1 Chemically, ATP is an adenine nucleotide bound to three phosphates. 2  There is a lot of energy stored in the bond between the second and third phosphate groups that can be used to fuel chemical reactions. 3  When a cell needs energy, it breaks this bond to form adenosine diphosphate (ADP) and a free phosphate molecule.
calf muscle pump. calf muscle pump. an action of the calf (soleus) muscles in which the muscles contract and squeeze the popliteal and tibial veins, forcing the blood in those veins to move upward toward the heart. Also called soleus pump.
Muscle cells, however, don't use sugar, fats or proteins to contract. Rather, our cells convert the energy stored in those nutrient molecules into energy stored within ATP (adenosine triphosphate). That's the universal energy molecule for living cells. ATP, in turn, provides the energy needed for muscular contraction. Exercising muscle gobbles up billions of ATP molecules every second. In this lesson, we'll describe how our muscle cells utilize energy from nutrients to make ATP and, therefore, contract.