Absorption and assimilation of proteins, carbohydrates and fats

Absorption is the step where the small molecules produced by digestion actually cross the intestinal lining into the blood and lymph, and the villi lining the small intestine — each built around a central lacteal surrounded by a network of blood capillaries — are the structures specialised to do this. Different substances cross by different routes. Small quantities of glucose, amino acids and electrolytes such as chloride move by simple diffusion, driven purely by their concentration gradient. Some substances, such as fructose, are ferried across with the help of carrier ions like Na+ in a process called facilitated transport. Others — glucose, amino acids and Na+ itself among them — are pumped across against their concentration gradient by active transport, which needs energy. Fats follow a distinct path: insoluble fatty acids, glycerol and fat-soluble vitamins are first packaged into tiny water-soluble droplets called micelles, absorbed into the intestinal mucosal cells, and then re-assembled inside those cells into protein-coated fat globules called chylomicrons; these are released by exocytosis into the lacteals of the villi, travel through the lymphatic ducts, and are eventually emptied into the bloodstream — which is why fat absorption relies on the lymphatic route rather than blood capillaries directly. Absorption is not confined to one organ: some occurs in the mouth, some simple sugars, alcohol and certain medicines are absorbed directly through the stomach wall, a limited amount happens through the tongue and mouth's mucosa for some drugs, and the large intestine reabsorbs substantial water along with some vitamins, minerals and drugs — but by far the greatest share of a …