ATP

What is ATP? Why is it considered energy? I thought we ate FOOD for energy?


ATP stands for Adenosine Triphosphate. Here's what the chemical structure looks like..

ATP generally consists of 3 negatively charged phosphate groups, an adenine (a purine nitrogenous base, just like in your DNA) and a ribose (a type of sugar monomer). 


ATP carries a special form of energy within the 3 phosphate groups called potential energy. Because each phosphate is negatively charged, and negative charges repel each other, energy is stored within the bonds of these 3 phosphates. Think of what might happen when a phosphate group is broken off. Do you think it would be a favorable reaction? Of course it would be! Those negative phosphate groups do NOT like being grouped together, thus if one phosphate is broken off, making Adenosine Diphosphate (ADP), the negative charges left can spread out more, repelling each other less. 


When a phosphate group is broken off ATP, making ADP, about 7.3 kilocalories (kcal) of energy per mole of ATP is released. What is this energy used for? Pretty much for all cellular functions! For instance, ATP can be used to move molecules, pump ions, even help a cell in moving itself. Imagine a sperm cell, that flagella needs a LOT of ATP in order to continue to whip back and forth. 


But if we use ATP for energy, why is it said that we eat food for energy? We need food in order to make ATP. Glucose, a sugar monomer, is the most favorable molecule to make ATP in a cell. ATP is made from glucose in a process called cellular respiration. However, if glucose is not available (perhaps you haven't eaten any carbohydrates in awhile) then other major molecules from food can be used to make ATP as well - like lipids (fats) or if your body is desperate, proteins.