Adenosine diphosphate and adenosine triphosphate are organic molecules, known as nucleotides, found in all plant and animal cells. ADP is converted to ATP for the storing of energy by the addition of a high-energy phosphate group. The conversion takes place in the substance between the cell membrane and the nucleus, known as the cytoplasm, or in special energy-producing structures called mitochondria.
The conversion of ADP to ATP can be written as ADP + Pi + energy → ATP or, in English, adenosine diphosphate plus inorganic phosphate plus energy gives adenosine triphosphate. Energy is stored in the ATP molecule in the covalent bonds between the phosphate group, particularly in the bond between the second and third phosphate groups, known as the pyrophosphate bond.
The conversion of ADP to ATP in the inner membranes of mitochondria is technically known as chemiosmotic phosphorylation. Membranous sacs on the walls of the mitrochondria contain an estimated 10,000 enzyme chains, which derive energy from food molecules or photosynthesis – the synthesis of complex organic molecules from carbon dioxide, water and inorganic salts – in plants, via what is known as the electron transport chain.
Cellular oxidation in a cycle of enzyme-catalyzed metabolic reactions, known as the Krebs cycle, creates a build-up of negatively charged particles called electrons, which pushes positively charged hydrogen ions, or protons, across the inner mitochondrial membrane into the inner chamber. The energy released by the electrical potential across the membrane causes an enzyme, known as ATP synthase, to become attached to ADP. ATP synthase is a huge molecular complex and its function is to catalyze the addition of a third phosphorous group to form ATP. A single ATP synthase complex can generate over 100 molecules of ATP each second.
Living cells use ATP as if it were power from a rechargeable battery. Converting ADP to ATP adds power, while almost all other cellular processes involve the breakdown of ATP and tend to discharge power. In the human body, a typical ATP molecule enters the mitochondria for recharging as ADP thousands of times a day, such that the concentration of ATP in a typical cell is about 10 times higher than that of ADP. Skeletal muscles require large amounts of energy for mechanical work, so muscle cells contain more mitochondria than the cells of other tissue types.