Where It Starts – Photosynthesis

1. Where It Starts – Photosynthesis Chapter 7 Hsueh-Fen Juan Oct. 02 & 09, 2012

2. ABC video: Solar Power

3. Impacts, Issues:Biofuels • Coal, petroleum, and natural gas were once ancient forests, a limited resource; biofuels from wastes are a renewable resource

4. 7.1 Sunlight as an Energy Source • Photosynthetic organisms use pigments to capture the energy of sunlight • Photosynthesis • The synthesis of organic molecules from inorganic molecules using the energy of light • 光合作用定義：利用光能將無機分子轉變為有機分子的代謝途徑

5. Properties of Light • Visible light is part of an electromagnetic spectrum of energy radiating from the sun • Travels in waves • Organized into photons • Wavelength • The distance between the crests of two successive waves of light (nm)

6. Electromagnetic Spectrum of Radiant Energy

7. The Rainbow Catchers • Different wavelengths form colors of the rainbow • Photosynthesis uses wavelengths of 380-750 nm • Pigment • An organic molecule that selectively absorbs light of specific wavelengths • Chlorophyll a • The most common photosynthetic pigment • Absorbs violet and red light (appears green) • 因此平常葉子呈綠色即因葉綠素含量豐富之故，但一到秋天，葉綠素大量破壞流失，其他色素反而較穩定，因此葉子呈現黃、紅等色

8. Photosynthetic Pigments • Collectively, chlorophyll and accessory pigments absorb most wavelengths of visible light • The light-trapping part of a pigment is an array of atoms, and electrons of these atoms occupy one large orbital that spans all of the atoms • Electrons in such arrays in pigment molecules absorb photons of light energy, boosting electrons to a higher energy level • Energy is captured and used for photosynthesis

9. Some Pigments in Photosynthesizers

10. Two Photosynthetic Pigments

11. 7.2 Exploring the Rainbow • Engelmann identified colors of light that drive photosynthesis (violet & red,光合色素所吸收的) by using a prism to divide light into colors • Algae (藻類) using these wavelengths gave off the most oxygen (使用好氧菌來測試) • An absorption spectrum (吸收光譜) shows which wavelengths a pigment absorbs best • Organisms in different environments use different pigments (地球不同地方能收到的陽光波長也不同) • 為啥藻類喜歡吸收500-600nm的光？因為海水對此波長的光吸收率最差，因此，水生藻類大多富含能吸收此段波長光的光合色素

12. Photosynthesis and Wavelengths of Light

13. 7.1-7.2 Key Concepts:The Rainbow Catchers • The flow of energy through the biosphere starts when chlorophylls and other photosynthetic pigments absorb the energy of visible light

14. 7.3 Overview of Photosynthesis • Chloroplast • An organelle that specializes in photosynthesis in plants and many protists • Stroma • A semifluid matrix surrounded by the two outer membranes of the chloroplast • Sugars are built in the stroma • Stroma contains DNA and some ribosomes

15. Overview of Photosynthesis • Thylakoid membrane (囊狀膜) • Folded membrane that make up thylakoids (葉綠囊) • The space inside all of the disks is a single, continuous compartment • Contains clusters of light-harvesting pigments that absorb photons of different energies • Photosystems (type I and type II) • Groups of molecules that work as a unit to begin the reactions of photosynthesis • Convert light energy into chemical energy

16. Overview of Photosynthesis • Light-dependent reactions • Light energy is transferred to ATP and NADPH • Water molecules are split, releasing O2 • Light-independent reactions • Energy in ATP and NADPH drives synthesis of glucose and other carbohydrates from CO2 and water

17. Animation: Photosynthesis overview

18. Summary: Photosynthesis

19. Sites of Photosynthesis in Plants

20. Fig. 7-5a, p. 111

21. Fig. 7-5b, p. 111

22. Fig. 7-5c, p. 111

23. Animation: Sites of photosynthesis

24. 7.4 Light-Dependent Reactions • In the first stage of photosynthesis, light energy drives electrons out of photosystems • The electrons may be used in a noncyclic or cyclic pathway of ATP formation

25. Capturing Energy for Photosynthesis • Photons boost electrons in pigments to higher energy levels • Light-harvesting complexes absorb the energy • Electrons are released from special pairs of chlorophyll a molecules in photosystems

26. The Thylakoid Membrane

27. Cyclic and Noncyclic Pathways • Electrons from photosystems take noncyclic or cyclic pathways, forming ATP

28. Replacing Lost Electrons • Electrons lost from photosystem II are replaced by photolysis of water molecules, which dissociate into hydrogen ions and oxygen • Photolysis (光解) • Process by which light energy breaks down a molecule such as water

29. Electron Flow in a Noncyclic Pathway • Electrons lost from a photosystem enter an electron transfer chain in the thylakoid membrane • Electron transfer chains • Organized arrays of enzymes, coenzymes, and other proteins that accept and donate electrons in a series • 分清楚：light-harvesting complex 和 electron transfer chains

