Final answer:
When oxygen is not available in human muscle cells, lactic acid fermentation occurs to continue producing energy. This allows muscles to sustain activity by regenerating NAD+ for glycolysis, resulting in the production of lactic acid, which can contribute to muscle fatigue.
Step-by-step explanation:
When oxygen is not available in human muscle cells, the process that occurs is c) Lactic acid fermentation. This form of anaerobic respiration allows cells to produce energy in the absence of oxygen by converting glucose into lactic acid.
During intense exercise when oxygen delivery to muscles is limited, lactic acid fermentation facilitates the continuation of glycolysis by ensuring the regeneration of NAD+ from NADH. As a result, muscles can sustain short bursts of activity even in oxygen-deficient conditions, albeit at the cost of producing a net gain of two ATPs per molecule of glucose and leading to potential muscle fatigue due to lactic acid accumulation.
Lactic acid fermentation is specifically utilized by animal skeletal muscles and is distinct from alcoholic fermentation, which is not performed by human cells. This mechanism is critical during physical exertion when energy demand is high and oxygen supply is restricted. Excess lactic acid can be transported out of the muscles for further processing in the liver, demonstrating the body's ability to handle the byproducts of anaerobic energy production.