1. PHOTOSYNTHESIS, Part Un

2. Photosynthesis is a two step process: the light reactions and the Calvin cycle.

3. Photosystems A photosystem is like an antenna!
A protein complex with pigments to absorb photons
PSII (P680) and PSI (P700)
Allows electrons to be passed along- linear electron flow

5. The Light Reactions
Photosystem II (P680) absorbs a photon of sunlight, which excites an electron.

6. The Light Reactions
The electron is sent along an electron transport chain and water is split (photolysis)
PSII now has an electron “hole”

7. The Light Reactions
The splitting of water releases oxygen (waste product) and hydrogen ions (protons) into the lumen (space) of the thylakoid.

8. The Light Reactions
It also replaces the electron that was lost to the electron transport chain (PSII is so HUNGRY for it!!! This drives the splitting of water, which is a pretty stable molecule).

9. The Light Reactions
As the electron travels down the ETC, more protons are pumped into the lumen of the thylakoid, forming a proton gradient (chemiosmotic gradient).

10. The Light Reactions
The protons diffuse through ATP synthase, which uses the energy of the proton gradient to phosphorylate ADP  ATP (photophosphorylation).

11. The Light Reactions
Another photon of light strikes Photosystem I (P700), exciting another electron.

12. The Light Reactions
This electron reduces NADP+, forming NADPH. NADPH is the final electron acceptor in photosynthesis.

13. Summary of the Light Reactions
NADPH
ATP
Oxygen (waste)
Light
Water

14. 2 electron transport chains
First makes ATP
Second makes NADPH (no hydrogen gradient!)

15. Animation and Video
Virtual Cell