Mechanical and Chemical processes in Digestion

Mechanical and Chemical Digestion 
Mechanical digestion breaks food up into small pieces that are more accessible to digestive enzymes. It includes the chewing of food by teeth which is discussed in the Medical Focus on the next page. The churning of food in the stom­ach is another good example of mechanical digestion.
Chemical digestion refers to the action of the digestive enzymes. As mentioned previously, the various digestive juices contain enzymes that digest particular types of food. Each of these enzymes will now be considered in regard to the digestion of a ham sandwich, which con­tains starch (a carbohydrate in the bread), protein (in the ham), and fat (butter on the bread).
Salivary amylase, an enzyme present in saliva, acts on starch. The end product of this reaction-maltose­however, is not small enough to cross plasma mem­branes to any degree; therefore, further digestion is required before absorption is possible.
In the stomach, the enzyme pepsin is present in gastric juices, which act on protein. The end product of this reaction-peptides-is, again, too large to cross plasma membranes; therefore, further digestion is required before absorption is possible.
Pancreatic juice, which enters the small intestine at the duodenum, contains digestive enzymes for all major types of food. Pancreatic amylase, like salivary amylase, acts on starch. The enzymes trypsin and chymotrypsin, like pepsin, act on protein. And the enzyme lipase acts on fat droplets after fat has been emulsified by bile salts. The end products of the lipase reaction-glycerol and fatty acids-are small enough to enter the cells of the intestinal villi. In the villi, glycerol and fatty acids rejoin to form fat, which enters the lacteals, a branch of the lymphatic system.
The brush border of the small intestines contains en­zymes that complete the digestion of both starch and R'ro­tein, forming small molecules that can cross the cells of the intestinal villi. Glucose is the end product of starch break­down, and amino acids are the end product of protein breakdown. These end products enter the blood capillaries of the villi. These blood capillaries join to form venules and veins, which empty into the hepatic portal vein, a ves­sel that goes to the liver.
Digestive enzymes present in digestive juices break down food into the following nutrient molecules: glucose, amino acids, fatty acids, and glycerol. Glucose and amino acids are absorbed into the blood capillaries of the intestinal villi. Fatty acids and glycerol are also absorbed but then rejoin to produce fat, which enters the lacteals.