The absorption of glucose from the gut into intestinal mucosal cells is coupled to Na+,K+ ATPase. In contrast, the movement of glucose from the intestinal epithelial cells into the submucosal bloodstream occursthrough passive transpo. Given these facts, which of the following statements can be true at one time or another?
The plasma membranes of intestinal epithelial cells contain a sodium gradient that drives the active transpo of glucose. The rate and amount of glucose transpoed depend upon the sodium gradient maintained across the plasma membrane. Sodium ions entering the cell in the company of glucose are pumped out again by Na+,K+-ATPase. Once in the cytosol of the intestinal cell, the glucose moves across the cell and diffuses out of the cell into the interstitial fluid of the submucosa and then into the plasma of the capillaries underlying the intestinal epithelium. This occurs for the following reason: while glucose is maintained in blood plasma at an approximately constant level, it is always slowly moving out of the plasma into the cells of tissue that use it. Given that the diffusion from the intestinal cells into the plasma is passive, the intestinal cells and the plasma try to maintain an equilibrium. Thus, plasma glucose levels are always approximately equal to or slightly less than levels in the intestinal cells. Due to the passive maintenance of this equilibrium, it is highly unlikely that the concentration of glucose in the plasma can get much higher than that in the intestinal cell cytosol. It is also unlikely that the levels of glucose in other tissues of the body (for example, muscle) will have any bearing on those found in the intestinal cells
Levels of plasma glucose are approximately equalto levels in the cytosol of intestinal epithelial cells
Free glucose levels in the lumen of the intestine can never be higher than levels in intestinal cells
Plasma glucose levels are much higher than intestinal cell cytosolic levels of glucose
Levels of glucose in the intestinal lumen are always higher than those in the cytosol of intestinal epithelial cells