Lecture 3 Outline (Ch. 8)

2. Lecture 3 Outline (Ch. 8) • Photosynthesis overview • A. Purpose • B. Location • The light vs. the “dark” reaction • Chloroplasts pigments • A. Light absorption • B. Types • Light reactions • A. Photosystems • B. Photophosphorylation • V. The light independent reaction (“dark” reaction) • A. Carbon “fixation” • B. Reduction • C. Regeneration • VI. Alternative plants

3. Photosynthesis - overview

4. Photosynthesis - overview Overall purpose: • photosynthesis – light chemical energy • complements respiration Energy for all life on earth ultimately comes from photosynthesis

5. Cellular Respiration: (Exergonic) Photosynthesis: (Endergonic) Cellular Respiration vs. Photosynthesis

6. Photosynthesis – chloroplast recap Outer membrane Inner membrane Thylakoid membrane Stroma Thylakoid space Intermembrane space

7. Photosynthesis - overview • Photosynthesis - 1. light rxn: store energy & split water – “photo” NADPH & ATP 2. dark rxn: “fix” CO2 & make sugars – “synthesis” Calvin cycle

8. 6CO2 + 12H2O + light energy C6H12O6 + 6O2 + 6H2O Redox Reactions Equation for photosynthesis

9. CO2 H2O Light NADP+ ADP + P Calvin Cycle Light Reactions ATP NADPH Chloroplast [CH2O] (sugar) O2 Photosynthesis - overview • light reactions: • dark reactions: – thylakoid membrane – stroma – thylakoid space

10. Photosynthesis – light absorption • visible light ~380 to 750 nm • chloroplast pigments – abs blue-violet & red - transmit and reflect green

11. Photosynthesis – light absorption • pigments: • chlorophyll a -energy-absorbing ring -hydrocarbon tail • accessory pigments - chlorophyll b - carotenoids - photoprotective

12. Photosynthesis – light absorption • chlorophyll a – abs blue-violet, red 400-450, 650-700 nm • chlorophyll b & carotenoids – abs broadly blue-violet mid-400s • more wavelengths used for photosynthesis = more light energy absorbed

13. Photosynthesis – light absorption Pigments have two states: ground & excited • chlorophyll abs light • e- excited • more energy • energy transferred

14. Photosynthesis – light absorption Pigments are held by proteins in the thylakoid membranes light harvesting complex • energy absorbed from light - to pigments • to reaction center - two special chlorophyll a - proteins - 1° electron acceptor • light harvesting complex & reaction center = photosystem (PS)

15. STROMA Photosystem I Photosystem II Light Light THYLAKOID SPACE Thylakoid membrane Photosynthesis – energy transfer • Photosystem I (PS I) & PS II • Difference – light wavelength, proteins,  where e- from

16. Photosynthesis – energy transfer • PSII: absorbs 680 nm, splits water, powerful ETC, ATP made • PS I: absorbs 700 nm, (less energy) e- from PSII, short ETC, NADPH made

17. • e- from PS II electron transport chain (ETC) PS I • e- from PS I 2nd ETC e- carrier: NADP+ Photosynthesis – energy transfer • e- in PS II, from split H20  NADPH

18. Photosynthesis – chemiosmosis • How is ATP produced? Chemiosmosis • e- down ETC, H+ to thylakoid space • H+ conc. gradient • H+ down gradient, ATP synthase photophosphorylation

19. Light reaction - summary • inputs: light energy, H2O • PS II, ETC, PS I, ETC • outputs: ATP NADPH O2 (waste)

20. Self-Check Know figures of chloroplast reactions/locations! 

21. Photosynthesis – energy transfer

22. H2O CO2 Light NADP+ 6CO2 + 12H2O + light energy C6H12O6 + 6O2 + 6H2O ADP P + i RuBP 3-Phosphoglycerate Calvin Cycle ATP G3P Starch (storage) NADPH Chloroplast O2 Sucrose (export) “Dark” reaction (Light-independent Reaction) • “Dark” reaction: Calvin cycle • regenerative • anabolic • CO2 in, sugar out • during daylight

23. Carbon fixation • 3 stages of Calvin-cycle: • #1 – carbon fixation • CO2 link to 5-C • 5-C: ribulose bisphosphate (RuBP) - enzyme: Rubisco abundant • 6-C unstable – split  2(3-C)

24. Reduction • 3 stages of Calvin-cycle: • #2 – reduction • 3-C reduced • e- from NADPH • reduced 3-C: G3P

25. Regeneration of C-acceptor • 3 stages of Calvin-cycle: • #3 – regenerate C-acceptor • still 5 G3P  3 RuBP • multiple steps • uses ATP • every 3 cycles: 1 G3P made 3 RuBP regenerated • C3 plants – CO2 fixed into 3-C

26. Self-Check

27. Alternate methods of C fixation

28. H2O CO2 Light NADP+ ADP P + i RuBP 3-Phosphoglycerate Calvin Cycle ATP G3P Starch (storage) NADPH Chloroplast O2 Sucrose (export) Alternate methods of C fixation • CO2 in  stomata • open, lose water • hot, dry – open stomata less; lowers water loss, lowers CO2 • O2 fixed – photorespiration – inefficient • fix CO2 into 4-C molecules

29. Photosynthesis – summary • light reaction: Light energy + H2O O2, NADPH, ATP Thylakoids • light-independent: CO2, NADPH, ATP G3P (sugar), RuBP Stroma

30. Photosynthesis – summary Where do photosynthetic products go?

31. Photosynthesis – in context of big picture

32. Lecture 3 Summary • 1. Photosynthesis Overview (Ch. 8) • Purpose • Redox reactions • Electron carriers & sugars • 2. Light (Ch. 8) • Absorption pigments • Light spectra/wavelengths • 3. Locations of steps, inputs/outputs, purpose, description (Ch. 8) • PSI vs. PS II • Whole light reaction [includes chemiosmosis] • “dark” reaction/Calvin cycle [3 steps] • 4. Alternate modes of photosynthesis (Ch. 8) • 5. Photosynthesis context (Ch. 8) • - Uses for products • - Relationship of cell respiration and photosynthesis