Final answer:
The metabolic pathway common to both cellular respiration and fermentation is glycolysis, which breaks down glucose into pyruvate and is the first step in both aerobic respiration and anaerobic fermentation processes. Glycolysis occurs in the cytoplasm and does not require oxygen, thus functioning under both aerobic and anaerobic conditions.
Step-by-step explanation:
The metabolic pathway that is common to both cellular respiration and fermentation is glycolysis. Glycolysis is the process by which a six-carbon glucose molecule is broken down into two three-carbon pyruvate molecules. This process occurs in the absence of oxygen and does not require the mitochondria, making it possible in both aerobic environments — where it is followed by the citric acid cycle and oxidative phosphorylation — and anaerobic environments, such as fermentation where the electron transport chain is not utilized.
Fermentation is an anaerobic process that allows for the continuation of glycolysis by regenerating NAD+, which is a necessary component for glycolysis to proceed. The reactant, or starting material, shared by aerobic respiration and both types of fermentation — alcoholic fermentation and lactic acid fermentation — is glucose.
In humans, the type of metabolic process that predominantly occurs is aerobic cellular respiration. However, in conditions where oxygen is scarce, such as during intense exercise, cells can switch to lactic acid fermentation to generate ATP.