# Why is cellular respiration more efficient when oxygen is present in cells?

${O}_{2}$ is involved in electron transport chain that produces $N A {D}^{+}$ which is used in TCA cycle.
${O}_{2}$ is a terminal electron acceptor that is involved in electron transport chain. Ultimately, electron is removed from NADH, thereby oxidizing it to $N A {D}^{+}$. $N A {D}^{+}$ is an important oxidizing agent that acts on intermediates of glycolysis and TCA cycle.
Without ${O}_{2}$, pyruvate will not be transported into the mitochondria (i.e. where TCA cycle and electron transport chain occur). But instead, it will be fermented to either lactic acid or ethanol. Fermentation still produce several $N A {D}^{+}$ as bonds is still being broken. However, the quantity is not enough for TCA cycle.