1. Nutrients are chemicals the body needs to function properly.
a. We get the nutrients we need from the food we eat.
b. The food is not in a form that can be used.
c. The large molecules must be broken down into small molecules that cells can use.
d. This process is called digestion.
2. Digestion starts even before you put food in your mouth.
a. When you are hungry or smell food, you start to produce saliva.
3. Digestion involves both physical and chemical changes.
a. A physical change is a change in the properties of a substance.
b. A chemical change results in new substances.
c. All food contains vitamins, minerals, proteins, lipids, and/or carbohydrates.
i. Vitamins and minerals can be absorbed directly into the body from the
ii. Proteins, lipids, and carbohydrates must be digested before being absorbed.
Stages of Digestion
There are two stages in digestion:
1. Mechanical digestion is the physical breakdown of food.
a. It breaks food into smaller pieces but does not change the substances in food.
b. Food must be broken into smaller pieces so it can be swallowed.
c. Most mechanical digestion is done by chewing.
2. Chemical digestion is the breakdown of large food molecules into smaller molecules.
a. Chemical digestion is performed mostly by special proteins called enzymes.
b. Enzymes perform and speed up chemical reactions in the body.
c. Each enzyme can break down only one type of food molecule.
i. For example,
(1) Pasta is mostly starch. Starch is made of glucose, a simple sugar.
(2) Glucose is the main source of energy for cells but they cannot
(3) Chemical digestion must break the starch into glucose molecules.
d. In general,
i. Carbohydrates are changed to simple sugars.
ii. Proteins are changed to amino acids.
iii. Fats are changed to fatty acids.
Organs of the Digestive System 1. The mouth is the entrance to the digestive system.
a. Teeth are used to grind food into smaller pieces. This is the beginning of
b. The tongue moves food around in the mouth and pushes it where it can be
c. Saliva is mixed with food to soften and moisten it. This makes it easier to
d. Saliva also contains an enzyme that starts to chemically digest starch.
2. The esophagus is the passageway from the mouth to the stomach.
a. The walls of the esophagus produce mucus which lubricates the food so it moves
b. Muscles lining the esophagus contract to move the food along. These contractions
are called peristalsis.
c. When you swallow, the epiglottis covers the windpipe so that food does not enter
3. The stomach is a large bag-like organ that begins the breakdown of protein in food.
a. The stomach has muscles that cause it to churn the contents, contributing to
b. This churning usually starts automatically at meal times. This is what is
happening when you hear your stomach “growling.”
c. The churning also mixes the food with gastric juice.
i. Gastric juice is a mixture of digestive juice made by the stomach. It
(1) mucus - protects the lining of the stomach from being digested
(2) pepsin - is an enzyme that digests proteins