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Photosynthesis is the process by which carbon dioxide and water are converted into carbohydrates and oxygen in the presence of light. This process is found in plants that contain a green pigment called chlorophyll found in the chloroplasts of plant cells. Photosynthesis requires two photosynthetic reaction centers that absorb light of different wavelengths. The reaction centers are called Photosystem I and Photosystem II and absorb wavelengths of $700nm$ and $680nm$, respectively.

In photosynthesis, the role of ATP (adenosine triphosphate) and NADPH (nicotinamide adenine diphosphate) is to carry the energy of the Calvin cycle that occurs in the absence of light. They are also called light-independent reactions.

When the electrons obtained from the photolysis of water travel through the electron transport chain, energy is released each time the excited state shifts to the ground state. This allows the hydrogen ions produced by photolysis to be sent to the thylakoid membrane of the chloroplast. Hydrogen ions set an electrical gradient and form an energy molecule called ATP, which is the energy source for the Calvin cycle.

Further movement of the electron after being charged by Photosystem I adds more energy and electrons to the chain. The electrons allow a reduction reaction between ferredoxins and ${NADP}^{+}$produce an energetic molecule called NADPH. This molecule is the source of electrons and hydrogen in the Calvin cycle.