Final answer:
Under anaerobic conditions, muscle cells reduce pyruvate to lactate to regenerate NAD+ from NADH, allowing glycolysis to continue and to produce ATP despite the absence of oxygen.
Step-by-step explanation:
Muscle cells overcome the buildup of NADH and decrease of NAD+ under anaerobic conditions by reducing pyruvate to lactate. During vigorous exercise or other situations where oxygen becomes limited within muscle tissues, cells switch from aerobic respiration to anaerobic respiration to meet their energy requirements. Due to the lack of oxygen, NADH can't be oxidized through the electron transport chain and thus, is oxidized via lactate dehydrogenase, leading to the production of lactate from pyruvate. This process allows the cells to regenerate NAD+, which is necessary for glycolysis to continue and provide ATP, albeit in much smaller quantities compared to aerobic respiration.
The fate of pyruvate in cells is crucial since it acts as a crossroad, deciding whether the cell will undergo aerobic or anaerobic respiration. Under anaerobic conditions, the conversion of pyruvate to lactate in glycolysis is a prime example of the reduction that involves NADH. This shift helps to offset the energy deficit caused by the inability to use the electron transport chain to produce large amounts of ATP. The anaerobic catabolism of glucose yields a net two ATPs from a glucose molecule, which is significantly less than the approximate thirty ATPs produced during aerobic respiration.