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Active Transport Since the cell membrane is somewhat permeable to sodium ions, simple diffusion would result in a net movement of sodium ions into the cell, until the concentrations on the two sides of the membrane became equal. Sodium actually does diffuse into the cell rather freely, but as fast as it does so, the cell actively pumps it out again, against the concentration difference. The mechanism by which the cell pumps the sodium ions …

…hen oxygen is scarce. This occurs during the early stages of strenuous exercise, when sugar catabolism for ATP production outpaces the muscle's supply of oxygen from the blood. Under these conditions, the cells switch from aerobic respiration to fermentation. The lactate that accumulates as a waste product may cause muscle fatigue and pain, but it is gradually carried away by the blood to the liver. Lactate is converted back to pyruvate by the liver cells.