Adenosine triphosphate (ATP for short) is used to provide the energy for many cellular functions - by both animal and plant cells - as cells are unable to source their energy directly from sugars such as glucose. ATP is formed during respiration, when the energy released from ingested glucose powers the addition of a inorganic phosphate molecule to a molecule of adenosine diphosphate (ADP), the enzyme ATP synthase is responsible for this. ATP itself is a small, soluble molecule which can be easily broken down and transported around the cell. ATP is readily available within cells upon demand as stores are released in manageable amounts, meaning there is no wasted energy. The energy-containing phosphate bond is easily broken down to release the energy it holds and to further this ATP molecules are not lipid soluble and therefore they are unable to pass beyond the cell membrane, ensuring the cell always has an immediate energy source. Finally, ATP can be deemed a good energy source as it has the ability to transfer a phosphate group, and therefore energy, to other molecules.