Glucose transport into intestinal epithelial cells

The cells lining the lumen of the intestine are polarized, which is they have two distinct sides or domains that have different lipid and protein compositions. The brush  or apical  border  membrane  facing  the lumen  is highly  folded  into microvilli  to raise  the surface field available  for the absorption  of nutrients. The  other  of the  plasma  membrane  the  basolateral  surface  is in contact  with neighboring  the blood and cells capillaries. Movement among adjacent

                               Figure: The Na⁺  /K⁺   -ATPase, shown as an αβ heterodimer.

Figure: Ion-driven cotransport mechanisms. (a) Symport process involving a symporter (e.g. Na  /glucose transporter); (b) antiport process involving an antiporter (e.g. erythrocyte band 3 anion transporter).

epithelial cells is prevented through the formation of tight junctions around the cells near the apical domain. Therefore any nutrient molecules in the lumen of the intestine have to pass through the cytosol of the epithelial cell in order to enter the blood.

Glucose (or other sugars and amino acids) are transported across the apical membrane from a relatively low concentration in the lumen of the intestine to a relatively high concentration in the cytosol of the epithelial cell through a glucose/Na⁺ symporter protein in the above Figure. This  is a form  of ion-driven  active  transport;  the energy  for the movement  of glucose  against  its concentration  gradient  comes from the movement  of Na⁺ down its concentration  gradient. The blood flowing by  the capillaries  on the basolateral  side of the epithelial  cell maintains  a concentration  gradient of glucose across this membrane  allowing the glucose to move out of the cell by facilitated  diffusion  through  a glucose transporter  (a uniporter)  which  is similar  to the erythrocyte  glucose  transporter  (see above). The relatively low concentration of Na⁺inside the epithelial cell is maintained by a Na⁺ /K⁺-ATPase (see above) on the basolateral membrane and an instance of ATP-driven active transport in figure.

                               Figure:  Transport of glucose and water across intestinal epithelial cells.