Absorption of the products of carbohydrate digestion. The roles of diffusion, active transport and co-transport involving sodium ions.

Carbphydrates are digested from polysaccharides into monosaccharides. These monosaccharides are absorbed into the blood stream. This happens in the small intestine.

Monosaccharides are moving from the lumen (interior) of the small intestine into the epithelial cells of the small intestine (lining) into the blood in the capillaries. There is a good blood flow which takes away the monosaccharides in the blood. So the highest concentration is in the intestine and lowest in the blood making a concentration gradient so monosaccharides can diffuse into the blood stream.

Not all of the monosaccharides in the intestine can be absorbed by diffusion because it relies on the concentration gradient being correct.. so active transport might be used. Monosaccharide molecules can travel into the epithelial cell by a co-transport protein carrier, but only if they are with sodium ions. To make sure sodium ions are travelling into the epithelial cells, sodium ions are transported out of them on the opposite side (into the blood) so that there is a lower concentration in the cells than the lumen: this means that sodium ions will be travelling into the cells (along the concentration gradient). When they travel in, they bind to the co-transport protein, a monosaccharide now also binds to the protein changing  its shape so it can release the molecules on the other side. Now the monosaccharide is in the cell facilitated diffusion is used to transport it out the other side into the blood stream.