Digestion and Absorption in the Gastrointestinal Tract

Hydrolysis of Carbohydrates

Almost all the carbohydrates of the diet are either large polysaccharides or disaccharides, which are combinations of monosaccharides bound to one another by condensation. This phenomenon means that a hydrogen ion (H ⁺ ) has been removed from one of the monosaccharides, and a hydroxyl ion (OH ⁻ ) has been removed from the next one. The two monosaccharides then combine with each other at these sites of removal, and the H ⁺ and OH ⁻ then combine to form water (H ₂ O).

When carbohydrates are digested, this process is reversed, and the carbohydrates are converted into monosaccharides. Specific enzymes in the digestive juices of the gastrointestinal tract return the H ⁺ and OH ⁻ from H ₂ O to the polysaccharides and thereby separate the monosaccharides from each other. This process, called hydrolysis, is the following (in which R′′-R′ is a disaccharide):

Hydrolysis of Fats

Almost the entire fat portion of the diet consists of triglycerides (neutral fats), which are combinations of three fatty acid molecules condensed with a single glycerol molecule. During condensation, three molecules of water are removed.

Hydrolysis (digestion) of the triglycerides consists of the reverse process: the fat-digesting enzymes return three molecules of water to the triglyceride molecule and thereby split the fatty acid molecules away from the glycerol.

Hydrolysis of Proteins

Proteins are formed from multiple amino acids that are bound together by peptide linkages. At each linkage, a OH ⁻ has been removed from one amino acid and a H ⁺ has been removed from the succeeding one; thus, the successive amino acids in the protein chain are also bound together by condensation, and digestion occurs by the reverse effect: hydrolysis. That is, the proteolytic enzymes return H ⁺ and OH ⁻ from water molecules to the protein molecules to split them into their constituent amino acids.

Therefore, the chemistry of digestion is simple because, in the case of all three major types of food, the same basic process of hydrolysis is involved. The only difference lies in the types of enzymes required to promote the hydrolysis reactions for each type of food.

All the digestive enzymes are proteins. Their secretion by the different gastrointestinal glands was discussed in Chapter 65 .

Carbohydrate Foods of the Diet

Only three major sources of carbohydrates exist in the normal human diet. They are sucrose, which is the disaccharide known popularly as cane sugar; lactose, which is a disaccharide found in milk; and starches, which are large polysaccharides present in almost all nonanimal foods, particularly in potatoes and different types of grains. Other carbohydrates ingested to a slight extent are amylose, glycogen, alcohol, lactic acid, pyruvic acid, pectins, dextrins, and minor quantities of carbohydrate derivatives in meats.

The diet also contains a large amount of cellulose , which is a carbohydrate. However, enzymes capable of hydrolyzing cellulose are not secreted in the human digestive tract. Consequently, cellulose cannot be considered a food for humans.

Digestion of Carbohydrates Begins in the Mouth and Stomach

When food is chewed, it is mixed with saliva, which contains the digestive enzyme ptyalin (an α-amylase ) secreted mainly by the parotid glands. This enzyme hydrolyzes starch into the disaccharide maltose and other small polymers of glucose that contain three to nine glucose molecules, as shown in Figure 66-1 . However, the food remains in the mouth only a short time, so probably not more than 5% of all the starches become hydrolyzed by the time the food is swallowed.

Starch digestion sometimes continues in the body and fundus of the stomach for as long as 1 hour before the food becomes mixed with the stomach secretions. Activity of the salivary amylase is then blocked by acid of the gastric secretions because the amylase is essentially inactive as an enzyme once the pH of the medium falls below about 4.0. Nevertheless, on average, before food and its accompanying saliva become completely mixed with the gastric secretions, as much as 30% to 40% of the starches will have been hydrolyzed, mainly to form maltose.

Digestion by Pancreatic Amylase

Pancreatic secretion, like saliva, contains a large quantity of α-amylase that is almost identical in its function to the α-amylase of saliva but is several times as powerful. Therefore, within 15 to 30 minutes after the chyme empties from the stomach into the duodenum and mixes with pancreatic juice, virtually all the carbohydrates will have become digested.

In general, the carbohydrates are almost totally converted into maltose and/or other small glucose polymers before passing beyond the duodenum or upper jejunum.

Hydrolysis of Disaccharides and Small Glucose Polymers Into Monosaccharides by Intestinal Epithelial Enzymes

The enterocytes lining the villi of the small intestine contain four enzymes ( lactase , sucrase , maltase , and α - dextrinase ), which are capable of splitting the disaccharides lactose, sucrose, and maltose, plus other small glucose polymers, into their constituent monosaccharides. These enzymes are located in the enterocytes covering the intestinal microvilli brush border, so the disaccharides are digested as they come in contact with these enterocytes.

