Carbohydrate Absorption

The absorption of carbohydrate molecules is directly linked to the final stage of digestion (Figure 4). Luminal carbohydrate digestion produces largely the disaccharide molecules sucrose, lactose, and maltose. The enterocyte luminal membranes are richly endowed with disaccharidases, which hydrolyze the disaccharides to the monosaccharides glucose, fructose, and galactose. These membrane-bound enzymes are now considered to be associated with the protein transporter molecules described previously that facilitate the transport of sodium across the enterocyte luminal membrane. Sugars are cotransported with sodium ions against their own concentration gradients—a process of great nutritional value. Once concentrated within the enterocyte, the sugars pass by facilitated diffusion across the basolateral membrane into the interstitial spaces and from there diffuse into the capillaries of the intestinal villus.

Glucose Na+ Galactose

Glucose Na+ Galactose

Primary active transport

Capillary

Fructose

Primary active transport

Fructose

Fructose

Capillary

Capillary \

Figure 4 Carbohydrate absorption mechanisms. (Left) Secondary active transport: Glucose and galactose are absorbed using Na+ cotransport systems. Sodium ions are removed using the (Na+-K+)-ATPase pump (primary active transport). (Right) Facilitated diffusion: Fructose absorption is passive but relies on a carrier molecule. Solid arrow, movement against concentration gradient; broken arrow, diffusion down concentration gradient; ATP, ATP-dependent pump; C, carrier molecule.

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