Photosynthesis

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  • Figure 10.2 Photoautotrophs.
  • Figure 10.4 Zooming in on the location of photosynthesis in a plant.
  • Figure 10.4 Zooming in on the location of photosynthesis in a plant.
  • Figure 10.5 Tracking atoms through photosynthesis.
  • Figure 10.6 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle.
  • Figure 10.6 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle.
  • Figure 10.6 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle.
  • 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.
  • For the Cell Biology Video Space-Filling Model of Chlorophyll a, go to Animation and Video Files.
  • Figure 10.6 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle.
  • 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 10.19 The Calvin cycle.
  • Figure 10.19 The Calvin cycle.
  • Figure 10.6 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle.
  • Photosynthesis

    1. 1. E tra lect ch ns ron ai po Primary n rt El e acceptor Primary ctro 4 7 acceptor n tr ans Fd por e− Pq tc hai 2 n e− 8 e− e− NADP+ H2O Cytochrome2H+ NADP+ + H+ + complex 3 reductase O2 NADPH e− Pc e− P700 P680 5 Light Light 6 ATP Pigment molecules Photosystem I (PS I) Photosystem II (PS II) Photosynthesis Dr. Kristen Walker
    2. 2. The Process That Feeds the Biosphere • Photosynthesis is the process that converts solar energy into chemical energy • Photosynthesis occurs in plants, algae, photoplankton, and some prokaryotes
    3. 3. • Autotrophs sustain themselves without eating anything derived from other organisms – 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
    4. 4. Figure 10.2 (b) Multicellular (a) Plants alga (d) Cyanobacteria 40 µm (c) Unicellular 10 µm protists (e) Purple sulfur 1 µm bacteria
    5. 5. • Heterotrophs obtain their organic material from other organisms – consumers of the biosphere• Almost all heterotrophs, including humans, depend on photoautotrophs for food and O2
    6. 6. Photosynthesis converts light energy to thechemical 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
    7. 7. 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
    8. 8. • 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
    9. 9. Figure 10.4a Leaf cross section Chloroplasts Vein Mesophyll Stomata CO2 O2 Chloroplast Mesophyll cell 20 µm
    10. 10. Figure 10.4b Chloroplast Outer membrane Thylakoid Intermembrane Stroma Granum Thylakoid space space Inner membrane 1 µm
    11. 11. Photosynthesis • Photosynthesis is a complex series of reactions that can be summarized as the following equation: 6 CO2 + 6 H2O + Light energy → C6H12O6 + 6 O2 + 6 H2O
    12. 12. 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
    13. 13. Figure 10.5 Reactants: 6 CO2 12 H2O Products: C6H12O6 6 H2O 6 O2
    14. 14. Photosynthesis as a Redox Process • Photosynthesis – reverses the direction of electron flow compared to cellular respiration – is a redox process in which H2O is oxidized and CO2 is reduced – is an endergonic process; the energy boost is provided by light becomes reduced Energy + 6 CO2 + 6 H2O C6 H12 O6 + 6 O2 becomes oxidized
    15. 15. The Two Stages of Photosynthesis • Photosynthesis consists of the light reactions (the photo part) and Calvin cycle (the synthesis part)
    16. 16. The light reactions in thylakoids convertsolar energy to the chemical energy ofATP and NADPH • Goal of the light reactions (in the thylakoids) – Split H2O – Release O2 – Reduce NADP+ to NADPH – Generate ATP from ADP
    17. 17. Calvin Cycle • Calvin cycle takes place in the stroma • Uses energy made in light reaction stage (ATP and NADPH) to form glucose (sugar) • Calvin cycle begins with carbon fixation, incorporating CO2 into organic molecules
    18. 18. Figure 10.6-1 H2O Light NADP+ ADP +Pi Light Reactions Chloroplast
    19. 19. Figure 10.6-2 H2O Light NADP+ ADP +Pi Light Reactions ATP NADPH Chloroplast O2
    20. 20. Figure 10.6-3 H2O CO2 Light NADP+ ADP +Pi Calvin Light Cycle Reactions ATP NADPH Chloroplast O2
    21. 21. Figure 10.6-4 H2O CO2 Light NADP+ ADP +Pi Calvin Light Cycle Reactions ATP NADPH Chloroplast O2 [CH2O] (sugar)
    22. 22. 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 – determines the type of electromagnetic energy
    23. 23. • 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
    24. 24. 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
    25. 25. 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
    26. 26. Figure 10.8 Light Reflected light Chloroplast Absorbed Granum light Transmitted light
