The role of adenosine triphosphate (ATP) The energy required for cellular activities is provided directly by molecules of adenosine triphosphate (ATP). ATP is made of one adenosine molecule and three phosphate groups, called Pi for short. Each molecule of ATP stores a small quantity of chemical energy.
Complex organic food molecules such as sugars, fats, and proteins are rich sources of energy for cells because much of the energy used to form these molecules is literally stored within the chemical bonds that hold them together. Scientists can measure the amount of energy stored in foods using a device called a bomb calorimeter. With this technique, food is placed inside the calorimeter and heated until it burns. The excess heat released by the reaction is directly proportional to the amount of energy contained in the food.
