1. PHOTOSYNTHESIS
…………The Details

2. Photosynthesis is divided into 2 sequential processes:
The Light Dependent Reactions (stages 1 & 2)
The Light Independent Reactions (stage 3)
a.k.a. the Calvin Cycle

3. The Light Reactions
The light reactions begin when a photon strikes a photosynthetic membrane.
This process is divided into 3 parts:
Photoexcitation: absorption of a photon by an electron of chlorophyll
Electron Transport: transfer of electrons through a series of membrane bound electron carriers
Chemiosmosis: the movement of protons through ATP synthase to drive the phosphorylation of ATP

4. 1. Photoexcitation
Before a photon strikes an electron of chlorophyll, it is at the lowest possible potential energy level called ground state
During its interaction, the electron gains energy called excitation
As it returns, if it is not a part of a photosynthetic membrane, the loss in potential energy appears as heat and light  known as fluorescence

5. Most chlorophyll molecules do not fluoresce because the excited electron is captured by a special molecule called the primary electron acceptor (a compound embedded in the thylakoid membrane)
Therefore, chlorophyll will become oxidized and the primary electron acceptor will be reduced

6. Photosystems
Light is absorbed by chlorophyll and other accessory pigment molecules that are associated with proteins in clusters called photosystems
A photosystem consists of
Antenna Complex: composed of a number of pigment molecules including chlorophyll, set in a protein matrix in the thylakoid membrane. These pigments absorb photons and transfer the energy from pigment to pigment until it reaches a chlorophyll a molecule in the reaction center.
Reaction Centre: an electron of the chlorophyll molecule absorbs the energy and is raised to a high energy level. A redox reaction transfers the electron to a primary electron acceptor.

8. Photosystems act as the primary light harvesting units of chloroplasts!

9. Photosystems
There are 2 types of photosystems found in the thylakoid membranes:
Photosystem I (PSI): chlorophyll a (a molecule in the reaction centre) is called P700 because its best absorption happens with wavelengths of 700nm
Photosystem II (PSII): chlorophyll a (a molecule in the reaction centre) is called P680 because its best absorption happens with wavelengths of 680nm

10. 2. Electron transport 3. chemiosmosis
Non cyclic pathway
In this pathway, plants use PSI or PSII to produce ATP & NADPH

11. How does it work?
A photon strikes PSII and eventually excites an electron of chlorophyll P680
The excited electron is captured by the primary electron acceptor
Through redox reactions an electron is transferred to plastoquinone (Pq), then to cytochrome B6-F complex to plastocyanin (Pc), and to PSI
As plastoquinone accepts electrons from PSII, it also gains protons from the stroma. These are released into the lumen (creating an electrochemical gradient) when the electrons are donated.
Water-splitting complex (associated with PSII) splits water in oxygen, hydrogen ions and electrons. The electrons are used to replace the missing electrons in P680

12. Oxygen leaves the cell as a waste product and the protons remain in the thylakoid lumen causing an electrochemical gradient
PSI gets hit with photons at the same time as PSII, exciting an electron of P700
Through redox reactions the electron is transferred to ferredoxin (Fd), then to NADP reductase
NADP reductase then reduces NADP+ to NADPH
The electrochemical gradient in the lumen drives ADP to ATP using photophosphorylation

14. The ATP formed from this process will be used in the last stage (the Calvin Cycle) to synthesize glucose

15. Homework
Section 5.2 p
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