OLE 7.6B Physical and Chemical Changes in the Digestive System
The process by which your body breaks down food into small
nutrient molecules is called digestion. The organs of the digestive
system have three main functions: digestion, absorption, and
elimination. These functions occur one after the other in an
efficient, continuous process.
Digestion of food involves both physical and chemical changes.
Physical changes involve the physical tearing, grinding, crushing,
and churning of pieces of food. These processes increase surface
area, which helps speed up the chemical changes of digestion.
During chemical changes, chemicals break foods into their building
blocks. Churning, which is similar to stirring, also speeds up these
chemical reactions. The figure at the right shows the key parts of
the digestive system.
1. Identify the two types of changes that break down food into
molecules that can be used by the body.
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What physical and chemical changes occur in the mouth and stomach?
Your mouth and stomach are the main organs of digestion in your upper digestive tract. Both of these organs
produce physical and chemical changes in food.
Changes in the Mouth Food enters your body through your mouth, where physical and chemical changes
begin. Physical changes are brought about by the action of your teeth. Your teeth cut, tear, crush, and grind the
food into small pieces. As your teeth work, saliva is released into your mouth from salivary glands. Saliva has
three digestive functions. Two produce physical changes, and one produces a chemical change. One physical
change is the moistening of food, which eases its passage through the esophagus to the stomach. Another
physical change is the dissolving of some foods, such as salt. Saliva contains an enzyme that can break down
starches into sugars. This begins the chemical digestion of your food. An enzyme is a protein that speeds up
chemical reactions in your body.
Your body produces many different enzymes, some of which are part of the digestive system. Each enzyme
speeds up only one kind of chemical change, or chemical reaction. Different enzymes help in the digestion of
different groups of nutrients, such as carbohydrates (starches and sugars), proteins (nitrogen-containing
nutrients), and lipids (fats and oils).
When you swallow, the food enters the esophagus, a muscular tube that connects the mouth to the stomach.
Waves of involuntary muscle contractions, called peristalsis (pehr ih STAWL sis), push food toward the
stomach.
Changes in the Stomach When food leaves the esophagus, it enters the stomach. Most physical changes and
some chemical changes occur here.
Physical Changes Layers of smooth muscle in the stomach wall contract and produce
a churning motion. This action mixes the food with fluids in the stomach. The fluids contain mucus. Mucus
keeps the food moist and protects the lining of the stomach from being digested or injured by its own secretions.
These secretions include two substances that contribute to chemical changes of the food in the stomach.
Chemical Changes The chemical breakdown of food continues as the churning food mixes with chemicals
called gastric fluids. The term “gastric” refers to the stomach. Gastric fluids include the enzyme pepsin. Pepsin
chemically breaks down proteins into short chains of amino acids. Gastric fluids also include hydrochloric acid.
This is a strong acid that helps your stomach function in two ways. First, pepsin works best in an acid
environment. Second, the acid kills many bacteria that you swallow with your food.
Food usually stays in your stomach for a few hours. Then the food, now a thick liquid, enters the next part of
the digestive system.
2. Both physical and chemical changes occur in the digestive system. How is a physical change distinguished
from a chemical change? _____________________________________________________________________
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