Describe how pyruvate is chemically modified in the mitochondrial matrix before the Krebs cycle begins, and explain how the product of this modification enters the cycle.

In the absence of oxygen (anoxia), the terminal electron acceptor at the end of the electron transport chain is unavailable, preventing the oxidation of reduced NAD ($NADH$) and reduced FAD (${FADH}_2$). Explain why anoxia leads to a halt in the Krebs cycle.