ATP (adenosine triphosphate):

The major energy currency of all cells is a nucleotide called adenosine triphosphate (ATP), ATP was discovered in 1929 by Karl Lohmann, and was proposed to be the main energy-transfer molecule in the cell by the Nobel prize winner, Fritz Lipmann in 1941.

ATP: The Cell's Energy Currency:

ATP is the main energy source for the majority of the cellular functions like the synthesis of macromolecules (DNA, RNA, and proteins), movement, the transmission of nerve impulses, active transport, exocytosis, endocytosis, etc.

The ability of ATP to store and release energy is due to its molecular structure.

Sub-Units of ATP:

Each ATP molecule has three subunits:

(a) adenine -a double-ringed nitrogenous base;

(b) a ribose - a five-carbon sugar, and

(c) three phosphate groups in a linear chain.

ATP Transfers Energy Between Metabolic Reactions:

The covalent bond connecting two phosphates is indicated by the "tilde" (~) and it is a high-energy bond. The energy in this bond is released as it breaks and inorganic phosphate (Pi) gets separated from ATP.

Energy Released by breaking off one phosphate bond: The breaking of one phosphate bond releases about 7.3 kcal (7,300 calories) per mole of ATP as follows:

ATP + H₂O → ADP + P₁ + energy (7.3 kcal/mole)

The energy from ATP is sufficient to drive most of the cell's energy-requiring reactions. In common energy, I reactions only the outermost of the two high-energy bonds break. When this happens, ATP becomes ADP (adenosine diphosphate) and one P_i is released. In some cases, ADP is further broken down to AMP (adenosine monophosphate) and P: as follows:

ADP + H₂O → AMP + P₁ + energy (7.3 kcal/mole)

Recycling of ADP:

Cells constantly recycle ADP by recombining it with Pi to form ATP the synthesis of ATP from ADP and P (requires the expenditure of 7.3 kcal of energy per mole. This energy is obtained from the oxidation of foodstuff. So, we can summarize that ATP is generated by energy-releasing processes and is broken down by energy-consuming processes. In this way, ATP transfers energy between metabolic reactions.