What we call ‘photosynthesis’ is a collection of events that uses the energy contained in light and reducing CO₂. A simplified net reaction is shown below:

H₂O + CO₂ + light → (CH₂O)_n + O₂

On an annual basis, this collection of processes is responsible for fixing 7 × 10¹³ kg of CO₂. To help put this value in perspective, that is equal to 1% of all known fossil fuel reserves on the planet and roughly 10× the annual global energy consumption. In addition, photosynthesis also releases oxygen at a rate that replaces all O₂ in the atmosphere every 2000 years.

Photosynthesis consists of 2 interdependent processes known as the light reactions and the carbon reactions. For our purposes, we will further disassemble the light reactions into the processes of photochemistry and electron transport. Both of these parts of the light reactions take place across the system of membranes found within chloroplasts, known as the thylakoid membrane system.

As we will discuss in more detail elsewhere, light results in the polarization of this membrane due to the formation of a H⁺ gradient. This proton gradient is used to by ATP synthase to combine ADP and P_i to form ATP, which is one of the key products of the light reactions. The other major product of the light reactions is an energy carrier called NADPH. All together, the light reactions can be thought of as a series of oxidation-reduction reactions powered by light energy, with electrons supplied by H₂O and ultimately reducing NADP⁺ to NADPH.

In the carbon reactions, the energy from the light reactions, represented by ATP and NADPH, is used to reduce atmospheric CO₂ to carbohydrates. This takes place in a series of enzyme-mediated reactions that takes place in the stroma of the chloroplast. The series of reactions represents an example of a biochemical cycle, with the initial reactant regenerated by the pathway. The products of the cycle are excess molecules that are “exported” from the pathway.

It is fairly obvious from the above descriptions that the carbon reactions are dependent upon the light reactions for ATP and NADPH. But it is important to keep in mind that the ADP and NADP⁺ produced by the carbon reactions are absolutely essential for the light reactions to continue. Without the return of these substrates to the light reactions, the cell would experience significant damage due to continued exposure to light energy.