LIGHT & DARK REACTIONS

1. LIGHT & DARK REACTIONS OF PHOTOSYNTHESIS

2. How photosynthesis works “Dark Reactions” Light Dependent Reactions Light Independent Reactions Sugars H2O O2 Photosystem IPhotosystem II Non-cyclic é flow Cyclic é flow CO2 Water Splitting Reactions Carbon Fixation

3. Step 1: Photoexcitation • The structure of chlorophyll is very important to its function: • Notice the alternating double bonds • These é are said to be ‘delocalized’ • The polar chlorophyll head is found mixed with the phospholipids of the thylakoid membrane • This is the where photosynthesis starts...

4. Step 1: Photoexcitation • Before a photon of light strikes chlorophyll, its éare at their lowest energy level • ‘ground state’ • When a photon hits, an é gains energy • Becomes ‘excited’ • When an excited é returns to its original state it can: • Emit light  ‘fluorescence’ • Transfer é to another é carrier  ‘primary é acceptor’

5. Cool Note • If you separate a chlorophyll molecule from the thylakoid membrane... • The excited é will fluoresce as its energy lowers back to its ground energy The red fluorescence in the middle of the jellyfish comes from chlorophyll in the ingested algae

6. Step 2: Photosystems • Photosystems consist of: • Antenna complex • Reaction centre • Antenna complex • composed of a # of chlorophyll molecules and accessory pigments • Embedded in the thylakoid membrane • Photon is absorbedand transfers energybetween pigmentsuntil it reacheschlorophyll a

7. Step 2: Photosystems • Photosystems consist of: • Antenna complex • Reaction centre • Reaction Centre • An é on chlorophyll a absorbs energy from the antenna complex and becomes ‘excited’ • A redox reaction transfers the excited é to the primary é acceptor *There are 2 Photosystems:Photosystem IPhotosystem II

8. Step 3: Non-cyclic é flow • Process: • A photon strikes the antennae complex of photosystem II to excite an é • The excited é is captured by the primary é acceptor pheophytin • Through a series of redox reactions, é is transferred to plastoquinone (PQ) and then to the electron transport chain • The é powers a H+ pump allowing 4H+ to enter

9. Step 3: Non-cyclic é flow • At the same time, A ‘Z’ protein splits water into oxygen, H+ ions (protons), and é • The é from water are used to replenish the é lost in photosystem II

10. Step 3: Non-cyclic é flow • Photosystem I  NADPH • Two photons excite 2 é from photosystem I • é from photosystem I pass through another ETC containing ferredoxin, Fd • Finally, Fd, gives its é to NADP reductase that uses H+ from the stroma to reduce NADP+ to NADPH • NADPH is used in the Calvin cycle

11. Step 3: Non-cyclic é flow *** Meanwhile, there is a H+ gradient forming in the lumen that allows ATP to be produced as H+ ions pass though ATP synthase We have produced NADPH and ATP from non-cyclic é flow!

12. Step 3: Non-cyclic é flow

13. Step 3: Non-cyclic é flow • VIDEO • http://www.youtube.com/watch?v=eY1ReqiYwYs

14. cyclic é flow • In some cases, excited é can take a cyclic pathway that stays within photosystem I • The excited é is picked up by Fd b6-f  Pc • In the end, the excited é is returned to the chlorophyll it came from • This process adds to the H+gradient to help produce ATP

15. cyclic é flow • In some cases, excited é can take a cyclic pathway that stays within photosystem I • The excited é is picked up by Fd b6-f  Pc • In the end, the excited é is returned to the chlorophyll it came from • This process adds to the H+gradient to help produce ATP

16. How photosynthesis works “Dark Reactions” Light Dependent Reactions Light Independent Reactions Sugars H2O O2 Photosystem IPhotosystem II Non-cyclic é flow Cyclic é flow CO2 Water Splitting Reactions Carbon Fixation

17. Calvin Cycle (Dark Reactions)

18. Step 4: Carbon Fixation • 3 CO2 molecules that are absorbed through the stomata and spongy mesophyll cells are added to 3 molecules of RuBP • Ribulose – 1,6 – Bisphosphate (5-carbon molecule) • Together they form unstable 6-carbon compound • Which, instantly breaks down into 6 PGA molecules • Often called C3photosynthesis as the first product contains 3-carbon • C4 plants exist as well

19. Step 5: Reduction Reactions • Each of the 6 PGA molecules is phosphorylated by ATP • Producing 1,3-BPG • Then, NADPH gives each 1,3-BPG 2 é • REDUCING THEM to G-3-P • ONE of the six G-3-P molecules exits the cycle to produce sugars

20. Step 6: RUBP REGENERATION • We have five 3-carbon G3P’s left • We need to regenerate RuBP so we can continue the Calvin Cycle • The five G3P molecules rearrange to REFROM the three 5-carbon RuBP • Phosphorylation by 3 ATP molecules will finally regenerate the RuBP

21. Calvin Cycle TOtals • ATP USED = 9 • NADPH USED = 12 • ATP/NADPH are produced in the light reactions • In the light reactions, • 3 ATP and 2 NADPH are produced • Conveniently, the Calvin cycle uses 3 ATP and 2 NADPHper CO2

22. Calvin Cycle TOtals • ATP USED = 9 • NADPH USED = 12 • ATP/NADPH are produced in the light reactions • In the light reactions, • 3 ATP and 2 NADPH are produced • Conveniently, the Calvin cycle uses 3 ATP and 2 NADPHper CO2

23. How photosynthesis works “Dark Reactions” Light Dependent Reactions Light Independent Reactions Sugars H2O O2 Photosystem IPhotosystem II Non-cyclic é flow Cyclic é flow CO2 Water Splitting Reactions Carbon Fixation