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Ch. 10

Ch. 10. Photosynthesis. Feeds the Biosphere Converts solar E into chemical E. Plants and other autotrophs  Producers of the biosphere. Figure 10.1. Photoautotrophs Use E of sunlight to make organic molecules from water and CO 2. These organisms use light energy to drive the

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Ch. 10

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  1. Ch. 10 • Photosynthesis

  2. Feeds the Biosphere • Converts solar E into chemical E

  3. Plants and other autotrophs Producers of the biosphere

  4. Figure 10.1 • Photoautotrophs • Use E of sunlight to make organic molecules from water and CO2

  5. These organisms use light energy to drive the synthesis of organic molecules from carbon dioxide and (in most cases) water. They feed not only themselves, but the entire living world. (a) On land, plants are the predominant producers of food. In aquatic environments, photosynthetic organisms include (b) multicellular algae, such as this kelp; (c) some unicellular protists, such as Euglena; (d) the prokaryotes called cyanobacteria; and (e) other photosynthetic prokaryotes, such as these purple sulfur bacteria, which produce sulfur (spherical globules) (c, d, e: LMs). (a) Plants (c) Unicellular protist 10 m (e) Pruple sulfur bacteria 1.5 m Figure 10.2 (d) Cyanobacteria (b) Multicellular algae 40 m Photosynthesis • Occurs in plants, algae, certain other protists, and some prokaryotes

  6. Heterotrophs • Obtain organic material f/ other organisms • Consumers of the biosphere

  7. Photosynthesis converts light E to the chemical E of food

  8. Leaf cross section Vein Mesophyll CO2 O2 Stomata Figure 10.3 Chloroplasts: Site of Photosynthesis (plants) • Leaf • Major site of photosynthesis

  9. Mesophyll Chloroplast 5 µm Outer membrane Thylakoid Intermembrane space Thylakoid space Granum Stroma Inner membrane 1 µm • Chloroplasts • Contain thylakoids and grana

  10. Photosynthesis summary reaction 6 CO2 + 12 H2O + Light energy  C6H12O6 + 6 O2 + 6 H2 O

  11. Reactants: 12 H2O 6 CO2 6 H2O 6 O2 C6H12O6 Products: Figure 10.4 Chloroplasts split water into • H2 and O2, incorporating the e- of H2 into sugar molecules

  12. Photosynthesis as a Redox Process • Water is oxidized, CO2 is reduced

  13. The Two Stages of Photosynthesis: A Preview • Light reactions • Calvin cycle

  14. Light reactions • Occurs on thylakoid membranes • Converts solar E to chemical E

  15. Calvin cycle • Occurs in the stroma • Forms sugar from carbon dioxide, using ATP for energy and NADPH for reducing power

  16. H2O CO2 Light NADP  ADP + P LIGHT REACTIONS CALVIN CYCLE ATP NADPH Chloroplast [CH2O] (sugar) O2 Figure 10.5 • Overview of photosynthesis

  17. Light reactions convert solar E to the chemical E of ATP and NADPH

  18. The Nature of Sunlight • Form of electromagnetic E, travels in waves

  19. Wavelength (l) • Distance between the crests of waves • Determines the type of electromagnetic E

  20. 1 m 106 nm 10–5 nm 106 nm 1 nm 10–3 nm 103 nm 103 m Micro- waves Radio waves Gamma rays X-rays UV Infrared Visible light 380 450 500 550 600 650 700 750 nm Shorter wavelength Longer wavelength Lower energy Higher energy Figure 10.6 • Electromagnetic spectrum • Entire range of electromagnetic E, or radiation

  21. Visible light spectrum • Colors of light we can see • l’s that drive photosynthesis

  22. Photosynthetic Pigments: The Light Receptors • Substances that absorb visible light

  23. Light Reflected Light Chloroplast Absorbed light Granum Transmitted light Figure 10.7 • Reflect light, which include the colors we see

  24. Spectrophotometer • Machine that sends light through pigments and measures the fraction of light transmitted at each l

  25. Refracting prism Chlorophyll solution Photoelectric tube White light Galvanometer 2 3 1 0 100 4 Slit moves to pass light of selected wavelength Green light The high transmittance (low absorption) reading indicates that chlorophyll absorbs very little green light. 0 100 The low transmittance (high absorption) reading chlorophyll absorbs most blue light. Blue light Figure 10.8 • Absorption spectrum • A graph plotting light absorption versus l

  26. Absorption spectra of chloroplast pigments • Clues to the relative effectiveness of different l f/ driving photosynthesis

  27. Three different experiments helped reveal which wavelengths of light are photosynthetically important. The results are shown below. EXPERIMENT RESULTS Chlorophyll a Chlorophyll b Absorption of light by chloroplast pigments Carotenoids Wavelength of light (nm) (a) Absorption spectra. The three curves show the wavelengths of light best absorbed by three types of chloroplast pigments. Figure 10.9 • Absorption spectra of 3 types of pigments

  28. Rate of photosynthesis (measured by O2 release) (b) Action spectrum. This graph plots the rate of photosynthesis versus wavelength. The resulting action spectrum resembles the absorption spectrum for chlorophyll a but does not match exactly (see part a). This is partly due to the absorption of light by accessory pigments such as chlorophyll b and carotenoids. • Action spectrum of a pigment • Effectiveness of different l of radiation in driving photosynthesis

  29. Aerobic bacteria Filament of alga 500 600 700 400 (c) Engelmann‘s experiment. In 1883, Theodor W. Engelmann illuminated a filamentous alga with light that had been passed through a prism, exposing different segments of the alga to different wavelengths. He used aerobic bacteria, which concentrate near an oxygen source, to determine which segments of the alga were releasing the most O2 and thus photosynthesizing most. Bacteria congregated in greatest numbers around the parts of the alga illuminated with violet-blue or red light. Notice the close match of the bacterial distribution to the action spectrum in part b. Light in the violet-blue and red portions of the spectrum are most effective in driving photosynthesis. CONCLUSION • First demonstrated by Theodor W. Engelmann

