I

Protein

H — N — C — C — N — C —C — OH + I I I

H HR

H — N — C — C — OH + |H — N — C — C — OH

digesting enzyme I I

H — N — C — C — N — C —C — OH + I I I

H HR

H — N — C — C — OH + |H — N — C — C — OH

digesting enzyme I I

Lipid

C17H35COO C H

3H2O

Triglyceride-digesting enzyme

C17H35C

C17H35C

C17H35C

CH I

Water

Fatty acids

Glycerol

■ Figure 18.1 The digestion of food molecules through hydrolysis reactions. These reactions ultimately release the subunit molecules of each food category.

C17H35COO C H

molecules from food. Some of the ingested food molecules are needed for their energy (caloric) value—obtained by the reactions of cell respiration and used in the production of ATP—and the balance is used to make additional tissue.

Most of the organic molecules that are ingested are similar to the molecules that form human tissues. These are generally large molecules (polymers), which are composed of subunits (monomers). Within the gastrointestinal tract, the digestion of these large molecules into their monomers occurs by means of hydrolysis reactions (reviewed in fig. 18.1). The monomers thus formed are transported across the wall of the small intestine into the blood and lymph in the process of absorption. Digestion and absorption are the primary functions of the digestive system.

Since the composition of food is similar to the composition of body tissues, enzymes that digest food are also capable of digesting a person's own tissues. This does not normally occur, however, because a variety of protective devices inactivate digestive enzymes in the body and keep them away from the cytoplasm of the cells. The fully active digestive enzymes are normally confined to the lumen (cavity) of the gastrointestinal tract.

The lumen of the gastrointestinal tract is open at both ends (mouth and anus), and is thus continuous with the environment. In this sense, the harsh conditions required for digestion occur outside the body. Indigestible materials, such as cellulose from plant walls, pass from one end to the other without crossing the epithelial lining of the digestive tract; since they are not absorbed, they do not enter the body.

In Planaria (a type of flatworm), the gastrointestinal tract has only one opening—the mouth is also the anus. Each cell that lines the gastrointestinal tract is thus exposed to food, absorbable digestion products, and waste products. The two open ends of the digestive tract of higher organisms, by contrast, permit one-way transport, which is ensured by wavelike muscle contractions and by the action of sphincter muscles. This oneway transport allows different regions of the gastrointestinal tract to be specialized for different functions, as a "dis-assembly line." These functions of the digestive system include:

1. Motility. This refers to the movement of food through the digestive tract through the processes of a. Ingestion: Taking food into the mouth.

b. Mastication: Chewing the food and mixing it with saliva.

c. Deglutition: Swallowing food.

d. Peristalsis: Rhythmic, wavelike contractions that move food through the gastrointestinal tract.

2. Secretion. This includes both exocrine and endocrine secretions.

a. Exocrine secretions: Water, hydrochloric acid, bicarbonate, and many digestive enzymes are secreted into the lumen of the gastrointestinal tract. The stomach alone, for example, secretes 2 to 3 liters of gastric juice a day.

b. Endocrine secretions: The stomach and small intestine secrete a number of hormones that help to regulate the digestive system.

3. Digestion. This refers to the breakdown of food molecules into their smaller subunits, which can be absorbed.

4. Absorption. This refers to the passage of digested end products into the blood or lymph.

5. Storage and elimination. This refers to the temporary storage and subsequent elimination of indigestible food molecules.

Anatomically and functionally, the digestive system can be divided into the tubular gastrointestinal (GI) tract, or alimentary canal, and accessory digestive organs. The GI tract is approximately 9 m (30 ft) long and extends from the mouth to the anus. It traverses the thoracic cavity and enters the abdominal cavity at the level of the diaphragm. The anus is located at the inferior portion of the pelvic cavity. The organs of the GI tract include the oral cavity, pharynx, esophagus, stomach, small intestine, and large intestine (fig. 18.2). The accessory digestive organs include the teeth, tongue, salivary glands, liver,

Oral cavity. Teeth.

Tongue,

Oral cavity. Teeth.

Tongue,

Diaphragm Liver. Gallbladder. Duodenum

Ascending colon

Cecum, Appendix.

Cecum Appendix

Sigmoid colon .Rectum Anal canal

Anus,

Diaphragm Liver. Gallbladder. Duodenum

Ascending colon

Cecum, Appendix.

Sigmoid colon .Rectum Anal canal

Anus,

■ Figure 18.2 The organs of the digestive system. The digestive system includes the gastrointestinal tract and the accessory digestive organs.

© The McGraw-Hill Companies, 2003

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