Light-dependent reaction

When the chlorophyll molecule absorbs light energy, it boosts the energy of a pair of electrons within this chlorophyll molecule. This raises their energy level, and the electrons are 'excited'. They are emitted from PS II (photosystem 2) as a result of their increased energy level. The chlorophyll molecule is now ionised, as it has lost electrons. This process is called photoionisation. 

The electrons which have been emitted by the chlorophyll molecule are taken up by a molecule called an electron carrier. The electron carrier has gained electrons, therefore it has been reduced. The electrons are passed along a number of electron carriers in a chain of oxidation- reduction reactions. These electron carriers form a chain, which is located in the thylakoid membranes. Each carrier is at a successively lower energy level than the previous one, therefore enrgy is lost at each reaction. Some of this energy is used to combine an inorganic phosphate molecule with an ADP molecule to form ATP. 

The precise mechanism by which ATP is produced can be explained by the chemiosmotic theory. 

1. Each thylakoid is an enclosed chamber into which protons are pumped from the stroma using protein carriers in the thylakoid membrane called proton pumps.

2. The energy to drive this process comes from electrons released in photolysis

3. The photolysis of water also produces protons which further increases their concentration within the thylakoid space.

4. This creates/ maintains a concentration gradient of protons across the thylakoid membrane with a high concentration inside the thylakoid space and a low concentration in the stroma.

5. The protons can only cross the thylakoid membrane through ATP synthase channel proteins- the rest of the membrane is impermeable to protons. These channels from small granules on the membrane and are therefore known as stalked granules.

6. As the protons pass through ATP synthase channels they catalyse the combination of ADP + Pi with ATP. 

The loss of electrons when light hits a chlorophyll molecule causes a defecit of electrons. The chloropyhll molecule needs the elctrons to be replaced to allow it to continue absorbing light energy. This photolysis of water also yields protons. The equation for this process is:

Water --> protons + electrons + oxygen

2H2O --> 4H+ + 4e- + O2 

The oxygen by-product from the photolysis of water is either used in respirtion or diffuses out of the leaf as a waste product of photosynthesis. 

Protons pass out of the tylakoid through ATP synthase channels and along with protons emitted from PS I,  are taken up by an electron carrier called NADP. This reduces the NADP to NADPH. The NADPH is the main product of the light dependent stage, and it enters the light dependent reaction.