Absorption of Glucose
Let us first consider the absorption of glucose from the gut lumen. (Refer to LSE-01, Units 7 and 8). The molecule involved in absorption of glucose is known as cotransporter because it couples the transport of a glucose molecule with that of a sodium ion. The energy needed is provided by the movement of sodium ion along its gradient. The cotransporter enables cells lining the lumen of intestine to absorb even quite small traces of glucose from food even though the epithelial cells may already have high concentrations of glucose inside them. Once inside the cell, the sodium ion is pumped out by ATP energised active transport and the glucose molecule is transferred to the blood stream through another transporter molecule, Glu T2, along its concentration gradient. Glu T2 transports glucose in proportion to the sugar concentration present in the blood. If more glucose is present in the blood, transport is slowed and if glucose content of blood is low then transport is accelerated.
Figure: Suggested mechanism for absorption of glucose. Na+ and glucose are transported together through carrier molecule or cotransporter located in the membrane. Inside the cell, sodium moves out by ATP pump and glucose is taken by a transporter molecule to the blood.
Experimental evidence shows that at least 4 transport processes for amino acids occur in the mammalian gut. Two for neutral amino acids, one for basic and one for acidic amino acid. Another separate transport system exists for dipeptides and tripeptides. Once inside the cell, these breakdown into constituent amino acids by intracellular peptidases. The sugar and amino acids reach the circulatory system from where similar mechanisms use the sodium gradient to transport amino acids and - glucose to the various tissues of the body.