Lesson 1: Physical and Chemical Changes in Digestion
Physical and Chemical Changes
Physical changes occur when a substance has a change in one of its physical
properties, such as density, size, smell, or luster. Physical changes do not alter the
chemical makeup of the substance. Examples of a physical change include tearing
paper, adding food color to water, or chewing food to break it into smaller pieces.
If a substance has a change in its chemical properties, a chemical change occurs.
New substances
form during a chemical change. Examples of a chemical change
include paper burning, organic material decomposing, and vinegar reacting with
baking soda. Photosynthesis is an example of a chemical change that occurs in
plants. Physical and chemical changes occur during digestion in humans.
The Digestive System
Our cells need energy and nutrients from food to survive and grow. Food that we eat
must be broken down into molecules that can be used by our cells. Large molecules
of carbohydrates, proteins, and fat are broken down into even smaller molecules
during digestion.
Digestion begins in the mouth. Chewing
food physically breaks it into smaller
pieces. The act of chewing also releases
saliva from glands in the mouth. Saliva
contains an enzyme that begins breaking
down food molecules. Complex
carbohydrates like starches are broken
down into smaller simple sugars. The
cracker tastes sweet because the complex
starch is being broken down into glucose,
a simple sugar.
The tongue moves the food particles
around to shape it into a ball, called a
bolus. The bolus is swallowed, moves
down the esophagus by a squeezing
action, and enters the stomach. The
muscles of the stomach physically mix
the bolus and gastric juices
, which include
hydrochloric acid
and enzymes. The acid
and enzymes continue to chemically break
down the food into even smaller pieces to
form a smooth paste called chyme.
Figure 5.1. The Digestive System
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