How Cells Acquire Energy

1. How Cells Acquire Energy Chapter 7

2. Overview: I. Places A. Chloroplasts : photosynthesis organelle B. Thylakoid: discs i. light-dependent: sunlight to chemical energy ii. ETC and water split = forming H+, e- , ATP: oxygen is by-product C. Stroma: fliud interior i. light-independent: makes glucose ii. carbon dioxide reduced to form glucose sunlight 12H2O + 6CO2 ————> 6O2 + C6H12O6 + 6H2O

3. Photoautotrophs – “self-nourishing” • Carbon source is carbon dioxide • Energy source is sunlight • Capture sunlight energy and use it to carry out photosynthesis • Plants, Some bacteria, Many protistans • Heterotrophs • Get carbon and energy by eating autotrophs or one another

4. II. Sunlight A. photoautotrophs intercept only about 1% of solar energy B. wavelength C. Electromagnetic spectrum Short-----------------------------------------------Long Gamma, x-ray, UV Visible infrared, microwaves, radio Too much energy just right energy not enough energy VIBGYOR

5. Photons • Packets of light energy • Each type of photon has fixed amount of energy • Photons having most energy travel as shortest wavelength (blue-violet light)

6. Chloroplast Structure two outer membranes stroma inner membrane system (thylakoids connected by channels) Figure 7.3d, Page 116

7. Photosynthesis Equation LIGHT ENERGY 12H2O + 6CO2 6O2 + C2H12O6 + 6H2O Water Carbon Dioxide Oxygen Glucose Water In-text figurePage 115

8. E. Pigments: molecules that absorb photons i. the color you see is the color reflected ii. absorption spectrum -chlorophylls: absorb all except green -carotenoids: accessory pigments: absorb blue-violet and blue-green -xanthophylls and beta-carotene -abundant in fruits, flowers and vegetables -can be seen in leaves during autumn -anthocyanins: blue or red accessory pigment -phycobilins: blue accessory pigment iii. photosystems: organization of pigments -proteins and 200-300 pigment molecules -PSII (p680) -PSI (p700)

9. Pigments in Photosynthesis • Bacteria • Pigments in plasma membranes • Plants • Pigments and proteins organized into photosystems that are embedded in thylakoid membrane system

10. Arrangement of Photosystems water-splitting complex thylakoid compartment H2O 2H + 1/2O2 P680 P700 acceptor acceptor pool of electron carriers PHOTOSYSTEM II stroma PHOTOSYSTEM I Figure 7.10Page 121

11. Light-Dependent Reactions I. General A. Thylakoid B. 3 Basic steps i. photosystems harvest sunlight ii. solar energy converted to chemical energy ATP iii. NADP+ (coenzyme) picks up e- and H+ II. “Random Walk” of Energy A. sunlight is trapped and randomly passed from chlorophyll b and carotenoids to reaction center (chlorophyll a) B. Low energy electrons become “excited” and is passed to e- acceptor C. e- can follow 2 pathways

12. Photosystem Function: Harvester Pigments • Most pigments in photosystem are harvester pigments • When excited by light energy, these pigments transfer energy to adjacent pigment molecules • Each transfer involves energy loss

13. Photosystem Function: Reaction Center • Energy is reduced to level that can be captured by molecule of chlorophyll a • This molecule (P700 or P680) is the reaction center of a photosystem • Reaction center accepts energy and donates electron to acceptor molecule

14. Pigments in a Photosystem reaction center Figure 7.11Page 122

15. Electron Transfer Chain • Adjacent to photosystem • Acceptor molecule donates electrons from reaction center • As electrons pass along chain, energy they release is used to produce ATP

16. Cyclic Electron Flow • Electrons • are donated by P700 in photosystem I to acceptor molecule • flow through electron transfer chain and back to P700 • Electron flow drives ATP formation • No NADPH is formed

17. Cyclic Electron Flow e– electron acceptor Electron flow through transfer chain sets up conditions for ATP formation at other membrane sites. electron transfer chain e– e– ATP e– Figure 7.12Page 122

18. Noncyclic Electron Flow • Two-step pathway for light absorption and electron excitation • Uses two photosystems: type I and type II • Produces ATP and NADPH • Involves photolysis - splitting of water

19. Machinery of Noncyclic Electron Flow H2O second electron transfer chain photolysis e– e– ATP SYNTHASE first electron transfer chain NADPH NADP+ ATP ADP + Pi PHOTOSYSTEM II PHOTOSYSTEM I Figure 7.13aPage 123

20. III. Cyclic (1 word) Pathway • A. PSI (700) – one photosystem • B. ATP – one product • C. oldest pathway • IV. Non-Cyclic (2 words) Pathway • A. PSII to PSI (2 photosystems) • B. ATP and NADPH (2 products) • C. uses water (photolysis) • D. Evolution of oxygen

21. V. Chemiosmotic model for ATP formation A. hydrogen ions released by photolysis accumulate inside the thylakoid i. oxygen is given off as waste B. more H+ accumulates from the ETC C. the build up of H+ ions inside the thylakoid results in 2 gradients: concentration and electric i. these 2 gradients force H+ into the stroma through ATP synthase forming ATP

