Final answer:
When oxygen levels are low or absent, cells revert to a) anaerobic metabolism, which occurs in the cytosol and efficiently produces ATP for short bursts of energy through the conversion of glucose into pyruvate and lactic acid.
Step-by-step explanation:
When oxygen levels are low or absent, the cells revert to a process of a) anaerobic metabolism as their energy-producing mechanism.
Anaerobic metabolism occurs in the cytosol of the cells where it converts glucose into pyruvate and lactic acid, and only a small amount of ATP is produced.
This metabolic pathway is efficient for short bursts of energy when aerobic respiration, which happens in the mitochondria and produces the majority of ATP, cannot occur due to a lack of oxygen.
Aerobic metabolism is a much slower process than its anaerobic counterpart, but it yields more ATP since it can use carbohydrates, proteins, or fats as fuel sources.
During strenuous exercise or in conditions where oxygen is scarce, cells, including muscle cells and erythrocytes (which lack mitochondria), rely on anaerobic metabolism.
The process of fermentation allows pyruvate to be converted into lactic acid, which keeps glycolysis going by oxidizing NADH back into NAD+, necessary for continued glycolysis.
This form of respiration, anaerobic respiration, helps to produce ATP quickly over short periods, ranging from seconds to a few minutes.