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Substrate$-$level phosphorylation is used only when there is no oxygen available to the cell. When there is oxygen available, oxidative phophorylation is used instead.

Substrate$-$level phosphorylation involves phosphorylation of ATP to form ADP, while oxidative phosphorylation converts ADP to ATP.

Substrate$-$level phosphorylation is an indirect process of ATP production that occurs in other parts of the cell from the reactions that break down glucose. Oxidative phosphorylation is the more direct production of ATP during the breakdown of glucose by oxygen.

Substrate level phosphorylation occurs directly during certain steps of glucose breakdown, while oxidative phosphorylation is a more complicated process that occurs in other parts of the cell and uses an external electron acceptor.