Lactose splits into a molecule of galactose and a molecule of glucose. Sucrose splits into a molecule of fructose and a molecule of glucose. Maltose and other small glucose polymers all split into multiple molecules of glucose . Thus, the final products of carbohydrate digestion are all monosaccharides. They are all water soluble and are absorbed immediately into the portal blood.

In the ordinary diet, which contains far more starches than all other carbohydrates combined, glucose represents more than 80% of the final products of carbohydrate digestion, and galactose and fructose each seldom represent more than 10%.

The major steps in carbohydrate digestion are summarized in Figure 66-1 .

Proteins of the Diet

Dietary proteins are chemically long chains of amino acids bound together by peptide linkages. A typical linkage is the following:

The characteristics of each protein are determined by the types of amino acids in the protein molecule and by the sequential arrangements of these amino acids. The physical and chemical characteristics of different proteins important in human tissues are discussed in Chapter 70 .

Digestion of Proteins in the Stomach

Pepsin, an important peptic enzyme of the stomach, is most active at a pH of 2.0 to 3.0 and is inactive at a pH above about 5.0. Consequently, for this enzyme to cause digestion of protein, the stomach juices must be acidic. As explained in Chapter 65 , the gastric glands secrete a large quantity of hydrochloric acid. This hydrochloric acid is secreted by the parietal (oxyntic) cells in the glands at a pH of about 0.8, but by the time it is mixed with the stomach contents and with secretions from the non-oxyntic glandular cells of the stomach, the pH then averages around 2.0 to 3.0, a highly favorable range of acidity for pepsin activity.

One of the important features of pepsin digestion is its ability to digest the protein collagen, an albuminoid type of protein that is affected little by other digestive enzymes. Collagen is a major constituent of the intercellular connective tissue of meats; therefore, for the digestive enzymes to penetrate meats and digest the other meat proteins, it is necessary that the collagen fibers be digested. Consequently, in people who lack pepsin in the stomach juices, the ingested meats are less well penetrated by the other digestive enzymes and, therefore, may be poorly digested.

As shown in Figure 66-2 , pepsin only initiates the process of protein digestion, usually providing only 10% to 20% of the total protein digestion to convert the protein to proteoses, peptones, and a few polypeptides. This splitting of proteins occurs as a result of hydrolysis at the peptide linkages between amino acids.

Most Protein Digestion Results From Actions of Pancreatic Proteolytic Enzymes

Most protein digestion occurs in the upper small intestine, in the duodenum and jejunum, under the influence of proteolytic enzymes from pancreatic secretion. Immediately upon entering the small intestine from the stomach, the partial breakdown products of the protein foods are attacked by the major proteolytic pancreatic enzymes trypsin, chymotrypsin, carboxypolypeptidase, and elastase, as shown in Figure 66-2 .

Both trypsin and chymotrypsin split proteins into small polypeptides; carboxypolypeptidase then cleaves individual amino acids from the carboxyl ends of the polypeptides. Proelastase, in turn, is converted into elastase, which then digests elastin fibers that partially hold meats together.

Only small percentages of the proteins are digested all the way to their constituent amino acids by the pancreatic juices. Most remain as dipeptides and tripeptides.

Digestion of Peptides by Peptidases in the Enterocytes That Line the Small Intestinal Villi

The last digestive stage of proteins in the intestinal lumen is achieved by enterocytes that line the villi of the small intestine, mainly in the duodenum and jejunum. These cells have a brush border that consists of hundreds of microvilli projecting from the surface of each cell. In the membrane of each of these microvilli are multiple peptidases that protrude through the membranes to the exterior, where they come in contact with the intestinal fluids.

Two types of peptidase enzymes are especially important, aminopolypeptidase and several dipeptidases. They split the remaining larger polypeptides into tripeptides and dipeptides and a few into amino acids. The amino acids, dipeptides, and tripeptides are easily transported through the microvillar membrane to the interior of the enterocyte.

Finally, inside the cytosol of the enterocyte are multiple other peptidases that are specific for the remaining types of linkages between amino acids. Within minutes, virtually all the last dipeptides and tripeptides are digested to the final stage to form single amino acids, which then pass on through to the other side of the enterocyte and thence into the blood.

More than 99% of the final protein digestive products that are absorbed are individual amino acids, with only rare absorption of peptides and very rare absorption of whole protein molecules. Even these few absorbed molecules of whole protein can sometimes cause serious allergic or immunologic disturbances, as discussed in Chapter 35 .