Photosynthesis

The light dependent reactions begin with a structure called Photosystem I.  Photosystem I is an integral membrane protein complex on the thylakoid membrane that absorbs sunlight with a photosynthetic pigment.  The optimal wavelength that this pigment absorbs is P700.  Within PS1, there is what is called the reaction center.  This is where an electron sits, and when the pigment absorbs a photon with the right energy, the electron becomes excited.  

 

This leads to the next step, in which the electrons are transported via the electron transport chain, which is made up of protein transporters.  The electrons power a pump that pumps hydrogen ions into the lumen, the inside of the thylakoid. 

 

The next step is for the hydrogen ions to pass back out of the lumen due to the concentration gradient.  They do so by passing through the ATP synthase.  The protons knock ATP off of the active site of the protein, freeing it for the cell's use.  This is very similar to the function of a dam.  A high concentration of water is purposefully built up on one side of the dam, then released to turn turbines that create electical energy that we can use.  The point of cyclic electron flow is just that: build up a concentration gradient of protons that then flow across ATP synthase to free ATP molecules for the 

cell's energy use.