Where does the light cycle take place and what are the reactants/products?
There is no simple answer to this question; look below.
Although we know the light cycle to use Chlorophyll, it is quite unique and in fact uses more than one version of Chlorophyll. Something interesting to know is that Chlorophyll uses magnesium in the center of its structure, which also acts as an energy collector of sorts.
The chlorophyll absorbs light causing the magnesium ring to get excited with electrons. Different wavelengths effect it differently.
The energy in the 'excited electrons' can be passed from one chlorophyll molecule to another, but in the end it will just be lost as fluorescence, unless the excited electron itself can be ejected from the chlorophyll molecule.
This process of electron ejection takes place only in chlorophyll molecules which are specifically held in a special protein complex called a "reaction center".
Heres a simplified equation for you to follow:
*Note that the sunlight is the source of the electrons in the photos above.