ATP
The simple sugar glucose can be metabolized to release some energy and store the rest as ATP (adenosine triphosphate). ATP is the universal energy currency of life. All cells store surplus energy as ATP, which acts like a rechargeable battery for cellular energy.
Chemical energy transport within cells transpires by shuttling ATP. Cells use ATP not only for energy but also for communication within and between cells – a cellular coin of the realm.
In releasing energy to a cell, ATP turns into ADP (adenosine diphosphate). In storing energy for later use, ADP becomes ATP.
ATP is a nucleotide, comprising 3 main structures: a nitrogenous base (adenine); a sugar (ribose); and a chain of 3 phosphate groups (triphosphate), bound to the ribose.
The adenine ring and ribose sugar form adenosine, which is a purine nucleoside. In animals, adenosine acts as a neuromodulator, promoting sleep and suppressing arousal. On its own, adenine (A) is a nucleobase.
Nucleobases (nucleic acid bases) are nitrogen-based, ring-shaped molecules that comprise the cornerstones of nucleotides. Nucleobases comprise the individual units of the nucleic acids DNA and RNA.
The active part of ATP is the triphosphate. The 3 phosphorous groups are connected to each other by oxygen atoms, with side oxygens connected to the phosphorous atoms.
ATP is formed by adding a 3rd phosphate group to ADP, turning the diphosphate (ADP) into a triphosphate (ATP).
When extra energy is available – from food (in animals) or sunshine (in plants) – ADP is charged into ATP via glycolysis: an energy reservoir to provide a source of power when needed. Photosynthesizers power photophosphorylation from the energy of sunlight. Otherwise, via the internally powered redox of cellular respiration, cells turn ADP into ATP. Generally, ATP production has an energy efficiency of ~54%, well above that of the most efficient machines.
Energy can be gained from ATP by dephosphorylation: ditching at least 1 phosphate group via hydrolysis, turning ATP into ADP. The negatively charged side oxygen atoms in phosphate are uneasy in proximity and would like to escape the association; the escaping oxygen atom dragging its attached phosphate with it.
An ATP molecule would be content with just 2 phosphate groups instead of the 3 that it is saddled with. ATP (triphosphate) is just itching to become ADP (diphosphate).
ATP + H2O → ADP + Pi
Via hydrolysis, the easily triggered conversion of ATP into ADP releases energy that cells use to power themselves. Hydrolysis is the process of splitting H2O into H+ and OH–; a redox reaction that creates usable energy. Conversely, when extra energy is available – from food (in animals) or sunshine (in plants) – ADP is charged into ATP via glycolysis, stocking a reserve to provide power when needed.
