Final answer:
Oxygen is not always necessary for ATP production, as cells can produce ATP through anaerobic respiration or fermentation. Glycolysis and lactic acid fermentation are two processes that allow ATP production without oxygen, although they are less efficient than aerobic respiration.
Step-by-step explanation:
No, oxygen is not always necessary for the production of ATP in cells. While oxygen is crucial in the final stages of cellular respiration, such as the Krebs cycle and electron transport chain where it acts as an 'electron magnet' driving the electron flow that allows for a large production of ATP, there are conditions under which cells can produce ATP without oxygen. This occurs through the process known as anaerobic respiration or fermentation.
Glycolysis is an anaerobic process that can proceed without oxygen. It produces a minimal amount of ATP. However, in an oxygen-depleted environment, such as in your muscle cells during intense exercise, your body performs lactic acid fermentation. This process also generates ATP without the presence of oxygen, albeit in much smaller quantities compared to aerobic respiration.
The production of ATP without oxygen is significantly less efficient. For each molecule of glucose, cellular respiration in the presence of oxygen produces a hefty 36-38 ATP molecules, while anaerobic processes produce only about two ATP molecules.