NGSS HS-LS2-3: Construct and revise an explanation based on evidence for the cycling of matter and flow of energy in aerobic and anaerobic conditions.
Oxygen is plentiful. The cell can fully break down glucose to extract maximum energy.
When oxygen is present, cells can fully oxidize glucose into Carbon Dioxide and Water. This complete breakdown cycles matter back to the environment in forms usable by plants, and transfers a massive amount of energy (up to 36 ATP) to power complex life processes.
When oxygen is absent or depleted (e.g., heavy sprinting), the cell must use fermentation. The glucose is only partially broken down into Lactic Acid (or Ethanol in yeast). Matter is not fully cycled into simple gases, and only a tiny fraction of energy (2 ATP) is transferred, making it unsustainable for long periods in complex organisms.