Digestion

Food consists of nutrients, liquids, minerals and vitamins. Individual nutrients (saccharides, fats and protein) are used by the organism as building blocks. The process of digestion is necessary for these substances, usually taken up as complicated organic compounds, to be absorbed by the small intestine.

Process of Digestion
By digestion we understand the chemical process of dissolution of matter by specific enzymes. Digestive enzymes are produced by gland cells in the oral cavity, the stomach and the exocrine part of the pancreas. The small intestine is important because it facilitates the transport of amino acids, fatty acids and sugars through its mucosa into blood or lymph vessels. This process is called resorption. The mucosa of the small intestine consists mainly of two types of cells: Multiple mechanisms allow the absorption of nutrients from the lumen of the small intestine into the extracellular fluid, such as diffusion, facilitated diffusion, active transport and pinocytosis.
 * Enterocytes – responsible for absorption, resorption and secretion
 * Goblet cells – secreting mucus and protecting the surface epithelia

Digestion of Saccharides
Saccharides are taken up mainly in the form of polysaccharides, disaccharides and monosaccharides. The most important alimentary polysaccharide is vegetable starch, composed of amylopectin and amylose. Its glucose molecules are ordered into straight or slightly branched chains and are linked by 1,4-α-glycosidic bonds. Glycogen is the alimentary polysaccharide of animals. It is composed of glucose units linked by 1,6-α-glycosidic bonds into branching chains. Disaccharides include sucrose and lactose. Glucose and fructose are examples of monosaccharides.

Cellulose, hemicellulose and pectin are some of indigestible plant polysaccharides. They are not important as a source of nutrients, but rather as a component of fiber. While indigestible by humans, it is being consumed by the bacteria of the colon. It also lowers cholesterol levels in blood and plays a role in the prevention of colon cancer. It influences the regulation of colon functions as well.

The digestion of starch begins in the mouth under the effect of ptyalin, a salivary enzyme. Its activity is diminished by the acidic contents of the stomach, but later continues in the duodenum under the influence of the pancreatic α-amylase. The basic pancreatic juice neutralizes the acidic chyme flowing into the duodenum from the stomach, which allows the pancreatic amylase to finish the cleaving of starch and glycogen into shorter saccharides (maltose, maltotriose, and oligosaccharides like α-dextrin). The resorption of sacccharides concerns only monosaccharides, which means the enzymes included in the brush border of enterocytes must finalize the cleaving of oligosaccharides created there.