CHAPTER 8

1. CHAPTER 8 Photosynthesis: Energy from the Sun

2. Photosynthesis • Biochemical process in which light energy is converted to chemical energy • Photos = light synthesis = to put together • In plants, photosynthesis takes place in chloroplasts. • It involves many enzyme controlled steps

3. Photosynthetic Reactants and Products 6 CO2 + 12 H2O + light  C6H12O6 + 6 O2 + 6 H2O

4. figure 08-01.jpg Figure 8.1 Figure 8.1

5. Photosynthesis • Photosynthesis can be divided into two pathways: • The light reaction - driven by light energy captured by chlorophyll. Consists of Photosystem I and Photosystem II. It produces ATP and NADPH + H+. • The Calvin–Benson cycle - does not use light directly. It uses ATP, NADPH + H+, and CO2 to produce sugars.

6. Properties of Light and Pigments • Light is the source of energy that drives photosynthesis • Molecules that absorb light energy in the visible range are called pigments.

7. figure 08-05.jpg Figure 8.5 Figure 8.5

8. Properties of Light and Pigments When light and a pigment molecule meet, one of 3 things happen • Reflection – the light bounces off the molecule • Transmission – the light passes through the molecule • Excitation – the light is absorbed by the molecule. If absorbed, the molecule goes from its ground state to and excited state of higher energy • An electron is boosted to another orbital

9. Pigments • When a beam of white light shines on an object, and the object appears to be red in color, it is because it has absorbed all other colors from the white light except for the color red. • In the case of chlorophyll, plants look green because they absorb green light less effectively than the other colors found in sunlight and reflect the green light not absorb

10. Properties of Light and Pigments • Different pigment molecules absorb different wavelengths of light • The particular set of wavelengths that a pigment absorbs is called its absorption spectrum • Review Figures 8.7

11. figure 08-07.jpg Figure 8.7 Figure 8.7

12. Properties of Light and Pigments • Chlorophylls are the most important pigments in photosynthesis • Chlorophyll a is the primary pigment in photosynthesis. • Chlorophylls and accessory pigments trap light and transfer energy to a reaction center

13. Chlorophyll

14. An excited pigment molecule may • lose its energy by emitting light of longer wavelength or • transfer the absorbed energy to another pigment molecule as a redox reaction.

15. Figure 8.8 Energy Transfer and Electron Transport

16. There are two different systems for transport of electrons in photosynthesis. 1. Noncyclic electron transport produces NADPH + H+ and ATP and O2. 2. Cyclic electron transport produces only ATP.

17. Noncyclic • In noncyclic electron transport, two photosystems are required. • Photosystems consist of many chlorophyll molecules and accessory pigments bound to proteins.

18. Photosystem I • Photosystem I uses light energy to reduce NADP+ to NADPH + H+. • The reaction center contains a chlorophyll a molecule called P700 because it best absorbs light at a wavelength of 700 nm.

19. Photosystem II • Photosystem II uses light energy to split water, producing electrons, protons, and O2. • The reaction center contains a chlorophyll a molecule called P680 because it best absorbs light at a wavelength of 680 nm. • To keep noncyclic electron transport going, both photosystems must constantly be absorbing light.

20. After absorbing light energy: • an energized electron leaves the Chl* in the reaction center and participates in a series of redox reactions. • the electron flows through a series of carriers in the thylakoid membrane. • producing ATP

21. Figure 8. 9 Noncyclic Electron Transport Uses Two Photosystems (Part 1)

22. Figure 8. 9 Noncyclic Electron Transport Uses Two Photosystems (Part 2)

23. Cyclic Electron Transfer • Cyclic electron transport produces only ATP. • The electron passes from an excited P700 molecule and cycles back to the same P700 molecule. • No O2 is released. • In cyclic electron flow, photosystem I acts on its own.

