The Process of Digestion

What occurs during digestion?

The human digestive system, like those of other chordates, is built around an alimentary canal—a one-way tube that passes through the body. During digestion, food travels through the mouth, esophagus, stomach, and small intestine. Mechanical digestion and chemical digestion are the two processes by which food is reduced to molecules that can be absorbed. Both mechanical digestion and chemical digestion start in the mouth.

The Mouth As you take a forkful of food into your mouth, the work of the digestive system begins. Teeth and saliva start to work on your food first. Chewing begins the process of mechanical digestion. Chemical digestion begins as digestive enzymes in saliva start the breakdown of complex carbohydrates into smaller molecules.

FIGURE 30–11 The Mouth Digestion begins in the mouth, where the tongue, teeth, and saliva form food into a moist lump that can be swallowed. Infer How do human teeth reflect an omnivorous diet?

▸ Teeth The teeth, shown in Figure 30–11, are anchored in the bones of the jaw. The surfaces of the teeth are protected by a coating of mineralized enamel. The teeth do much of the mechanical work of digestion. The incisors, cuspids, and bicuspids cut into and tear at food. The molars grind and crush food into a fine paste that can be swallowed. Meanwhile, your tongue moves food around so that it comes in contact with your teeth.

▸ Saliva As the teeth cut and grind the food, the salivary glands secrete saliva, which helps to moisten the food and make it easier to chew. The release of saliva is under the control of the nervous system and can be triggered by the scent of food—especially when you are hungry!

Saliva not only eases the passage of food through the digestive system but also begins the process of chemical digestion. Saliva contains an enzyme called amylase that begins to break the chemical bonds in starches, forming sugars. If you chew on a starchy food like a cracker long enough, it will begin to taste sweet—the result of amylase's work in breaking down starches into sugars. Saliva also contains lysozyme, an enzyme that fights infection by digesting the cell walls of many bacteria that may enter the mouth with food.

Once food is chewed, the combined actions of the tongue and throat muscles push the clump of food, called a bolus, down the throat. When you swallow, the bolus first enters the area at the back of the throat called the pharynx. As this occurs, a flap of connective tissue called the epiglottis closes over the opening to the trachea. This action prevents food from moving into the air passageways to the lungs as it passes through the pharynx and into the esophagus.

In Your Notebook Explain in your own words two protective functions of the mouth and throat.

Table of Contents