Biology of Disease

with respect to the outside. The membrane potential of the apical membrane

of the enterocyte can be harnessed to facilitate the uptake of a variety of nutri-

ents from the lumen of the GIT. Both amino acids and monosaccharides are

transported across the enterocyte luminal membrane in a Na+-dependent

fashion. A variety of different membrane transporter proteins are responsible

for the absorption of specific sugars and different groups of amino acids.

Glucose and galactose are transported across the enterocyte luminal mem-

brane in an active, Na+-dependent fashion by the same transporter. One mol-

ecule of these sugars can only move through the transporter into the cell if

Na+ ions move in at the same time (Figure 11.17). The concentrations of the

sugars can build up within the cytoplasm, such that they are able to leave the

cell through the basolateral membrane by facilitated diffusion. Other mono-

saccharides, for example fructose and trehalose, are absorbed only by facili-

tated diffusion and are absorbed to a much lesser extent.

The major initial products of protein digestion are small peptides and these

are absorbed by enterocytes of the jejunum at their luminal surfaces by pep-

tide transporter protein, called the PepT1 (Figure 11.18). This occurs in a H+-

dependent fashion that resembles the uptake of glucose and galactose. Within

the cytoplasm, the peptides are hydrolyzed to amino acids, ensuring a con-

tinuous sink is present to facilitate peptide uptake by the cells. The exit of the

amino acids from the cells on the basolateral side also occurs down their con-

centration gradients. However, as peptides are moved further along the GIT,

they are hydrolyzed by peptidases to free amino acids and their absorption

occurs in the ileum using a number of Na+-dependent transporters (Figure

11.18), which have specificities for different amino acid side chains. Peptides

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Na+/K+-ATPase

Na+/glucose transporter

Tight junction

Capillary

GIT lumen

3Na+2K+

3Na+2K+

ATP ADP

Fructose Trehalose

Fructose Trehalose

Fructose Trehalose

Fructose permease

Trehalose permease

Glucose

Galactose

Glucose

Galactose

Na+

Na+

Glucose Galactose

Figure 11.17Overview of the absorption of

monosaccharides by an enterocyte. Transport

movements are denoted by colored lines.See text

for general details.