1. PHOTOSYNTHESIS Chapter 10

2. BASIC VOCABULARY Autotrophs – producers; make their own “food” Heterotrophs – consumers; cannot make own food

3. LEAF STRUCTURE Stomata (stoma) – microscopic pores that allow water, carbon dioxide and oxygen to move into/out of leaf Chloroplasts – organelle that performs photosynthesis Found mainly in mesophyll – the tissue of the interior leaf Contain chlorophyll (green pigment) Stroma – dense fluid in chloroplast Thylakoid membrane – inner membrane of chloroplast Grana (granum) – stacks of thylakoid membrane

4. Figure 10.2 Focusing in on the location of photosynthesis in a plant

5. PHOTOSYNTHESIS SUMMARY 6CO 2 + 6H 2 0 + light energy C 6 H 12 O 6 + 6O 2 Oxygen comes from water, not CO 2 Two parts: Light Reactions The Calvin Cycle (Dark Reactions or Light Independent)

6. Figure 10.3 Tracking atoms through photosynthesis

7. Figure 10.4 An overview of photosynthesis: cooperation of the light reactions and the Calvin cycle

8. LIGHT Photons – discrete packets of light energy Chlorophyll a – (blue-green)only pigment that is directly used in light reactions Chlorophyll b – (yellow-green) accessory pigment Carotenoids - (yellow-orange)

9. Figure 10.6 Why leaves are green: interaction of light with chloroplasts

10. Figure 10.8 Evidence that chloroplast pigments participate in photosynthesis: absorption and action spectra for photosynthesis in an alga

11. PHOTOEXCITAION When photons hit chlorophyll and other pigments, electrons are excited to an orbital of higher energy In solution when the excited electrons fall, they give off energy (a photon) and fluoresce

12. Figure 10.9 Location and structure of chlorophyll molecules in plants

13. LIGHT REACTIONS Photosystems: Made of proteins and other molecules surrounding chlorophyll a Contain a primary electron acceptor Photosystem I – P700 Photosytem II – P680

14. Figure 10.11 How a photosystem harvests light

15. Require light to occur Two pathways: Noncyclic (predominant route) Cyclic Noncyclic animation Another animation

16. NONCYCLIC ELECTRON FLOW Photosystem II absorbs light Two electrons excited and captured by primary electron acceptor “Hole” in photosystem II is filled by 2 electrons that come from the splitting of water H 2 O 2H + + ½ O2 + 2e-

17. Figure 10.12 How noncyclic electron flow during the light reactions generates ATP and NADPH (Layer 1)

18. Oxygen is released Excited electrons pass from primary electron acceptor down an electron transport chain to photosystem I (filling its “hole”) ATP is made by photophosphorylation as electrons fall down ETC

19. Figure 10.12 How noncyclic electron flow during the light reactions generates ATP and NADPH (Layer 2)

20. Figure 10.12 How noncyclic electron flow during the light reactions generates ATP and NADPH (Layer 3)

21. Photons excite 2 electrons from Photosystem I and are captured by its primary electron acceptor Electrons then move down another ETC to ferredoxin (Fd) Fd gives electrons to NADP + (nicotinamide dinucleotide phosphate) making NADPH The enzyme that helps this transfer of e- is called NADP + reductase

22. Figure 10.12 How noncyclic electron flow during the light reactions generates ATP and NADPH (Layer 4)

23. Figure 10.12 How noncyclic electron flow during the light reactions generates ATP and NADPH (Layer 5)

24. Figure 10.13 A mechanical analogy for the light reactions

25. Figure 10.14 Cyclic electron flow

26. CYCLIC ELECTRON FLOW Only Photosystem I is used Fd passes electrons back to Photosystem I via ETC Some ATP made No NADPH made No oxygen released Used when cell needs more ATP than NADPH

27. ETC Food (chemical energy) to ATP (chemical energy) ATP synthase Pumps H+ into intermembrane space Light energy to ATP (chemical energy) ATP synthase Pumps H+ into thylakoid space MITOCHONDRIACHLOROPLAST

28. Figure 10.15 Comparison of chemiosmosis in mitochondria and chloroplasts

29. Figure 10.17 The Calvin cycle (Layer 1)

30. Figure 10.17 The Calvin cycle (Layer 2)

31. Figure 10.17 The Calvin cycle (Layer 3)

32. CALVIN CYCLE Also called Dark Reactions because light is not needed; however products from light reactions are needed. Carbon Fixation – initial incorporation of carbon into organic molecules CO 2 attaches to a 5-carbon sugar called ribulose bisphosphate (RuBP) The enzyme that catalyzes this is called rubisco Calvin cycle animation

33. Immediately splits into two 3-carbon molecules called 3-phosphoglycerate 3-phosphoglycerate is phosphorylated by ATP (from light reactions) making 1,3-bisphosphoglycerate 1,3-bisphosphoglycerate is reduced by taking electrons from NADPH making glyceraldehyde 3-phosphate (G3P) One G3P molecule leaves cycle to be used by plant The remaining G3P’s are converted into RUBP in several steps and by getting phosphorylated by ATP

34. Recall, G3P is the sugar formed by splitting glucose in glycolysis G3P can be made into glucose, sucrose, cellulose etc. by plant

35. C 3 PLANTS – have a problem Examples : rice, wheat, and soy beans Problem - produce less food when stomata are closed during hot days because low CO 2 starves Calvin Cycle and rubisco can accept O 2 instead of CO 2 High oxygen levels = O 2 passed to RUBP (not CO 2 ) and Calvin cycle stops When this oxygen made product splits, it makes a molecule that is broken down by releasing CO 2

36. This process is called photorespiration. Occurs during daylight (photo) Uses O 2 and makes CO 2 (respiration) NO ATP made (unlike respiration) and NO food made Early earth had low O 2 so this would not have mattered as much Photorespiration drains away as much as 50% of carbon fixed by Calvin Cycle in many plants.

37. C 4 PLANTS – have a solution Examples: sugarcane, corn and grasses Leaves contain bundle-sheath cells and mesophyll cells Bundle sheath surrounds veins of leaf (location of Calvin cycle) Mesophyll – between bundle and surface

38. In mesophyll cells: CO 2 fixed to phosphoenolpyruvate (PEP) PEP carboxylase is the enzyme that does this PEP carboxylase has higher affinity for CO 2 than rubisco so less danger of O 2 interfering The fixed CO 2 is then taken to Calvin cycle (in bundle-sheath) as part of a 4-carbon molecule (malate) Malate gives CO 2 to Calvin cycle

39. Figure 10.18 C 4 leaf anatomy and the C 4 pathway

40. CAM PLANTS – have another solution (crassulacean acid metabolism) Examples: succulent plants (pineapples and cacti etc.) Open stomata at night and close during day At night CO 2 is fixed into organic acids in mesophyll and then taken to Calvin cycle (also in mesophyll) during day.

41. Figure 10.19 C 4 and CAM photosynthesis compared

42. PHOTOSYNTHESIS FACTS 50% of organic material made is used by plant in respiration Organic molecules often leave leaves as sucrose Large amounts of cellulose are made (for cell walls) “And no process is more important than photosynthesis to the welfare of life on Earth.” (Campbell and Reece, 2005)

43. Figure 10.20 A review of photosynthesis