$H_{2}O$ or Water is a very important component of the cellular respiration. However, it does not act in the process of cellular respiration directly, but is produced at the end of the cellular respiration during Oxidative Phosphorylation or Electron Transport System.

Basically, what happens is during the stages ETS, the NADH and $FADH_{2}$ gets converted into NAD and FAD whereas the $H^{+}$ ion accumulates into the inter-membrane space, which then passes through the ATP synthase molecule from the inter-membrane space to the matrix thus, stimulating the ATP synthase molecule to produce ATP the most important product of cellular respiration. This $H^{+}$ ion when enters the matrix gets utilized to form $H_{2}O$ by getting bound to $O_{2}$ . But if no $H_{2}O$ is produced in the final stage and the $H^{+}$ ion gets accumulated in the matrix then the cellular machinery will malfunction and the production of ATP will stop. Thus, by this fact, we know that $H_{2}O$ has a very important role in cellular respiration.