BIOL 101 Chp 10: Photosynthesis

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This is a lecture presentation for my BIOL 101 General Biology I students on Chapter 10: Photosynthesis. (Campbell Biology, 10th Ed. by Reece et al).

Rob Swatski, Associate Professor of Biology, Harrisburg Area Community College - York Campus, York, PA. Email: rjswatsk@hacc.edu

Please visit my website for more anatomy and biology learning resources: http://robswatski.virb.com/

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BIOL 101 Chp 10: Photosynthesis

  1. 1. BIOL 101 Chapter 10 Photosynthesis Rob Swatski Associate Professor of Biology HACC – York Campus
  2. 2. The Process That Feeds the Biosphere Photosynthesis: - converts solar energy into chemical energy - directly or indirectly feeds entire living world - in plants, algae & other protists, some prokaryotes
  3. 3. Autotrophs: survive without eating anything derived from other organisms - Producers: make organic molecules from inorganic molecules (H2O and CO2) - Photoautotrophs: use solar energy to make organic molecules
  4. 4. (a) Plants (c) Unicellular protist 10 µm (e) Purple sulfur bacteria (b) Multicellular alga (d) Cyanobacteria 40 µm 1.5 µm
  5. 5. Heterotrophs: obtain their organic material from other organisms - Consumers of the biosphere - heterotrophs depend on photoautotrophs for food and O2 Can an organism be both autotrophic and heterotrophic?
  6. 6. Chloroplasts are structurally similar to and likely evolved from photosynthetic bacteria - their structure enables the chemical reactions of photosynthesis to occur – how so?
  7. 7. Leaves: the major organs of photosynthesis - chlorophyll: green pigment inside chloroplasts - absorbs light energy that powers the synthesis of organic molecules CO2 enters and O2 exits the leaf through stomata
  8. 8. Chloroplasts: found in mesophyll cells in the leaf’s interior - 30-40 chloroplasts per mesophyll cell Chlorophyll is in the thylakoid membranes of chloroplasts - thylakoids are stacked into grana (granum) - stroma
  9. 9. Leaf cross section Vein Mesophyll Stomata Chloroplast CO2 O2 Mesophyll cell 5 µm
  10. 10. Chloroplast Outer membrane Thylakoid Stroma Granum Thylakoid space Intermembrane space Inner membrane 1 µm
  11. 11. Photosynthesis summary equation: 6 CO2 + 12 H2O + Light energy  C6H12O6 + 6 O2 + 6 H2O Photosynthesis is a redox process: - H2O is oxidized - CO2 is reduced
  12. 12. Chloroplasts split H2O into hydrogen and oxygen, incorporating the e- of H into glucose Reactants: Products: 12 H2O 6 CO2 C6H12O6 6 H2O 6 O2
  13. 13. The Two Stages of Photosynthesis: A Preview Light reactions: the “photo” part Calvin cycle: the “synthesis” part Light Reactions (in thylakoids): - split H2O - release O2 - reduce NADP+ to NADPH - generate ATP from ADP by photophosphorylation
  14. 14. H2O Light NADP+ ADP + P Light Reactions Chloroplast i
  15. 15. H2O Light NADP+ ADP + P i Light Reactions ATP NADPH Chloroplast O2
  16. 16. Calvin cycle (in stroma) - forms glucose from CO2 - uses ATP and NADPH Begins with carbon fixation - incorporate CO2 into organic molecules
  17. 17. CO2 H2O Light NADP+ ADP + P i Light Reactions ATP NADPH Chloroplast O2 Calvin Cycle
  18. 18. CO2 H2O Light NADP+ ADP + P i Light Reactions Calvin Cycle ATP NADPH Chloroplast O2 [CH2O] (sugar)
  19. 19. The Nature of Sunlight Light is a form of electromagnetic energy (radiation) - travels in rhythmic waves Wavelength: distance between crests of waves - determines the type of electromagnetic energy
  20. 20. Electromagnetic spectrum: the entire range of electromagnetic radiation Visible light: the wavelengths that produce colors we can see - includes wavelengths that drive PSN - behaves as though it consists of discrete energy “particles” (photons)
  21. 21. 10–5 nm 10–3 nm 103 nm 1 nm Gamma X-rays rays UV 106 nm Infrared 1m (109 nm) Microwaves 103 m Radio waves Visible light 380 450 500 Shorter wavelength Higher energy 550 600 650 700 750 nm Longer wavelength Lower energy
