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Life is an energy intensive process. It takes energy to operate muscles, extract wastes, make new cells, heal wounds, even to think. It’s in an organism’s cells where all this energy is spent. In some cells, as much as half of a cell’s energy output is used to transfer molecules across the cell membrane, a process called ‘active transport.’

Cell movements require energy and thousands of energy-hungry chemical reactions go on in every living cell, every second, every day. The kind of energy cells use is chemical bond energy, the shared electrons that holds atoms together in molecules.

Most cell processes use the same energy source, the rechargeable energy carrier, adenosine tri phosphate -- ATP.

ATP has this arrangement: a molecular unit of adenosine coupled to a chain of three phosphate groups, thus the name, adenosinetri phosphate. The phosphate groups are held to each other by very high energy chemical bonds. But under certain conditions, the end phosphate can break away and the energy released to the energy-hungry reactions that keep a cell alive. When the end phosphate is released, what is left is ADP, adenosine diphosphate. This change from tri to di is taking place constantly as ATPs circulate through cells.