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Photosynthesis: The Details Part one

Photosynthesis: The Details Part one. Section 3.3 Pg. 156. Photosynthesis takes place in the chloroplasts. Three stages:. “The light reactions”. The Calvin Cycle. The light reactions. Photoexcitation Electron transport Chemiosmosis (oxidative phosphorylation ).

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Photosynthesis: The Details Part one

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  1. Photosynthesis: The DetailsPart one Section 3.3 Pg. 156

  2. Photosynthesis takes place in the chloroplasts. Three stages: “The light reactions” The Calvin Cycle

  3. The light reactions • Photoexcitation • Electron transport • Chemiosmosis (oxidative phosphorylation)

  4. Goal of the light reactions: Transfer light energy to chemical energy: • ATP and NADPH

  5. Some quantum mechanics... When a molecule is in its normal state, and has not received any energy input, its electrons exist at their ground state.

  6. When the molecule is energized, its electrons are raised to a higher potential energy level.

  7. Excited state As the electrons return to their ground states, they re-emit the energy as a photon of light • Fluorescence

  8. Photoexcitation

  9. Photoexcitation The electrons of chlorophyll molecules become excited when they absorb photons.

  10. But plants don’t glow red... In a plant cell, the chlorophyll does not fluoresce because the excited electron is actually captured by a molecule in the thylakoid membrane. The capturing molecule is called the primary electron acceptor.

  11. Antenna complex web of chlorophyll and accessory pigments Reaction centre A trans- membrane protein complex containing chlorophyll a

  12. Photoexcitation – In depth Recall only chlorophyll a can pass light energy to the next reactions.

  13. i. An antenna pigment absorbs a photon and transfers the energy from pigment to pigment.

  14. ii. when the energy finally reaches the reaction centre, chlorophyll a absorbs the energy and an electron becomes excited.

  15. The electron is transferred via a redox reaction to the primary electron acceptor. Chlorophyll is left in an oxidized state. (missing an electron)

  16. Two types of photosystems Photosystem I and Photosystem II: • Their chlorophyll a’s absorb slightly different wavelengths (due to associated proteins in reaction centre). • Photosystem II – 680 nm • Photosystem I – 700 nm *note: Not the same as chlorophyll a vs. Chlorophyll b. Both photosystems contain chlorophyll a.

  17. Noncyclic electron flow

  18. Photosystem II Photosystem I

  19. Photosystem II Photosystem I

  20. Photosystem II Photosystem I

  21. Electrons move down the chain: • Free energy released is used to pump H+ into the lumen • H+ gradient builds up in lumen • NADP+ is the final electron acceptor

  22. chemiosmosis

  23. H+ builds up in the lumen = electrochemical gradient. • Proton-motiveforce drives H+ through an ATPase complex from the lumen into the stroma. • ADP  ATP • Photophosphorylation: 1 ATP per 4 H+

  24. Endresult of noncyclic electron flow + chemiosmosis: ADP + Pi  ATP NADP + + H + + 2e - s  NADPH + H+

  25. Homework: Pg. 166 #1-4, skip 4b

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