  22. 22. Photosynthetic Pigments: The Light Receptors Pigments: chemicals 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
  23. 23. Light Reflected light Absorbed light Granum Transmitted light
  24. 24. Spectrophotometer: measures a pigment’s ability to absorb different wavelengths - sends light through pigments - measures the fraction of light transmitted at each wavelength
  25. 25. TECHNIQUE Refracting prism White light Chlorophyll solution Photoelectric tube 2 1 Slit moves to pass light of selected wavelength 3 Galvanometer 4 Green light High transmittance (low absorption) reading -chlorophyll absorbs very little green light Blue light The low transmittance (high absorption) reading -chlorophyll absorbs most blue light
  26. 26. Absorption spectrum: graph plotting a pigment’s light absorption vs. wavelength - the absorption spectrum of chlorophyll a suggests that violet-blue and red light work best for PSN Action spectrum: profiles the relative effectiveness of different wavelengths of radiation in driving PSN
  27. 27. RESULTS Chlorophyll a Chlorophyll b Carotenoids (a) Absorption spectra 400 500 600 700 Wavelength of light (nm) (b) Action spectrum Aerobic bacteria Filament of alga (c) Engelmann’s experiment 400 500 600 700
  28. 28. Chlorophyll a: the main photosynthetic pigment Accessory pigments: - Chlorophyll b: broadens the PSN spectrum - Carotenoids: absorb excess light that would damage chlorophyll
  29. 29. CH3 CHO in chlorophyll a in chlorophyll b Porphyrin ring: light-absorbing “head” of molecule -Mg atom at center Hydrocarbon tail: interacts with hydrophobic regions of proteins inside thylakoid membranes of chloroplasts (H atoms not shown)
  30. 30. Excitation of Chlorophyll by Light When a pigment absorbs light, it goes from a stable ground state to an unstable excited state When excited e- fall back to the ground state, photons are given off (fluorescence) - if illuminated, a chlorophyll solution will fluoresce, giving off light and heat
  31. 31. Energy of electron e– Excited state Heat Photon e– Chlorophyll molecule Photon (fluorescence) Ground state (a) Excitation of isolated chlorophyll molecule (b) Fluorescence
  32. 32. Photosystem consists of: - Reaction-center complex surrounded by - Light-harvesting complexes (pigments bound to proteins) Funnels energy of photons to the reaction center
  33. 33. Primary electron acceptor (in reaction center complex) - accepts an excited e- from chlorophyll a - the 1st step of the light reactions
  34. 34. STROMA Photosystem Photon Thylakoid membrane Light-harvesting complexes Reaction-center complex Primary electron acceptor e– Transfer of energy Special pair of chlorophyll a molecules (P680 or P700) Pigment molecules THYLAKOID SPACE (INSIDE THYLAKOID)
  35. 35. Two Types of Photosystems: Photosystem II (PS II) functions first: - best at absorbing wavelengths of 680 nm P680: the reaction-center chlorophyll a of PS II
  36. 36. Photosystem I (PS I) functions second: - best at absorbing wavelengths of 700 nm P700: the reaction-center chlorophyll a of PS I
  37. 37. Linear Electron Flow Two possible routes for e- flow during the light reactions: cyclic and linear Linear electron flow: the primary pathway - involves both photosystems I and II - 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 e- from P680 is transferred to the primary electron acceptor
  39. 39. Primary acceptor e– e- 2 P680 1 Light Pigment molecules Photosystem II (PS II)
  40. 40. P680+ = P680 with one less e- is a very strong oxidizing agent - H2O is split by enzymes - its e- are transferred from H atoms to P680+ - reduces P680+ to P680 - O2 is released as a by-product of this reaction
  41. 41. Primary acceptor 2 H+ + 1/ O 2 2 e– H2O 3 2 ee– e–e e- P680 1 Light Pigment molecules Photosystem II (PS II)
  42. 42. Each e- “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 - the diffusion of H+ (protons) across the membrane drives ATP synthesis
