Most of the important polysaccharides are made from hexose units, so are called hexosans. Many are made from glucose itself. So are called glucosans. Glucose units can from long chains in three ways. Amylose and cellulose are completely straight. Starch is a mixture of both amylose and amylopectin. Glycogen is amylopectin with very short lengths between its branches.
Digestion involves the breakdown of large, insoluble food molecules into smaller, soluble compounds, which can be absorbed. For example, starch is broken down into smaller monosaccharides and disaccharides; proteins are broken down into polypeptides, then into tripeptides, dipeptides and finally amino acids.
Human digestion begins with physical digestion – the action of teeth chewing or mastication. This is when the mouth takes in the saccharide units in starch. Physical digestion is also started by the contractions of the alimentary canal, particularly the stomach and by bile salts. It helps mix the food with the digestive juices and breaks the food into smaller pieces so increasing the surface area available for the enzyme attack.
Chemical digestion involves the type of chemical reaction called hydrolysis - the chemical bonds which hold the large food molecules together are broken by adding water to them.
The saccharide units are digested through the stomach then these small molecules are left in the lumen of the intestine; i.e. outside the body. To be of any use they must be moved from the lumen into the bloodstream or lymph – the process of absorption. Nutrient molecules are then transported to the tissues where they are assimilated and used either for energy, growth, repair or maintenance.
Small molecules can be absorbed from the food in the bucal cavity and the stomach. For example, the effects of drinking alcohol are experienced quickly because the absorption occurs in the stomach. However, most absorption occurs in the small intestine, which in adults humans is about 6 metres long. Frequent folds in the wall of the small intestine increases its surface area. The folds themselves have tiny projections called villi, the mucosal cells of which have microvilli.
Assimilation is the absorption of food materials into the body. The water – soluble products of digestion are passed from the epithelial cells to the blood capillaries within each villus. The transfer of monosaccharides is aided by sodium ions. From the villus, these nutrients are carried to the liver by the hepatic portal vein. Immediately after a meal the concentration of these nutrients may be he very high but at other times it may be low. The activities of the liver ensure that blood passed to the rest of the body does not suffer these fluctuations in concentration.
