Photosynthesis

1. Photosynthesis

2. I. Overview A. Background 1. The conversion of light energy (from the sun) into chemical energy (stored in sugar & organic molecules. 2. Plants, algae (protists), cyanobacteria, phytoplankton 3. Plants are primary producers – produce organic molecules from CO2 & H2O. They are the bottom of the terrestrial food chain. All complex organisms on land depend on plants (ultimately) for food & O2.

3. B. Photosynthetic Structures Fig 10.3

4. 1. Cyanobacteria 3. Plants 2. Protists a. Chloroplast i. Location ii. Structure 2 membranes (inner & outer) Thylakoid membranes – site of the light reactions Stroma – site of the Calvin cycle, contains rubisco

5. C. PhotoSyNthesis (PSN) - overview 1. Reaction a. 6CO2 + 6H20 C6H12O6 + 6O2 O2 is released as a by-product from the splitting of water 2. 2 interdependent pathways: a. Light reactions – chlorophyll absorbs light energy to create high energy molecules: ATP & NADPH + H+. These are used to drive: b. Calvin cycle – carbon fixation

6. Fig 10.5

7. D. Characteristics of Light 1. What is light?? = electromagnetic energy (radiation) 2. Consists of groups of particles called photons 3. Travels in waves 4. Wavelength = distance between peaks in wave 5. Only visible light drives PSN: 380 to 750nm

8. Fig 10.6

9. E. Plant Pigments 1. Molecular substances that absorb visible light. 2. Chlorophyll a – main PSN pigment, absorbs red & blue light (reflects green) to initiate the light reactions 3. Accessory pigments – absorb light energy & transfer it to Chlorophyll a, Chlorophyll b, Carotenoids, Xanthophylls

11. Fig 10.9

12. II. Photosystems A. Light Dependent Reactions 1. Background a. The functional units of chlorophyll & accessory pigments that work together to absorb a photon of light. “Kicks” electrons to an excited state (high energy) b. 2 main components: i. Light-harvesting complex – accessory pigments absorb light, pass their excited electrons to ii. Reaction center complex – a pair of chlorophyll molecules. Passes the excited electrons to a primary electron acceptor.

13. Fig 10.12

14. Fig 10.11

15. c. Types of photosystems in the thylakoid membranes: i. Photosystem I (P700) – absorbs 700nm wavelengths best (far-red) ii. Photosystem II (P680) – absorbs 680nm wavelengths best (red)

16. 2. Non-cyclic Photophosphorylation a. Structure and Location b.Process or Steps i. PS II absorbs light, splitting water into H+, electrons, and O2. ii. Further light absorption by PS II kicks the electrons up to an excited state. iii. Excited electrons are pass from PS II to PS I along an electron transport chain (PSII  Pq  cytochrome complex  Pc  PS I). Energy released at every step used to drive ATP synthesis iv. When electron reaches PS I, light absorption kicks it up to excited state again. v. Excited electron is passed to Fd to NADP+ - the terminal electron acceptor. Reduced to NADPH

17. Fig 10.13

18. Figure 10.17

19. c. ATP Production • i. Pathway • Non-cyclic electron flow produces ATP & NADPH + H+. • Energy released during electron transfer drives proton pump at • cytochrome complex. • H+ pumped from stroma into thylakoid lumen, creating an • electrochemical gradient • H+ diffusion back out into stroma drives ATP synthesis in Stroma. d. Electron Carrier (NADPH + H+) • i. Pathway

20. 3. Cyclic Photophosphorylation • a. Structure and Location • b. Process or Steps • i. Produces ATP only • ii. electrons transferred from Fd back to the • cytochrome complex (instead of to NADP+) • iii. thus more H+pumped into lumen thus more ATP • produced

21. c. Why cyclic flow? i. Non-cyclic flow produces equal amounts of ATP & NADPH + H+. ii. But… Calvin cycle requires more ATP than NADPH + H+. So…. NADPH + H+builds up in the stroma, triggering The shift to cyclic phosphorylation

22. Fig 10.15

23. Fig 10.5

24. B. Light Independent Reactions • 1. The Calvin Cycle • a. Location • The use of ATP & NADPH + H+ to convert CO2 to • carbohydrates • 2. Process and Steps: • a. Carbon fixation: RuBP + CO2 3PGA • b. Reduction: • 3PGA + 2ATP + NADPH + H+G3P • c. Regeneration of RuBP from G3P • d. G3P leaves the chloroplast to become ?!?! Rubisco

25. Fig 10.18

26. III. Problems and Limitations A. Overall efficiency < 35% B. Wavelength (λ) of light – shorter λ have greater energy. PSN use of longer λ means more photons needed C. Cyclic phosphorylation – need extra photons just for extra ATP production

27. D. Photorespiration • 1. Steps • a. light-dependent inhibition of C fixation • b. Cause: increased O2 concentration in leaf – • How? Stomata are closed • c. Rubisco adds O2 to RuBP to make a compound • (phosphoglycolate) that is converted back to CO2 in the • mitochondria. • d. No carb’s produced, but energy required. • e. Wastes about 50% of C compounds in the • chloroplast

28. IV. Plant Adaptations A. C3 Plants senescent 1. Strategy 2. Examples

29. B. C4 plants • 1. Strategy • a. 4-C compound instead of a 3-C compound (3PGA) • as first product of Calvin cycle • b. CO2 + PEP Oxaloacetate Malate • c. Malate exported to bundle sheath cells • d. CO2 released from malate to enter Calvin cycle PEP carboxylase 2. Examples

30. Fig 10.18

31. 3. Why is the C4 pathway more efficient? a. PEP carboxylase has much higher affinity for CO2 than Rubisco b. CO2 is stored as malate in the bundle-sheath cells a. thus plenty of CO2 even when stomata closed c. Benefits? i. Increased WUE (?) ii. Little photorespiration

32. C. Crassulacean Acid Metabolism (CAM) 1. Strategy a. Stomata open only at night b. CO2 is stored in organic acids in mesophyll cells. c. During day, CO2 released from organic acids and enters Calvin cycle in same mesophyll cells d. Same benefits as C4 pathway, but even greater WUE. 2. Examples

33. Fig 10.20