Digestion is the chemical breakdown of food into smaller molecules. In an individual cell, digestion is accomplished by intracellular digestion when a lysosome containing digestive enzymes merges with a food vacuole. In most animals, however, the food ingested is too large to be engulfed by individual cells. Thus, food is first digested in a gastrovascular cavity by extracellular digestion and then absorbed by individual cells. During digestion, four different groups of molecules are commonly encountered. Each is broken down into its molecular components by specific enzymes, as follows:

1. Starches are broken down into glucose molecules.

2. Proteins are broken down into amino acids.

3. Fats (or lipids) are broken down into glycerol and fatty acids.

4. Nucleic acids are broken down into nucleotides.

In humans and other mammals, digestion follows the following sequence of events. In particular, note which kinds of molecules are digested (broken down) and by which enzymes. Since enzymes are specific for different bonds, only a representative few of the numerous enzymes are given.

1. Mouth. Salivary amylase, secreted into the mouth by the salivary glands, begins the breakdown of starch into maltose (a disaccharide). Chewing reduces the size of food particles, thereby increasing the surface area upon which amylase and subsequent enzymes can operate. Food is shaped into a ball, or bolus, and then swallowed.

2. Pharynx. When food is swallowed and passed into the throat, or pharynx, a flap of tissue, the epiglottis, blocks the trachea so that solid and liquid material enter only the esophagus.

3. Esophagus. Food moves through the esophagus, a tube leading to the stomach, by muscular contractions called peristalsis.

4. Stomach. The stomach secretes gastric juice, a mixture of digestive enzymes and hydrochloric acid (HCl), and serves a variety of functions, as follows:

• Storage. Because of its accordionlike folds, the wall of the stomach can expand to store two to four liters of material.

• Mixing. The stomach mixes the food with water and gastric juice to produce a creamy medium called chyme.

• Physical breakdown. Muscles churn the contents of the stomach, physically breaking food down into smaller particles. In addition, HCl from the gastric juice denatures (or unfolds) proteins and loosens the cementing substances between cells of the food. Also, the HCl kills most bacteria that may accompany the food.

• Chemical breakdown. Proteins are chemically broken down (digested) by the enzyme pepsin. Stomach cells producing pepsin are protected from self-digestion because they produce and secrete an inactive form, pepsinogen. Pepsinogen is activated into pepsin by HCl, which is produced by other stomach cells. Thus, only after pepsinogen is secreted into the stomach cavity can protein digestion begin. Once protein digestion begins, the stomach is protected by a layer of mucus secreted by still other cells in the stomach lining. Failure of the mucus to protect the stomach can lead to lesions, or peptic ulcers. Long believed to be caused by stress, diet, or other factors, most ulcers are now known to be caused by bacteria and can be successfully treated with antibiotics.

• Controlled release. Movement of chyme into the small intestine is regulated by a valve at the end of the stomach, the pyloric sphincter.

5. Small intestine. The first twenty-five cm of the small intestine, the duodenum, continues the digestion of starches and proteins (which began in the mouth and stomach, respectively) as well as all remaining food types (including fats and nucleotides). Enzymes for these various processes originate from the following sources:

• Small intestine. The wall of the small intestine is the source of various enzymes, including proteolytic enzymes (or proteases, enzymes that digest proteins, such as aminopeptidase), maltase and lactase (for the digestion of disaccharides), and phosphatases (for the digestion of nucleotides).

• Pancreas. The pancreas produces various enzymes, including trypsin and chymotrypsin (proteases), lipase (digestion of fats), and pancreatic amylase (digestion of starch). These and other enzymes, packaged in an alkaline solution that serves to neutralize the HCl in the chyme, enter the duodenum through the pancreatic duct.

• Liver. The liver produces bile, which functions to emulsify fats. Emulsification is the breaking up of fat globules into smaller fat droplets, increasing the surface area upon which fat-digesting enzymes (lipase, for example) can operate. Since bile does not chemically change anything, it is not an enzyme. Bile is also alkaline, serving to help neutralize the HCl in the chyme. The bile is stored adjacent to the liver in the gallbladder and flows through the bile duct where it merges with the pancreatic duct.

• The remainder of the small intestine (nearly six meters) absorbs the breakdown products of food. It is characterized by villi and microvilli, fingerlike projections of the intestinal wall that increase its total absorptive surface area. Amino acids and sugars are absorbed into blood capillaries, while most of the fatty acids and glycerol are absorbed into the lymphatic system.

6. Large intestine. The main function of the large intestine, or colon, is the reabsorption of water to form solid waste, or feces. Feces are stored at the end of the large intestine, in the rectum, and excreted through the anus. Various harmless bacteria live in the large intestine, including some that produce vitamin K, which is absorbed through the intestinal wall. At the beginning of the large intestine, there is a short branch to a dead-end pouch which bears a fingerlike projection called the appendix. Other than a possible role in the immune response, the appendix is significant only when it becomes inflamed, causing appendicitis. In herbivores, the dead-end pouch is much enlarged and is called the cecum. It harbors bacteria that help in the digestion of cellulose. Hormones are involved in the digestive process. Three important hormones are described below:

1. Gastrin is produced by cells in the stomach lining when food reaches the stomach or when the nervous system, through smell or sight, senses the availability of food. Gastrin enters the blood stream and stimulates other cells of the stomach to produce gastric juices.

2. Secretin is produced by the cells lining the duodenum when food enters. Secretin stimulates the pancreas to produce bicarbonate which, when deposited into the small intestine, neutralizes the acidity of the chyme.

3. Cholecystokinin is produced by the small intestine to stimulate the gallbladder to release bile and the pancreas to release its enzymes.