The epithelial cells lining the ileum possess microvilli, which are finger-like projections of the cell-surface membrane.
The microvilli provide more surfacearea for the insertion of carrier proteins through which diffusion, facilitated diffusion and active transport can occur.
Another mechanism to increase transport across membranes is to increase the number of protein channels and carrier proteins in any given area of membrane.
As carbohydrates and proteins are being digested continuously, there is normally a greater concentration of glucose and amino acids within the ileum than in the blood. This creates a concentration gradient for glucose to move via facilitated diffusion from the ileum into the blood.
Since blood is being constantly circulated by the heart, the glucose absorbed into it is continuously being removed by cells as they use it up during respiration. This helps to maintain the concentration gradient between the inside of the ileum and the blood. This increases the rate of facilitated diffusion across the epithelial cell-surface membranes.
Diffusion only results in the concentration either side of the intestinal epithelium becoming equal (achieving equilibrium). This means that not all available glucose and amino acids are absorbed in this way and some may pass out of the body. This is why co-transport, an indirect form of active transport, is used as well.
The specific mechanism used to absorb amino acids and glucose from the small intestine is known as co-transport, whereby either glucose or amino acids are drawn into cells alongside sodium ions that have been actively transported out by the sodium-potassium pump.
