmosis inChloroplasts 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© 2011 Pearson Education, Inc.
    55. 55. • 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© 2011 Pearson Education, Inc.
    56. 56. Figure 10.17 Mitochondrion Chloroplast MITOCHONDRION CHLOROPLAST STRUCTURE STRUCTURE H+ Diffusion Intermembrane Thylakoid space space Electron Inner Thylakoid transport membrane membrane chain ATP synthase Matrix Stroma ADP + P i ATP Key Higher [H ] + H + Lower [H+ ]
    57. 57. • 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© 2011 Pearson Education, Inc.
    58. 58. Figure 10.18 STROMA (low H+ concentration) Cytochrome NADP+ complex Photosystem I reductase Photosystem II Light Light 4 H+ 3 NADP+ + H+ Fd Pq NADPH Pc 2 H2O 1 /2 O2 1 THYLAKOID SPACE +2 H+ 4 H+ (high H+ concentration) To Calvin Cycle Thylakoid membrane ATP synthase ADP + ATP STROMA Pi (low H+ concentration) H+
    59. 59. The Calvin cycle uses the chemical energyof 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 • The cycle builds sugar from smaller molecules by using ATP and the reducing power of electrons carried by NADPH© 2011 Pearson Education, Inc.
    60. 60. • 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)© 2011 Pearson Education, Inc.
    61. 61. Figure 10.19-3 Input 3 (Entering one CO2 at a time) 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 3 6 P P ATP 1,3-Bisphosphoglycerate 6 NADPH Phase 3: Regeneration of 6 NADP+ the CO2 acceptor 6 Pi (RuBP) P 5 G3P 6 P Glyceraldehyde 3-phosphate Phase 2: (G3P) Reduction 1 P G3P Glucose and (a sugar) other organic Output compounds
    62. 62. Alternative mechanisms of carbon fixationhave 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 an apparently wasteful process called photorespiration© 2011 Pearson Education, Inc.
    63. 63. Photorespiration: An Evolutionary Relic? • 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 consumes O2 and organic fuel and releases CO2 without producing ATP or sugar© 2011 Pearson Education, Inc.
    64. 64. • 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 can drain as much as 50% of the carbon fixed by the Calvin cycle on a hot, dry day • However, studies have shown that photorespiration is essential for plant growth as demonstrated by photorespiration mutants that are inviable in normal air and only grow in elevated CO2 conditions© 2011 Pearson Education, Inc.
    65. 65. 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© 2011 Pearson Education, Inc.
    66. 66. Figure 10.20 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
    67. 67. 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© 2011 Pearson Education, Inc.
    68. 68. Figure 10.21 Sugarcane Pineapple C4 CAM CO2 CO2 1 CO2 incorporated Mesophyll Organic acid Organic acid Night cell (carbon fixation) CO2 CO2 Bundle- 2 CO2 released Day sheath Calvin Calvin Cycle to the Calvin Cycle cell cycle Sugar Sugar (a) Spatial separation of steps (b) Temporal separation of steps
    69. 69. 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© 2011 Pearson Education, Inc.