Where It Starts – Photosynthesis

1. Where It Starts – Photosynthesis Chapter 7 Part 2

2. 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

3. Carbon Fixation • Carbon fixation • Extraction of carbon atoms from inorganic sources (atmosphere) and incorporating them into an organic molecule • Builds glucose from CO2 • Uses bond energy of molecules formed in light-dependent reactions (ATP, NADPH)

4. The Calvin-Benson Cycle • Enzyme rubisco attaches CO2 to RuBP • Forms two 3-carbon PGA molecules • PGAL is formed • PGAs receive a phosphate group from ATP, and hydrogen and electrons from NADPH • Two PGAL combine to form a 6-carbon sugar • Rubisco is regenerated

5. Inputs and Outputs of the Calvin-Benson Cycle

6. The Calvin-Benson Cycle

7. A Six CO2 in air spaces inside of a leaf diffuse into a photosynthetic cell. Rubisco attaches each to a RuBP molecule. The resulting intermediates split, so twelve molecules of PGA form. B Each PGA molecule gets a phosphate group from ATP, plus hydrogen and electrons from NADPH. Twelve intermediate molecules (PGAL) form. 6CO2 C Two of the PGAL combine and form one molecule of glucose. The glucose may enter reactions that form other carbohydrates, such as sucrose and starch. A B ATP 6 RuBP 12 12 PGA 6 ADP 12 ADP + 12 Pi Calvin–Benson Cycle ATP 6 NADPH 12 D The remaining ten PGAL get phosphate groups from ATP. The transfer primes them for endergonic reactions that regenerate the 6 RuBP. 4 Pi 12 NADP+ D 12 PGAL 10 PGAL C other molecules glucose Fig. 7-11, p. 115

8. A Six CO2 in air spaces inside of a leaf diffuse into a photosynthetic cell. Rubisco attaches each to a RuBP molecule. The resulting intermediates split, so twelve molecules of PGA form. B Each PGA molecule gets a phosphate group from ATP, plus hydrogen and electrons from NADPH. Twelve intermediate molecules (PGAL) form. 6CO2 A B 12 PGA 6 RuBP ATP 12 6 ADP D The remaining ten PGAL get phosphate groups from ATP. The transfer primes them for endergonic reactions that regenerate the 6 RuBP. 12 ADP + 12 Pi Calvin–Benson Cycle ATP 6 12 NADPH C Two of the PGAL combine and form one molecule of glucose. The glucose may enter reactions that form other carbohydrates, such as sucrose and starch. 4 Pi 12 NADP+ D 12 PGAL 10 PGAL C other molecules glucose Stepped Art Fig. 7-11, p. 115

9. Animation: Calvin-Benson cycle

10. 7.7 Adaptations: Different Carbon-Fixing Pathways • Environments differ, and so do details of photosynthesis • C3 plants • C4 plants • CAM plants

11. Stomata • Stomata • 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

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

13. Photorespiration • When stomata are closed, CO2 needed for light-independent reactions can’t enter, O2 produced by light-dependent reactions can’t leave • Photorespiration • At high O2 levels, rubisco attaches to oxygen instead of carbon • CO2 is produced rather than fixed

14. 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

15. C3 and C4 Plant Leaves

16. Fig. 7-12a, p. 116

17. palisade mesophyll cell spongy mesophyll cell A C3 plant leaves. Chloroplasts are distributed evenly among two kinds of mesophyll cells in leaves of C3 plants such as basswood (Tilia americana). The light-dependent and light-independent reactions occur in both cell types. Fig. 7-12a, p. 116

18. Fig. 7-12b, p. 116

19. bundle-sheath cell mesophyll cell B C4 plant leaves. In C4 plants such as corn (Zea mays), carbon is fixed the first time in mesophyll cells, which are near the air spaces in the leaf, but have few chloroplasts. Specialized bundle-sheath cells ringing the leaf veins closely associate with mesophyll cells. Carbon fixation occurs for the second time in bundle-sheath cells, which are stuffed with rubisco-containing chloroplasts. Fig. 7-12b, p. 116