24. Figure 8.10 Cyclic Electron Transport Traps Light Energy as ATP

25. “Z” Scheme • Photosystem I & II (P680 & P700) work together to generate ATP and NADPH. • This pathway is called the “Z” scheme. • Noncyclic

26. Noncyclic Electron Flow or Z Scheme • In Photosystem II chlorophyll a absorbs light energy to become energized chloropyll a • 2 electrons are released and caught by the primary electron acceptor. • H20  ½ O2 + 2 e- + 2H+

27. The electrons pass through a redox chain for chemiosmotic ATP production. • The electron transport chain pumps protons across the membrane into the thylakoid space. • The protons accumulate establishing a proton concentration gradient • ATP synthases open and the protons diffuse to generate ATP from ADP.

28. Z Scheme Cont’d • The electrons are passed to P700 chlorophyll • P700 loses electrons to Ferredoxin (Fd) • NADP combines with H to form NADPH. • NADPH is the source of H used to make C6H12O6

29. Figure 8.11 Chloroplasts Form ATP Chemiosmotically

30. The Calvin–Benson Cycle • The Calvin–Benson cycle makes sugar from CO2 • ATP and NADPH provide the needed energy • This pathway was elucidated through use of radioactive tracers

31. The Calvin–Benson Cycle Three phases: • 1. Carbon Fixation– RuBP + CO2 6 carbon sugar  3 PG (first stable product) • The reaction is catalyzed by rubisco (ribulose bisphosphate carboxylase). • 2. Series of reactions to produce G3P • 3. Regeneration of RuBP (7 enzymatic steps) • RuBP (ribulose biphosphate) is the initial CO2 acceptor

32. The Calvin–Benson Cycle • The end product of the cycle is glyceraldehyde 3-phosphate, G3P. • There are two fates for the G3P: • One-third ends up as starch, which is stored in the chloroplast and serves as a source of glucose. • Two-thirds is converted to the disaccharide sucrose, which is transported to other organs.

33. Figure 8.13 The Calvin-Benson Cycle

34. Rubisco • Rubisco is a carboxylase, adding CO2 to RuBP. It can also be an oxygenase, adding O2 to RuBP. • These two reactions compete with each other. • When RuBP reacts with O2, it cannot react with CO2, which reduces the rate of CO2 fixation.

35. Photorespiration • A specialized metabolic pathway in which rubisco reacts with O2 instead of CO2 • Occurs under stress conditions of hot, dry, bright days when the internal leaf concentration of O2 is greater than CO2 concentration. • Glucose production is reduced thereby limiting plant growth

36. C3 Plants • Most common type of plants on earth. • Grow best in temperate zones • Includes rice, wheat, soybeans, bluegrass • On hot days the stomata close, O2 builds up and photorespiration occurs. • The first product is the 3-C molecule of 3PG • CO2 + RuBP  3 phosophoglycerate (3 C compound)

37. Figure 8.16 Leaf Anatomy of C3 and C4 Plants

38. C4 Plants • C4 plants have 2 enzymes (PEP carboxylase & rubisco) for CO2 fixation in 2 different parts of the leaf. • PEP carboxylase does not have an affinity for O2 and fixes CO2 even at very low CO2 levels. • What is the significance of this fact? • C4 plants include sugarcane, corn and other plants that grow in hot, dry climates.

39. C4 Plants Cont’d • CO2 + PEP carboxylase  Oxaloacetate (4 C compound). • Occurs in cells near top of leaf • Oxaloacetate diffuses into bundle sheath cells in the interior of the cells. • Here oxaloacetate loses a C forming CO2 • CO2 enters the Calvin-Benson Cycle

40. Crassulacean Acid Metabolism (CAM) • CAM plants are succulents or water storing plants. • Include cacti and pineapples • CAM plants open their stomata only at night • CO2 enters and forms malic acid which is stored as an acid in the vacuoles until morning • In daylight, the CO2 is released from the acid and enters the Calvin Benson Cycle.

41. Stomates • Stomates close when weather is hot & dry. • O2 concentration increases, CO2 concentration decreases. • Why? • Ribulose requires high concentrations of CO2 • If sufficient CO2 is unavailable, photorespiration occurs.

42. Metabolic Pathways in Plants • Both photosynthesis and respiration occurs in plants. • Compare photosynthesis and respiration.