  30. CH3 in chlorophyll a in chlorophyll b CHO CH2 CH3 CH H C C C Porphyrin ring: Light-absorbing “head” of molecule note magnesium atom at center C C CH3 C C H3C CH2 C N C N H C C Mg H N C C N H3C C C CH3 C C C C C H H CH2 H C C O CH2 O O C O O CH3 CH2 Hydrocarbon tail: interacts with hydrophobic regions of proteins inside thylakoid membranes of chloroplasts: H atoms not shown Figure 10.10 • Chlorophyll a • Main photosynthetic pigment • Chlorophyll b • Accessory pigment

  31. Other accessory pigments • Absorb different ls of light and pass the E to chlorophyll a

  32. Excited state e– Heat Energy of election Photon (fluorescence) Ground state Chlorophyll molecule Photon Figure 10.11 A Excitation of Chlorophyll by Light • When a pigment absorbs light it goes f/ a ground state to an excited state (unstable)

  33. Figure 10.11 B • Isolated chlorophyll fluoresce

  34. Thylakoid Photosystem Photon STROMA Light-harvesting complexes Reaction center Primary election acceptor e– Thylakoid membrane Special chlorophyll a molecules Transfer of energy Pigment molecules THYLAKOID SPACE (INTERIOR OF THYLAKOID) Figure 10.12 A Photosystem • Composed of a reaction center surrounded by a number of light-harvesting complexes

  35. Light-harvesting complexes • Pigment molecules bound to proteins • Funnel the E of photons of light to the reaction center

  36. When a reaction-center chlorophyll molecule absorbs E • Electrons gets bumped up to a primary e- acceptor

  37. Thylakoid membrane • 2 types of photosystems, I and II

  38. Noncyclic Electron Flow • Primary pathway

  39. H2O CO2 Light NADP+ ADP CALVIN CYCLE LIGHT REACTIONS ATP NADPH Electron Transport chain O2 [CH2O] (sugar) Primary acceptor 7 Primary acceptor 4 Fd Electron transport chain Pq 2 e 8 e– e H2O NADP+ + 2 H+ Cytochrome complex 2 H+ NADP+ reductase + 3 NADPH O2 PC e– + H+ P700 e– 5 Light P680 Light 1 6 ATP Photosystem-I (PS I) Photosystem II (PS II) Figure 10.13 • Produces NADPH, ATP, and O2

  40. e– ATP e– e– NADPH e– e– e– Mill makes ATP Photon e– Photon Photosystem I Photosystem II Figure 10.14 

  41. Primary acceptor Primary acceptor Fd Fd NADP+ Pq NADP+ reductase Cytochrome complex NADPH Pc Photosystem I ATP Photosystem II Figure 10.15 • Cyclic e- flow • Only photosystem I is used • Only ATP is produced

  42. Chemiosmosis in Chloroplasts v. Mitochondria • Chloroplasts and mitochondria • Generate ATP by the same basic mechanism: chemiosmosis • Use different sources of E to accomplish this

  43. Key Higher [H+] Lower [H+] Chloroplast Mitochondrion MITOCHONDRION STRUCTURE CHLOROPLAST STRUCTURE H+ Diffusion Thylakoid space Intermembrance space Electron transport chain Membrance ATP Synthase Stroma Matrix ADP+ P ATP H+ Figure 10.16 • Spatial organization of chemiosmosis

  44. In both • Redox reactions of e- transport chains generate a H+ gradient across membrane • ATP synthase • Uses proton-motive force to form ATP

  45. H2O CO2 LIGHT NADP+ ADP CALVIN CYCLE LIGHT REACTOR ATP NADPH STROMA (Low H+ concentration) O2 [CH2O] (sugar) Cytochrome complex Photosystem II Photosystem I NADP+ reductase Light 2 H+ 3 NADP+ + 2H+ Fd NADPH + H+ Pq Pc 2 H2O 1⁄2 O2 THYLAKOID SPACE (High H+ concentration) 1 2 H+ +2 H+ To Calvin cycle ATP synthase Thylakoid membrane STROMA (Low H+ concentration) ADP ATP P H+ Figure 10.17 • Light reactions and chemiosmosis

  46. Calvin cycle • Uses ATP and NADPH to convert CO2 to sugar • Similar to the citric acid cycle • Occurs in the stroma

  47. 3 phases • Carbon fixation • Reduction • Regeneration of CO2 acceptor

  48. H2O Input CO2 Light 3 (Entering one at a time) NADP+ CO2 ADP CALVINCYCLE LIGHTREACTION ATP NADPH Rubisco O2 [CH2O] (sugar) 3 P P Short-livedintermediate P 6 3 P P Ribulose bisphosphate(RuBP) 3-Phosphoglycerate 6 ATP 6 ADP CALVIN CYCLE 3 ADP 6 P P 3 ATP 1,3-Bisphoglycerate 6 NADPH 6 NADPH+ 6 P P 5 (G3P) 6 P Glyceraldehyde-3-phosphate (G3P) P 1 G3P(a sugar)Output Glucose andother organiccompounds Figure 10.18 • Calvin cycle Phase 1: Carbon fixation Phase 3:Regeneration ofthe CO2 acceptor(RuBP) Phase 2:Reduction

  49. Alternative mechanisms of carbon fixation have evolved in hot, arid climates

  50. On hot, dry days, plants close their stomata • Conserving water but limiting access to CO2 • Causing O2 to build up

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