When atp is used by a cell to provide the energy for cellular work, the usual end products left over from the atp molecule are

When atp is used by a cell to provide the energy for cellular work, the usual end products left over from the atp molecule are ADP

Explanation:

Like every other living thing that needs energy to survive the cells also needs energy for the proper functioning of the organism. Through Cellular respiration the cells derive energy in the form of ATP molecules.  

Adenosine triphosphate (ATP) releases energy to the required cell by transferring one phosphate and then becoming Adenosine diposphate (ADP).

Then after it releases energy it is all set up to absorb another phosphate molecule to become ATP. And this is the simple process of acquiring and transferring energy to the cells.

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b. the carbon cycle ensures living things have a source of hydrogen