The body needs oxygen because oxygen is involved in the last steps of ATP synthesis. ATP is a molecule is all of the body's cells that provide the cell with energy to carry out its tasks. One of the most important of these is a pump on the cell surface that regulates the concentration of molecules in the cell. Without ATP, therefore, the cells would die. Without cells, the basic building block of tissues and organs, the body itself cannot function. Thus, oxygen is vital to make this ATP.
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