Photosynthesis is divided into two parts:

• Light-dependent reactions (light reactions)

• Light-independent reactions (dark reactions).

The first overall principle of photosynthesis is that the light energy from the sun is transformed into chemical energy and stored in the bonds of glucose (the sugar carbohydrate) for later use by the plant and/or organism that eat the plant.

The second overall principle of photosynthesis is that carbon, oxygen, and hydrogen atoms are taken from carbon dioxide and water molecules and are broken up and rearranged into new substances: carbohydrate (specifically glucose) and oxygen gas (so we can breathe, whew!). This reaction represents the transfer of matter: carbon dioxide from the atmosphere, water from the soil or atmosphere, into sugar in the plant and oxygen back into the atmosphere.

Light reactions need light to produce organic energy molecules (ATP and NADPH). They are initiated by colored pigments, mainly green colored chlorophylls.

Dark reactions make use of these organic energy molecules (ATP and NADPH). This reaction cycle is also called Calvin Benison Cycle, and it occurs in the stroma. ATP provides the energy while NADPH provides the electrons required to fix the CO₂ (carbon dioxide) into carbohydrates. ATP and NADPH combine with CO₂ and water to make the end product of glucose. The ADP and NADPH+ are recycled to the light-dependent side to start the process over.

This means Dark reactions will fail to continue if the plants are deprived of light for too long since they use the output of the initial light-dependent reactions.