1. Photosynthesis Light reactions

2. Pigments: substances that absorb light
Photons: light that has a fixed quantity of energy

3. Photoexcitation of chlorophyll: absorption of a photon causes electrons to jump from their outer most shell (when in a grounded/stable state) into an orbit of higher energy (excited state)

4. Photosystems: Light Harvesting Complexes of the Thylakoid Membrane
Photosystems: small organic bundles that contain chlorophyll and proteins
- have a light gathering antennae complex which consists of chlorophyll a, chlorophyll b, and carotenoid molecules

7. Pathway of energy:
Photon (light)  to pigments (chlorophyll b and carotenoid)  chlorophyll a  reaction centre  primary electron acceptor

9. Chlorophyll a loses an electron to the primary electron acceptor (due to electron being in its excited state/higher orbit) and energy is trapped

10. Thylakoid Membrane has Two Types of Photosystems
Photosystem I: reaction centre P700 (absorbs wavelength 700nm-redish)
Photosystem II: reaction centre P680 (absorbs wavelength 680nm-redish)
**P700 and P680 are identical chlorophyll a molecules but contain different proteins

12. Two Routes Electrons can Flow
Noncyclic Electron Flow (predominant)
Cyclic Electron Flow
Noncyclic/Linear Electron Flow:
i.) Photosystem II absorbs light
Electron goes into excited state and at the reaction centre chlorophyll P680 the electron is captured by the primary electron acceptor
Chlorophyll P680 wants an electron back

13. Enzyme extracts electrons from H2O and supplies P680 electrons
This splits H2O molecule (2H+ ions and ½ O O2)
Each photoexcited electron moves from the primary electron acceptor of Photosystem II to Photosystem I via an electron transport chain
ATP is synthesized by photophosphorylation (ATP made by light) and chemiosmosis

15. v.) electrons travel down ETC and fills in the “hole” (missing electron) because the P700 chlorophyll a was excited from light and lost an electron to primary electron acceptor in Photosystem I

18. vi.) primary electron acceptor of Photosystem I passes the photoexcited electrons to a second ETC vii) Photosystem II produces equal amounts of ATP and NADPH

19. Cyclic Electron Flow ** Photosystem I (P700) Does not produce O2
The electron is replaced by the electrons in the ETC from P680
Does not produce O2
Does not produce NADPH
Does produce ATP

21. light reaction summary