Photosynthesis: The Process That Powers Life on Earth

1. Chapter 10 Photosynthesis

2. Overview: The Process That Feeds the Biosphere • Photosynthesis • Is the process that converts solar energy into chemical energy

3. Plants and other autotrophs • Are the producers of the biosphere

4. Figure 10.1 • Plants are photoautotrophs • They use the energy of sunlight to make organic molecules from water and carbon dioxide

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 their organic material from other organisms • Are the consumers of the biosphere

7. Concept 10.1: Photosynthesis converts light energy to the chemical energy of food

8. Leaf cross section Vein Mesophyll CO2 O2 Stomata Figure 10.3 Chloroplasts: The Sites of Photosynthesis in Plants • The leaves of plants • Are the major sites of photosynthesis

9. Mesophyll Chloroplast 5 µm Outer membrane Thylakoid Intermembrane space Thylakoid space Granum Stroma Inner membrane 1 µm • Chloroplasts • Are the organelles in which photosynthesis occurs • Contain thylakoids and grana

10. Tracking Atoms Through Photosynthesis: Scientific Inquiry • Photosynthesis is summarized as 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 The Splitting of Water • Chloroplasts split water into • Hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules

12. Photosynthesis as a Redox Process • Photosynthesis is a redox process • Water is oxidized, carbon dioxide is reduced

13. The Two Stages of Photosynthesis: A Preview • Photosynthesis consists of two processes • The light reactions • The Calvin cycle

14. The light reactions • Occur in the grana • Split water, release oxygen, produce ATP, and form NADPH

15. The 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 • An overview of photosynthesis

17. Concept 10.2: The light reactions convert solar energy to the chemical energy of ATP and NADPH

18. The Nature of Sunlight • Light • Is a form of electromagnetic energy, which travels in waves

19. Wavelength • Is the distance between the crests of waves • Determines the type of electromagnetic energy

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 • The electromagnetic spectrum • Is the entire range of electromagnetic energy, or radiation

21. The visible light spectrum • Includes the colors of light we can see • Includes the wavelengths that drive photosynthesis

22. Photosynthetic Pigments: The Light Receptors • Pigments • Are 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. The spectrophotometer • Is a machine that sends light through pigments and measures the fraction of light transmitted at each wavelength

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 • An absorption spectrum • Is a graph plotting light absorption versus wavelength

26. The absorption spectra of chloroplast pigments • Provide clues to the relative effectiveness of different wavelengths for 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 • The absorption spectra of three types of pigments in chloroplasts

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. • The action spectrum of a pigment • Profiles the relative effectiveness of different wavelengths 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 • The action spectrum for photosynthesis • Was 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

31. Is the main photosynthetic pigment • Chlorophyll b • Is an accessory pigment

32. Other accessory pigments • Absorb different wavelengths of light and pass the energy to chlorophyll a

33. 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 from a ground state to an excited state, which is unstable

34. Figure 10.11 B • If an isolated solution of chlorophyll is illuminated • It will fluoresce, giving off light and heat

35. A Photosystem: A Reaction Center Associated with Light-Harvesting Complexes

36. 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 • Is composed of a reaction center surrounded by a number of light-harvesting complexes

37. The light-harvesting complexes • Consist of pigment molecules bound to particular proteins • Funnel the energy of photons of light to the reaction center

38. When a reaction-center chlorophyll molecule absorbs energy • One of its electrons gets bumped up to a primary electron acceptor

39. The thylakoid membrane • Is populated by two types of photosystems, I and II

40. Noncyclic Electron Flow • Noncyclic electron flow • Is the primary pathway of energy transformation in the light reactions

41. 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 oxygen

42. e– ATP e– e– NADPH e– e– e– Mill makes ATP Photon e– Photon Photosystem I Photosystem II Figure 10.14  • A mechanical analogy for the light reactions

43. Cyclic Electron Flow • Under certain conditions • Photoexcited electrons take an alternative path

44. Primary acceptor Primary acceptor Fd Fd NADP+ Pq NADP+ reductase Cytochrome complex NADPH Pc Photosystem I ATP Photosystem II Figure 10.15 • In cyclic electron flow • Only photosystem I is used • Only ATP is produced

45. A Comparison of Chemiosmosis in Chloroplasts and Mitochondria • Chloroplasts and mitochondria • Generate ATP by the same basic mechanism: chemiosmosis • But use different sources of energy to accomplish this

46. Key Higher [H+] Lower [H+] Chloroplast Mitochondrion CHLOROPLAST STRUCTURE MITOCHONDRION STRUCTURE H+ Diffusion Thylakoid space Intermembrance space Electron transport chain Membrance ATP Synthase Stroma Matrix ADP+ P ATP H+ Figure 10.16 • The spatial organization of chemiosmosis • Differs in chloroplasts and mitochondria

47. In both organelles • Redox reactions of electron transport chains generate a H+ gradient across a membrane • ATP synthase • Uses this proton-motive force to make ATP

48. 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 • The light reactions and chemiosmosis: the organization of the thylakoid membrane

49. Concept 10.3: The Calvin cycle uses ATP and NADPH to convert CO2 to sugar • The Calvin cycle • Is similar to the citric acid cycle • Occurs in the stroma

50. The Calvin cycle has three phases • Carbon fixation • Reduction • Regeneration of the CO2 acceptor