UNIT 5: Structure and Function of Organisms, Part 2
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.
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
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.
Hydrochloric acid breaks down food and also kills bacteria and viruses that
are ingested with the food.
The chyme moves into the small intestine, and different digestive enzymes from
the liver and pancreas continue breaking the molecules down.
6–8 hours after the food enters the mouth, it has been physically and chemically
broken down into simple molecules that are easily absorbed into the blood
through the small intestine’s villi. The circulatory system then transports the
digested food to the cells of the body.
Villi increase the surface area in the intestines for nutrient absorption.
Figure 5.1. The Digestive System
Lesson 1: Physical and Chemical Changes in Digestion
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