Two types of chemical reactions are vital to the efficient use and storage of cellular energy, oxidation-reduction reactions and phosphorylation reactions. Together, they produce the compounds that are the main energy carriers in cells.

Oxidation-Reduction Equations
Oxidation-reduction, or redox, reactions, involve the transfer of electrons between the outer shells of atoms. When an electron moves from one atom to another, so does the energy it contains. The atom that loses the electron(s) is said to be oxidized. The atom that gains the electron(s) is reduced.

Photosynthesis and cellular respiration can both be thought of as redox reactions. In photosynthesis, carbon dioxide is reduced, forming sugar, while water is oxidized to release oxygen. Energy is stored in the sugars that are formed in the process. During respiration the process is essentially reversed and sugar is oxidized and oxygen is reduced, producing carbon dioxide and water. Energy is released from the sugars that are broken down.

A great many kinds of atoms are capable of participating in redox reactions. But three pairs of compounds are of prime importance in cellular energy reactions:

NAD+ / NADH
NADP+ / NADPH
FAD / FADH2

The first atom in each pair accepts electrons readily. When they do, they gain a negative charge and consequently acquire an H+ as well, forming the second atom. These molecules act both as energy carriers and as hydrogen carriers. NAD+ / NADH and FAD / FADH2 are important components of cellular respiration, while NADP+ / NADPH are used in photosynthesis.

Phosphorylation
A phosphorylation reaction bonds a phosphate group to another molecule. A phosphate group is made of one phosphorous and four oxygen atoms, but it is often represented by a simple P or Pi in chemical equations. Two phosphate groups can be joined by a phosphate bond, which is both high in energy and easily formed and broken.

Because phosphate bonds are energy-rich and easy to manipulate, they are part of the most important molecule in cellular energy, ATP, or adenosine triphosphate. ATP contains three phosphate groups. When the end phosphate bond is broken, turning ATP into ADP (adenosine diphosphate), energy is released. Energy is also released when ADP loses a phosphate group and becomes AMP, adenosine monophosphate. When a phosphate group is added to AMP and ADP, energy is stored for later use.

ADP and ATP are universally present in all living organisms. ATP is so widespread and heavily used that it is sometimes referred to as the “energy currency” of life.