  43. 43. e- - e- ee Primary - acceptor ee e- 2 2 H+ 1/ 2 + O2 Pq e– H2O 3 4 Cytochrome complex ee– e–e Pc e- 5 P680 e- 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 e- to an electron acceptor P700+: P700 that is missing an e- - accepts an e- passed down from PS II via the electron transport chain
  45. 45. e- - e- ee Primary - acceptor ee e- 2 2 H+ 1/ 2 + O2 3 4 e– Pq e– H2O Primary acceptor Cytochrome complex ee– e–e Pc eP700 e- 5 P680 e- e- 1 Light Light 6 ATP Pigment molecules Photosystem II (PS II) Photosystem I (PS I)
  46. 46. Each e- “falls” down an electron transport chain from the primary electron acceptor of PS I to the protein ferredoxin (Fd) - the e- are then transferred to NADP+ and reduce it to NADPH - the e- of NADPH are available for the reactions of the Calvin cycle
  47. 47. 4 Primary acceptor 2 H+ + 1/ O 2 2 e– H2O Primary acceptor 2 Fd e– Pq 7 e– Cytochrome complex 8 e– NADP+ reductase 3 Pc e– e– P700 5 P680 Light 1 Light 6 ATP Pigment molecules Photosystem II (PS II) Photosystem I (PS I) NADP+ + H+ NADPH e-
  48. 48. e– ATP e– e– NADPH e– e– e– Mill makes ATP e– Photosystem II Photosystem I
  49. 49. A Comparison of Chemiosmosis in Chloroplasts and Mitochondria Both 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
  50. 50. Spatial Organization of Chemiosmosis: Mitochondria: protons are pumped into the intermembrane space - diffusion of protons back into the matrix drives ATP synthesis Chloroplasts: protons are pumped into the thylakoid space - diffusion of protons back into the stroma drives ATP synthesis
  51. 51. Chloroplast Mitochondrion CHLOROPLAST STRUCTURE MITOCHONDRION STRUCTURE H+ Intermembrane space Inner membrane Diffusion Electron transport chain Thylakoid space Thylakoid membrane ATP synthase Matrix Key Stroma ADP + P [H+] Higher Lower [H+] i H+ ATP
  52. 52. ATP and NADPH are produced on the side facing the stroma, where the Calvin cycle takes place In summary: - light reactions generate ATP - they increase the potential energy of e- by moving them from H2O to NADPH
  53. 53. STROMA (low H+ concentration) Cytochrome complex Photosystem II 4 H+ Light Photosystem I Light Fd NADP+ reductase H2O THYLAKOID SPACE (high H+ concentration) 1 e– Pc 2 1/ 2 NADP+ + H+ NADPH Pq e– 3 O2 +2 H+ 4 H+ To Calvin Cycle Thylakoid membrane STROMA (low H+ concentration) ATP synthase ADP + Pi ATP H+
  54. 54. The Calvin cycle regenerates its starting material as molecules enter and leave the cycle - analagous to the citric acid cycle - builds sugar from smaller molecules using ATP and the reducing power of e- carried by NADPH
  55. 55. Carbon enters the Calvin cycle as CO2 Carbon leaves as the sugar, glyceraldehyde-3phospate (G3P) For net synthesis of 1 G3P: - the Calvin cycle must turn 3X, fixing 3 molecules of CO2
  56. 56. Three Phases of the Calvin Cycle 1. Carbon fixation (catalyzed by rubisco) 2. Reduction 3. Regeneration of the CO2 acceptor (RuBP)
  57. 57. Input 3 CO2 (Entering one at a time) Phase 1: Carbon fixation Rubisco 3 P Short-lived intermediate 3 P Ribulose bisphosphate (RuBP) P P 6 P 3-Phosphoglycerate
  58. 58. Input 3 CO2 (Entering one at a time) Phase 1: Carbon fixation Rubisco 3 P Short-lived intermediate 3 P P 6 P Ribulose bisphosphate (RuBP) P 3-Phosphoglycerate 6 ATP 6 ADP Calvin Cycle 6 P P 1,3-Bisphosphoglycerate 6 NADPH 6 NADP+ 6 Pi 6 P Glyceraldehyde-3-phosphate (G3P) 1 Output P G3P (a sugar) Glucose and other organic compounds Phase 2: Reduction
  59. 59. Input 3 CO2 (Entering one at a time) Phase 1: Carbon fixation Rubisco 3 P Short-lived intermediate 3 P P 6 P Ribulose bisphosphate (RuBP) P 3-Phosphoglycerate 6 ATP 6 ADP 3 ADP 3 Calvin Cycle 6 P P 1,3-Bisphosphoglycerate ATP 6 NADPH Phase 3: Regeneration of the CO2 acceptor (RuBP) 6 NADP+ 6 Pi P 5 G3P 6 P Glyceraldehyde-3-phosphate (G3P) 1 Output P G3P (a sugar) Glucose and other organic compounds Phase 2: Reduction
  60. 60. H2O CO2 Light NADP+ ADP + P i Light Reactions: Photosystem II Electron transport chain Photosystem I Electron transport chain RuBP 3-Phosphoglycerate Calvin Cycle ATP NADPH G3P Starch (storage) Chloroplast O2 Sucrose (export)

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