30. Harvesting Electron Energy • Light energy is converted to chemical energy • Entry of electrons from a photosystem into the electron transfer chain is the first step in light-dependent reactions • ATP forms in the stroma • Electron energy is used to build up a H+ gradient across the membrane • H+flows through ATP synthase (ATP合成酶), which attaches a phosphate group to ADP • ATP synthase is a membrane transport protein (H+ cannot simply diffuse through a lipid bilayer )

31. Noncyclic Pathway of Photosynthesis to second stage of reactions The Light-Dependent Reactions of Photosynthesis ATP synthase light energy ATP NADPH light energy ADP + Pi electron transfer chain photosystem II photosystem I NADP+ thylakoid compartment stroma A Light energy drives electrons out of photosystem II. C Electrons from photosystem II enter an electron transfer chain. E Light energy drives electrons out of photosystem I, which accepts replacement electrons from electron transfer chains. G Hydrogen ions in the thylakoid compartment are propelled through the interior of ATP synthases by their gradient across the thylakoid membrane. B Photosystem II pulls replacement electrons from water molecules, which dissociate into oxygen and hydrogen ions (photolysis). The oxygen leaves the cell as O2. D Energy lost by the electrons as they move through the chain causes H+ to be pumped from the stroma into the thylakoid compartment. An H+ gradient forms across the membrane. H H+flow causes the ATPsynthases to attach phosphate to ADP, so ATP forms in the stroma. F Electrons from photosystem I move through a second electron transfer chain, then combine with NADP+ and H+. NADPH forms. Fig. 7-8, p. 113

32. Electron Flow in a Cyclic Pathway

33. 7.5 Energy Flow in Photosynthesis • Energy flow in the light-dependent reactions is an example of how organisms harvest energy from their environment

34. Photophosphorylation

35. Energy Flow in Light-Dependent Reactions

36. Fig. 7-9a, p. 114

37. Fig. 7-9b, p. 114

38. 7.3-7.5 Key Concepts:Making ATP and NADPH • Photosynthesis proceeds through two stages in the chloroplasts of plants and many types of protists • In the first stage, sunlight energy is converted to the chemical bond energy of ATP • The coenzyme NADPH forms in a pathway that also releases oxygen (有NADPH有氧，反之則無)

39. 7.6 Light-Independent Reactions: The Sugar Factory • The cyclic, light-independent reactions of the Calvin-Benson cycle are the “synthesis” part of photosynthesis • Calvin-Benson cycle • Enzyme-mediated reactions that build sugars in the stroma of chloroplasts

40. Carbon Fixation • Carbon fixation (固碳) • Extraction of carbon atoms from inorganic sources (atmosphere) and incorporating them into an organic molecule • Builds glucose from CO2 • Uses bond energyof ATP and the reducing powerof NADPH formed in light-dependent reactions

41. The Calvin-Benson Cycle • Enzyme rubisco attaches CO2 to RuBP • Forms two 3-carbon PGA molecules (因六碳不穩定，馬上變成兩個三碳) • PGAL is formed • PGAs receive a phosphate group from ATP, and hydrogen and electrons from NADPH (PGA吃了這三樣玩意形成PGAL) • Two PGAL combine to form a 6-carbon sugar • Rubisco is regenerated (rubisco這酶要記一下)

42. Inputs and Outputs of the Calvin-Benson Cycle

43. The Calvin-Benson Cycle

44. Animation: Calvin-Benson cycle

45. 7.7 Adaptations: Different Carbon-Fixing Pathways • Environments differ, and so do details of photosynthesis • C3 plants • C4 plants • CAM plants

46. Stomata • Stomata (單數：stoma，氣孔) • Small openings through the waxy cuticle covering epidermal surfaces of leaves and green stems • Allow CO2 in and O2 out • Close on dry days to minimize water loss

47. C3 Plants • C3 plants • Plants that use only the Calvin–Benson cycle to fix carbon • Forms 3-carbon PGA in mesophyll cells (葉肉細胞) • Used by most plants, but inefficient in dry weather when stomata are closed

48. Photorespiration • When stomata are closed, CO2 needed for light-independent reactions can’t enter, O2 produced by light-dependent reactions can’t leave • Photorespiration (光呼吸) • At high O2 levels, rubisco attaches to oxygen instead of carbon • CO2 is produced rather than fixed • 光呼吸成因是O2累積過多，而非CO2無法進來

49. C4 Plants • C4 plants • Plants that have an additional set of reactions for sugar production on dry days when stomata are closed; compensates for inefficiency of rubisco • Forms 4-carbon oxaloacetate in mesophyll cells, then bundle-sheath cells make sugar • Examples: Corn, switchgrass, bamboo

50. C3 and C4 Plant Leaves