DIGESTION OF CARBOHYDRATES
Digestion of carbohydrate begins in the mouth, with the secretion of the enzyme salivary amylase from
the serous cells of the salivary gland. This enzyme breaks starch and glycogen into disaccharides. The mucous
cells of the salivary gland secrete a mucus, which causes the food to stick together, and acts as a lubricant to aid
in swallowing. The salivary glands are grouped into three categories: the parotid gland, submandibular glands,
and sublingual, all located near the mouth. The food (bolus) is forced into the pharynx by the tongue. As the
food is swallowed, it moves into the esophagus (a straight collapsible tube), which essentially provides a
passageway from the pharynx to the stomach. The mucous glands of the esophagus secrete mucus to aid in
moistening and lubricating the bolus. The bolus passes through the cardiac sphincter, into the first section of the
stomach. The stomach is divided into several regions: the cardiac region, body region, fundic region, and
pyloric region. The stomach works to mix and churn the food, which aids in further digestion of carbohydrates.
At this point, the bolus is converted into a semifluid paste of bolus and gastric juices called chyme.
The chyme then travels through the pyloric sphincter into the first section of the small intestines. The
small intestines is divided into three sections: the duodenum, jejunum, and ileum. The majority of digestion of
carbohydrates takes place in the small intestines. As the chyme moves into the duodenum, an enzyme called
pancreatic amylase is released through the pancreatic duct. This enzyme splits molecules of starch and glycogen
into disaccharides.
The liver plays an important role in the several metabolic activities. It is responsible for changing
glycogen to glucose to increase blood glucose or converting glucose to glycogen, thereby decreasing blood
glucose. The liver also converts other noncarbohydrates into glucose, if needed.
The interior wall of the small intestines is covered with tiny projections called the villi. These
projections increase the surface area of the intestines and play an important part in the process of absorption of
the nutrients. The epithelial cells of the villi contain even smaller extensions, called microvilli. Embedded in
the microvilli are digestive enzymes which are needed to further break down carbohydrates. The include
sucrase, maltase, and lactase, which break down the disaccharides into monosaccharide. These monosaccharide
are then absorbed by the villi and enter the blood capillaries to be transported to other parts of the body.
DIGESTION OF LIPIDS
As food containing fat travels through the digestive tract, digestion takes place in different locations.
The food containing fats essentially follow the same pathway as carbohydrate foods, with chemical digestion
primarily beginning in the stomach.
The surface of the inner lining of the stomach contains openings called gastric glands, which are made
up of three types of cells: mucous cells, chief cells, and parietal cells. A hormone called gastrin stimulates the
gastric glands to secrete their fluids. Gastric enzymes are secreted by the chief cells, while hydrochloric acid is
secreted by the parietal cells. The combination of mucus, hydrochloric acid and enzymes is referred to as gastric
juices. The gastric juices contain small amounts of gastric lipase, which begin the breaking down of specific
lipids. A hormone called somatostatin inhibits the release of acid. As the stomach churns, it mixes the bolus
and converts it into a semifluid referred to as chyme.
The liver also plays an important role in lipid metabolism by oxidizing fatty acids, synthesizing
lipoproteins, phospholipids, and cholesterol; and converting portions of carbohydrates and protein molecules
into fat molecules.
If fats are not being digested well enough in the stomach the hormone cholecystokinin slows gastric
motility. As the chyme passes into the duodenum, cholecystokinin stimulates the gallbladder to release a
substance called bile through the common bile duct. The function of bile is to emulsify fats, or break them into
smaller droplets. This allows the small droplets of fat to be digested more effectively. Cholecystokinin also
stimulates the release of pancreatic juices, which contains pancreatic lipase. This enzyme initiates the breaking
down of lipids. The cells of the small intestines release intestinal gastrin, which increases gastric secretions. The
mucosal cells release an enzyme called intestinal lipase, which splits fats into fatty acids and glycerol. The fatty
acids are then dissolved in the epithelial cell membranes of the villi and diffuse into them. Some fatty acids may
be absorbed directly into the blood capillary, without being converted back into fat, while others are
incorporated into chylomicrons (large molecules of lipoprotein) for transport.
DIGESTION OF PROTEIN
As food containing protein enters the mouth, mucus is secreted to bind the particles together. The food
then travels down the esophagus and more mucus is secreted by the wall of the esophagus.
The chief cells located in the stomach secrete gastric juices containing an enzyme precursor called
pepsinogen. As it comes in contact with hydrochloric acid, it is converted into an active form referred to as
pepsin. This begins the process of chemical digestion of dietary protein. A hormone called gastrin is
responsible for stimulating the secretion of gastric juices. After the food is mixed and churned in the stomach, it
is converted into chyme.
The liver plays a vital role in protein metabolism by deaminating amino acids and forming urea
nitrogen. It also is responsible for synthesizing certain blood proteins and converting certain amino acids to
other amino acids.
The chyme moves into the duodenum and enzymes begin to work on the protein. The hormone called
intestinal gastrin stimulates the gastric glands to increase secretion. When protein reaches the duodenum, a
hormone called cholecystokinin is released from the intestinal walls, which stimulates the release of pancreatic
juice. The pancreatic juice contains three protein splitting enzymes in inactive formscalled trypsinogen,
chymotrypsinogen, and procarboxypeptidase. Once trypsinogen comes in contact with an enzyme called
enterokinase, which is secreted by the mucosal cells of the small intestines, then trypsin is activated. The
presence of trypsin then activates the inactive procarboxypeptidase and chymotrypsinogen, and they are
converted into chymotrypsin and carboxypeptidase. The enzymes are then able to work on the protein. The
mucosal cells secrete an enzyme called peptidase, which splits peptide bonds into amino acids to allow for
digestion. Protein digestion is completed in the small intestines. Smaller particles of amino acids are absorbed
into the villi, and are carried away by the blood.