The process that will occur in both the presence and absence of oxygen is glycolysis.

Core idea

  • Glycolysis is the first step of cellular respiration, where glucose is broken down into pyruvate in the cytoplasm.
  • It does not require oxygen, so it happens in aerobic (with oxygen) and anaerobic (without oxygen) conditions.

How it fits with other processes

  • When oxygen is present , glycolysis is followed by the Krebs (citric acid) cycle and oxidative phosphorylation in the mitochondria (aerobic respiration).
  • When oxygen is absent , glycolysis is followed by fermentation (e.g., lactic acid fermentation in muscles or alcoholic fermentation in yeast).

So, if your options include glycolysis (or “the breakdown of glucose to pyruvate in the cytoplasm”), that is the process that occurs whether oxygen is present or not.