2. Light Reaction of Photosynthesis

3. -occurs in thylakoid (“stacks of paper plates”) -thylakoids found in chloroplast

4. H2OH2O LIGHT REACTIONS Chloroplast Light ATP NADPH O2O2 Light enters and is absorbed by photosystem II

5.  Photosystem II = consist of: pigments, proteins, and organic molecules. act as “antennas” that absorb light energy

6.  Photosystem II Photon energy Photon energy is transferred from pigment to pigment until it reaches chlorophyll a

9. CH 3 CHO in chlorophyll a in chlorophyll b Porphyrin ring: light-absorbing “head” of molecule; note magnesium atom at center Hydrocarbon tail: interacts with hydrophobic regions of proteins inside thylakoid membranes of chloroplasts; H atoms not shown PIGMENT

10. Fills in “lost” e - from H 2 0 being split

12. to the

13. Photosystem II Photo II aka. P680- absorbs wavelengths of light at 680nm

14. Light Reaction overview Light Reaction overview: 1) pigments absorb light photon & the photon is relayed around photo II 2) an e -, from chlorophyll a bounces to the next energy level & caught by the primary e- acceptor Solar E. --> C CC Chemical E.:

15. Light P680 e–e– Photosystem II (PS II) Primary acceptor [CH 2 O] (sugar) NADPH CALVIN CYCLE LIGHT REACTIONS NADP + Light H2OH2O CO 2 O2O2

16. Light Reaction overview Light Reaction overview: 3)meanwhile… an enzyme splits H 2 O -> H +, e -, O an e - fills in the e - lost by the P680 molecule O 2 goes up your nose Solar E. --> C CC Chemical E.:

17. Light P680 e–e– Photosystem II (PS II) Primary acceptor [CH 2 O] (sugar) NADPH CALVIN CYCLE LIGHT REACTIONS NADP + Light H2OH2O CO 2 Energy of electrons O2O2 2 H + H2OH2O O2O2 1/21/2

18. Light Reaction overview Light Reaction overview: 4) Each photexcited e - is now passed down an e - transport chain 5) The e- “fall” synthesizes ATP (H+ gradient from H 2 0 splitting) Solar E. --> C CC Chemical E.:

19. Light P680 e–e– Photosystem II (PS II) Primary acceptor [CH 2 O] (sugar) ATP CALVIN CYCLE LIGHT REACTIONS NADP + Light H2OH2O CO 2 Energy of electrons O2O2 2 H + H2OH2O O2O2 Pq Cytochrome complex Electron transport chain Pc ATP H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+

20. Light Reaction overview Light Reaction overview: 6) chlorophyll in PS I absorbs light photon & the photon is relayed an e - is bounced to the next energy level & caught by the Primary e- acceptor Solar E. --> C CC Chemical E.:

21. Photosystem I and II Photosystem I and II what makes them different? Photo II aka. P680- absorbs wavelengths of light at 680nm Photo I aka. P700- absorbs wavelengths of light at 700nm

22. Light P680 e–e– Photosystem II (PS II) Primary acceptor [CH 2 O] (sugar) NADPH CALVIN CYCLE LIGHT REACTIONS NADP + Light H2OH2O CO 2 2 H + H2OH2O O2O2 1/21/2 Pq Cytochrome complex Electron transport chain Pc ATP P700 e–e– Primary acceptor Photosystem I (PS I) Light

23. FYI: P680 and P700 are identical chlorophyll a molecules. difference in light absorption comes from association with different proteins in the thylakoid membrane.

24. Light Reaction overview Light Reaction overview: 7) e-’s passed down the second “chain” 8) 2 e-’s hop on an NADP + bus and form NADPH Solar EE. EE. --> Chemical E.:

25. NADPH off to the dark side…

26. ATP Photophosphorylation - Light energy generates ATP

27. +2 H + 2 H + +2 H + 2 H + +2 H + 2 H + +2 H + 2 H + +2 H + 2 H + +2 H + 2 H +

28. Purpose of Light Reactions (revisited): Generate ATP Generate NADPH these will provide E. & reducing power to the Calvin Cycle.

29. Light Reaction

30. e - carrier

32. via. ATP synthase

33. ATP Photosystem II e–e– e–e– e–e– e–e– Mill makes ATP e–e– e–e– e–e– Photon Photosystem I Photon NADPH

34. Go to the dark side

35. Cyclic Electron Flow - occurs under condition where there is not enough ATP for Calvin Cycle.occurs under condition where there is not enough ATP for Calvin Cycle. Uses photosystem I but not II. No production of NADPH No production of O 2

36. Cyclic Electron Flow - Rise in NADPH (b/c Calvin Cycle consumes more ATP) triggers this process, goal to make ATP “catch up” with NADPH. Side steps Photo II (P680).

37. Photosystem I ATP Pc Fd Cytochrome complex Primary acceptor

38. Cyclic Electron Flow - Excited electrons from Photo I are passed by electron acceptor to Fd (ferrodoxin). Then passed to ETC to make ATP. No NADPH is made and there is no release of O 2.