    27. 27. • 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
    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 Absorption of light by chloroplast pigments Chloro- phyll a Chlorophyll b Carotenoids 400 500 600 700 Wavelength of light (nm)
    29. 29. • 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 Absorption of light by chloroplast pigments Chloro- phyll a Chlorophyll b Carotenoids 400 500 600 700 Wavelength of light (nm)
    30. 30. BioFlix: Photosynthesis
    31. 31. Figure 10.6-4 H2O CO2 Light NADP+ ADP +Pi Calvin Light Cycle Reactions ATP NADPH Chloroplast O2 [CH2O] (sugar)
    32. 32. A closer look at light reactions… • 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
    33. 33. Figure 10.13a Photosystem STROMA Photon Light- Reaction- Primary harvesting center electron complexes complex acceptor Thylakoid membrane e− Transfer Special pair of Pigment of energy chlorophyll a molecules molecules THYLAKOID SPACE (INTERIOR OF THYLAKOID) (a) How a photosystem harvests light
    34. 34. • 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
    35. 35. • Two types of photosystems in the thylakoid membrane • Photosystem II and Photosystem I• Photosystem II (PS II) functions first (the numbers reflect order of discovery) and is best at absorbing a wavelength of 680 nm • reaction-center chlorophyll a of PS II is called P680
    36. 36. • Photosystem I (PS I) is best at absorbing a wavelength of 700 nm • reaction-center chlorophyll a of PS I is called P700
    37. 37. 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
    38. 38. • 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+)
    39. 39. Figure 10.14-1 Primary acceptor 2 e− P680 1 Light Pigment molecules Photosystem II (PS II)
    40. 40. • 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
    41. 41. 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)
    42. 42. • 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
    43. 43. Figure 10.14-3 El e Primary ctro 4 acceptor n tr ans por Pq tc hai 2 n H2O e − 2 H+ Cytochrome + complex1 /2 O2 3 e− Pc e− P680 5 1 Light ATP Pigment molecules Photosystem II (PS II)
    44. 44. • 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
    45. 45. Figure 10.14-4 Primary El e acceptor Primary ctro 4 acceptor n tr ans por e− Pq tc hai 2 n H2O e− 2H + 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)
    46. 46. • Each electron “falls” down an electron transport chain from the primary electron acceptor of PS I• 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
    47. 47. Figure 10.14-5 E tra lect ch ns ron ai po Primary n rt El e acceptor Primary ctro 4 7 acceptor n tr ans Fd por e− Pq tc hai 2 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)
    48. 48. Figure 10.15 e− e− e− Mill makes ATP NADPH e − e− e− n Photo e− ATP n Photo Photosystem II Photosystem I
    49. 49. 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 • Cyclic electron flow may protect cells from light-induced damage
    50. 50. Figure 10.16 Primary Primary acceptor Fd acceptor Fd Pq NADP+ NADP+ + H+ reductase Cytochrome NADPH complex Pc Photosystem I Photosystem II ATP
    51. 51. A Comparison of ATP Synthesis inChloroplasts and Mitochondria • Chloroplasts and mitochondria generate ATP, but use different sources of energy • Mitochondria transfer chemical energy from food to ATP • Chloroplasts transform light energy into the chemical energy of ATP
    52. 52. • 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
    53. 53. 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+ ]
    54. 54. Light Reaction Summary • 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
    55. 55. 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 O 2 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+
    56. 56. 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
    57. 57. • Carbon enters the cycle as CO2 and leaves as a sugar named glyceraldehyde 3-phosphate (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 enzyme, RuBisCO) – Reduction – Regeneration of the CO2 acceptor
    58. 58. Figure 10.19-1 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)
    59. 59. Figure 10.19-2 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 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 G3P Glucose (a sugar) Output
    60. 60. 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
    61. 61. Figure 10.6-4 H2O CO2 Light NADP+ ADP +Pi Calvin Light Cycle Reactions ATP NADPH Chloroplast O2 [CH2O] (sugar)

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