Facilitative glucose transporters galactose

Glucose and galactose enter the absorptive cells by way of secondary active transport. Cotransport carrier molecules associated with the disaccharidases in the brush border transport the monosaccharide and a Na+ ion from the lumen of the small intestine into the absorptive cell. This process is referred to as "secondary" because the cotransport carriers operate passively and do not require energy. However, they do require a concentration gradient for the transport of Na+ ions into the cell. This gradient is established by the active transport of Na+ ions out of the absorptive cell at the basolateral surface. Fructose enters the absorptive cells by way of facilitated diffusion. All monosaccharide molecules exit the absorptive cells by way of facilitated diffusion and enter the blood capillaries. 

Other cotransporters facilitate the transport of other sugars, osmolytes, and amino acids. In humans, a disorder of intestinal glucose and galactose absorption is due to a defective sodium-glucose transporter. 

Schematic representation of glucose (or galactose) transport by the enterocyte. Glucose binds to the receptor, facilitated by the simultaneous binding of two Na+ at separate sites. The glucose and Na" " are released in the cytosol as the receptor affinity for them decreases. The Na" " are actively extruded at the basolateral surface into the intercellular space by Na+,K+-ATPase, which provides the energy for the overall transport. Glucose is transported out of the cell into the intercellular space and thence to portal capillaries, both by a serosal carrier and by diffusion. (Reproduced with permission from G. M. Gray, Carbohydrate Absorption and Malabsorption in Gastrointestinal Physiology. Raven Press,...

Glucose, galactose, and fructose formed by the digestive enzymes are transported into the absorptive epithelial cells of the small intestine by protein-mediated Na -dependent active transport and facilitative diffusion. Monosaccharides are transported from these cells into the blood and circulate to the liver and peripheral tissues, where they are taken up by facilitative transporters. Facilitative transport of glucose across epithelial cells and other cell membranes is mediated by a family of tissue-specific glucose transport proteins (GLUT I-V). The type of transporter found in each cell reflects the role of glucose metabolism in that cell. 

Fig. 27.12. Na -dependent and facilitative transporters in the intestinal epithelial cells. Both glucose and fructose are transported by the facilitated glucose transpxrrters on the luminal and serosal sides of the absorptive cells. Glucose and galactose are transported by the Na -glucose cotransporters on the luminal (mucosal) side of the absorptive cells.

Fructose transport is distinct from glucose-galactose transport and requires a specific saturable membrane carrier (facilitated diffusion). 

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