22. Chemiosmotic Model for ATP Formation H+ is shunted across membrane by some components of the first electron transfer chain Gradients propel H+ through ATP synthases; ATP forms by phosphate-group transfer Photolysis in the thylakoid compartment splits water H2O e– acceptor ATP SYNTHASE ATP ADP + Pi PHOTOSYSTEM II Figure 7.15Page 124

23. Light-Independent Reactions • Synthesis part of photosynthesis • Can proceed in the dark • Take place in the stroma • Calvin-Benson cycle • need ATP (energy) and NADPH (H+ and e-) • Uses carbon dioxide

24. Overall reactants Carbon dioxide ATP NADPH Overall products Glucose ADP NADP+ Calvin-Benson Cycle Reaction pathway is cyclic and RuBP (ribulose bisphosphate) is regenerated

25. 6 CO2 (from the air) Calvin- Benson Cycle CARBON FIXATION 6 6 RuBP unstable intermediate 12 PGA 6 ADP 12 ATP 6 ATP 12 NADPH 4 Pi 12 ADP 12 Pi 12 NADP+ 10 PGAL 12 PGAL 2 PGAL Pi P Figure 7.16Page 125 glucose

26. II. Calvin-Benson Cycle A. Capture of carbon dioxide (Carbon Fixation – 1st step) i. stroma ii. enzyme (RuBP Carboxylase – Rubisco) + RuBP (ribulosebisphosphate) + CO2 yields 6-C intermediate that splits into 2 , 3-C PGA (phosphoglycerate) molecules B. Reduction of PGA i. PGA accepts one phosphate group from ATP plus hydrogen and electrons from the NADPH, resulting in the formation of the intermediate PGAL (phosphorglyceraldehyde)

27. C. Regeneration of RuBP i. to form 1 six carbon sugar requires 6 CO2 molecules and the formation of 12 PGAL’s ii. most of the PGAL get recycled back to RuBP, but 2 of the PGAL combine forming glucose with a phosphate group iii. when glucose is phosphorylated like this it is primed to enter reactions to become sucrose, cellulose and starch (the main plant carbohydrates) -the ADP, NADP+ and released phosphate diffuse through the stroma back to the light-dependent reactions

28. Two Stages of Photosynthesis sunlight water uptake carbon dioxide uptake ATP ADP + Pi LIGHT-DEPENDENT REACTIONS LIGHT-INDEPENDENT REACTIONS NADPH NADP+ glucose P oxygen release new water In-text figurePage 117

29. C4, C3 and CAM plants I. General A. since environments differ, photosynthesis is not the same in all plants B. We will compare the carbon-fixing adaptations in plants i. compared to C3 plants (most plants), C4 and CAM plants have modified ways of fixing carbon for photosynthesis during hot, dry conditions

30. C. Stomata: tiny openings in leaf surface through which gases diffuse i. closes on hot dry days to conserve water; when this happens CO2 gets used up and O2 builds up D. a high O2 concentration triggers photorespiration, a process that lowers a plant’s sugar making capacity i. rubisco (the enzyme in the calvin-benson cycle that attaches to CO2 in carbon fixation) also attaches to oxygen ii. competition between CO2 and O2 for rubisco iii. sugars are still formed, but at a lower rate

31. The C3 Pathway • In Calvin-Benson cycle, the first stable intermediate is a three-carbon PGA • Because the first intermediate has three carbons, the pathway is called the C3 pathway • C3 plants • bluegrass, beans, wheat, rice, oats (most plants) • In hot, dry conditions photorespiration occurs

33. III. C4 plants A. corn, bermuda grass, sugar cane B. oxaloacetate (4 carbons) is the first intermediate in these plants C. these plants fix carbon twice, in 2 types of photosynthetic cells, so CO2 levels do not decline as much

34. D. 2 types of photosynthetic cells i. mesophyll uses PEP (phenolpyruvate, 3-C) to fix CO2 ii. oxaloacetate forms, then malate which diffuses into adjoining bundle sheaths iii. in the bundle sheaths, pyruvate forms. CO2 is released and enters the C-B cycle iv. the pyruvate regenerates PEP for use in the C4 cycle in the mesophyll cells E. Advantages: small stomata, lose less water, make more glucose than C3 plants during hot, bright, dry days

35. CAM Plants • Carbon is fixed twice (in same cells) • Night • Carbon dioxide is fixed to form organic acids • Day • Carbon dioxide is released and fixed in Calvin-Benson cycle

36. IV. CAM (crassulacean acid metabolism) plants A. desert plants (cacti) B. Type of C4 i. closes stomates during day, open at night (fix CO2 at night) ii. at night, mesophyll cells use C4 cycle and store malate and other products until the next day iii. day time, stomata close: malate releases CO2 which enters the C-B cycle in the same cells -this way photosynthesis continues without water loss

37. light LIGHT-DEPENDENT REACTIONS 6O2 12H2O ATP NADP+ NADPH ADP + Pi PGA CALVIN-BENSON CYCLE PGAL 6H2O 6CO2 RuBP P C6H12O6 (phosphorylated glucose) end product (e.g., sucrose, starch, cellulose) Summary of Photosynthesis LIGHT-INDEPENDENT REACTIONS Figure 7.21Page 129