20. CAM Plants • CAM plants (Crassulacean Acid Metabolism) • Plants with an alternative carbon-fixing pathway that allows them to conserve water in climates where days are hot • Forms 4-carbon oxaloacetate at night, which is later broken down to CO2 for sugar production • Example: succulents, cactuses

21. A CAM Plant • Jade plant (Crassula argentea)

22. C3, C4, and CAM Reactions

23. Fig. 7-13a, p. 117

24. mesophyll cell CO2 O2 glycolate RuBP Calvin– Benson Cycle PGA sugar ATP NADPH A C3 plants. On dry days, stomata close and oxygen accumulates to high concentration inside leaves. The excess causes rubisco to attach oxygen instead of carbon to RuBP. Cells lose carbon and energy as they make sugars. Fig. 7-13a, p. 117

25. Fig. 7-13b, p. 117

26. mesophyll cell CO2 from inside plant oxaloacetate C4 Cycle bundle-sheath cell CO2 RuBP Calvin–Benson Cycle PGA sugar B C4 plants. Oxygen also builds up inside leaves when stomata close during photosynthesis. An additional pathway in these plants keeps the CO2 concentration high enough to prevent rubisco from using oxygen. Fig. 7-13b, p. 117

27. Fig. 7-13c, p. 117

28. mesophyll cell CO2 from outside plant oxaloacetate C4 Cycle night day CO2 RuBP Calvin–Benson Cycle PGA sugar C CAM plants open stomata and fix carbon using a C4 pathway at night. When stomata are closed during the day, the organic compounds made during the night are converted to CO2 that enters the Calvin–Benson cycle. Fig. 7-13c, p. 117

29. 7.6-7.7 Key Concepts:Making Sugars • The second stage is the “synthesis” part of photosynthesis, in which sugars are assembled from CO2 • The reactions use ATP and NADPH that form in the first stage of photosynthesis • Details of the reactions vary among organisms

30. 7.8 Photosynthesis and the Atmosphere • The evolution of photosynthesis dramatically and permanently changed Earth’s atmosphere

31. Different Food Sources • Autotrophs • Organisms that make their own food using energy from the environment and inorganic carbon • Heterotrophs • Organisms that get energy and carbon from organic molecules assembled by other organisms

32. Two Kinds of Autotrophs • Chemoautotrophs • Extract energy and carbon from simple molecules in the environment (hydrogen sulfide, methane) • Used before the atmosphere contained oxygen • Photoautotrophs • Use photosynthesis to make food from CO2 and water, releasing O2 • Allowed oxygen to accumulate in the atmosphere

33. Earth With and Without Oxygen Atmosphere

34. Fig. 7-15a, p. 118

35. Fig. 7-15b, p. 118

36. Effects of Atmospheric Oxygen • Selection pressure on evolution of life • Oxygen radicals • Development of ATP-forming reactions • Aerobic respiration • Formation of ozone (O3) layer • Protection from UV radiation

37. 7.8 Key Concepts:Evolution and Photosynthesis • The evolution of photosynthesis changed the composition of Earth’s atmosphere • New pathways that detoxified the oxygen by-product of photosynthesis evolved

38. 7.9 A Burning Concern • Earth’s natural atmospheric cycle of carbon dioxide is out of balance, mainly as a result of human activity

39. The Carbon Cycle • Photosynthesis locks CO2 from the atmosphere in organic molecules; aerobic respiration returns CO2 to the atmosphere • A balanced cycle of the biosphere • Humans burn wood and fossil fuels for energy, releasing locked carbon into the atmosphere • Contributes to global warming, disrupting biological systems

40. Fossil Fuel Emissions

41. 7.9 Key Concepts: Photosynthesis, CO2 & Global Warming • Photosynthesis by autotrophs removes CO2 from the atmosphere; metabolism by all organisms puts it back in • Human activities have disrupted this balance, and contribute to global warming

42. Animation: C3-C4 comparison

43. Animation: Harvesting photo energy

44. Animation: Light-dependent reactions

45. Animation: Photosynthesis overview

46. Animation: Structure of a chloroplast

47. Animation: Wavelengths of light

48. ABC video: Solar Power

49. Video: Biofuels