The second stage of cellular metabolism that breaks glucose down into energy that can be used by the body requires the presence of

Oxygen (O2)

Explanation:

The cellular metabolism described in the question is CELLULAR RESPIRATION, which involves breaking down glucose to release usable energy (ATP). The first stage of cellular respiration (GLYCOLYSIS) can occur in the absence of oxygen, which is why many organisms including anaerobic ones undergo this stage. It is where the actual breakdown of glucose occurs to yield pyruvate (3-carbon) and 2 molecules of ATP.

The second stage of cellular respiration is referred to as Kreb's cycle (named after Hans Kreb) or Citric acid cycle. In this stage, oxygen is required to oxidize the pyruvate produced in glycolysis to form Acetyl CoA. Acetyl CoA enters into the mitochondria of the cell and gets involved a series of reaction. NAD+ and FAD+ molecules gets reduced to 3 NADH and 1 FADH2 molecules and 1 ATP and GTP molecule respectively. The reduced electron carriers (NADH and FADH2) acts as the first electron donor in the Electron transport chain.

In a nutshell, oxygen is required for pyruvate to enter into the Kreb's cycle, hence, it cannot happen in the absence of oxygen.

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