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Name _________________________ Class ______ Date _________
TEKS
Grade 7
Lesson 7.6C
TEKS 7.6C Recognize how large molecules are broken down into
smaller molecules such as carbohydrates can be broken down into
sugars.
TEKS Lesson 7.6C: Digesting Your
Food
All living things need food to stay alive. Food provides your body with materials to grow
and to repair tissues. It also provides energy for everything you do. Exercising, reading,
and sleeping require energy. Even maintaining homeostasis takes energy.
Your body breaks down the food you eat into nutrients it can use. Nutrients are the
substances in food that provide the raw materials and energy the body needs to carry out
all its essential processes. People need six types of nutrients: carbohydrates, fats, proteins,
vitamins, minerals, and water. Of these nutrients, carbohydrates, proteins, and fats must
be broken down into small molecules before the body can use them. The digestive system
does not break down water, minerals, and vitamins. Your digestive system breaks down
the food you eat into nutrients it can use.
The Role of Enzymes Enzymes are biological catalysts. A catalyst speeds up, or even
simply makes possible, certain chemical reactions. A biological catalyst—that is, an
enzyme—speeds up or makes possible chemical reactions in the body. Some of these
reactions break down large nutrient molecules into smaller ones. The diagram below
shows how enzymes work. The mouth, stomach, pancreas, and small intestine all make
enzymes that help digest food.
1 An enzyme molecule
becomes available.
2 A starch molecule
fits the chemical shape
of the enzyme.
3 The starch molecule
binds to the enzyme.
4 The starch molecule
is broken down into two
sugar molecules.
5 The enzyme and the
molecules separate.
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Name _________________________ Class ______ Date _________
TEKS
Grade 7
Lesson 7.6C
How are carbohydrates broken down?
Carbohydrates are compounds that contain atoms of carbon, hydrogen,
and oxygen. These atoms are bonded to one another to form molecules of
starches and sugars.
Types of Carbohydrates The simple sugars that provide your body with energy are
called monosaccharides. Mono means “one.” Saccharide means “sugar.” So, the term
monosaccharide means “a carbohydrate made up of a single simple molecule of sugar.”
The carbohydrates you eat are mostly made up of complex sugars. Disaccharides, like
table sugar, consist of two molecules of a simple sugar linked together. Polysaccharides,
such as potatoes, consist of many linked sugar molecules. Your body cannot use these
complex sugars directly to produce energy. Instead, these molecules must be broken
down by the digestive system into monosaccharides. The most common simple sugar is
glucose.
Enzymes Break Down Carbohydrates The breakdown of large carbohydrate
molecules begins in the mouth. There, an enzyme in saliva breaks down starch into
smaller molecules. However, these molecules are not simple sugars that your body uses
for energy. Instead, they are disaccharides and even larger molecules.
In the stomach, no changes occur to carbohydrates. But a new set of enzymes acts on
them in the small intestine. Some of these enzymes are produced by the pancreas. Others
are produced by cells in the walls of the small intestine. Together, these enzymes work to
break down the still-complex carbohydrate molecules into disaccharides and eventually
into simple sugars.
1. Recognize Which organ is not involved in the breakdown of complex
carbohydrates into simple sugars?
A mouth
B small intestine
C pancreas
D stomach
How are proteins broken down?
Proteins are very large molecules that contain carbon, hydrogen, nitrogen, and oxygen
atoms. The molecules of the proteins you eat are large and complex. However, the
molecules in proteins that you can use are small and simple. Foods rich in proteins
include meat, fish, dairy products, eggs, beans, and peas.
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Name _________________________ Class ______ Date _________
TEKS
Grade 7
Lesson 7.6C
Amino Acids The simplest unit in a protein is an amino acid.
Proteins are made up of long chains of 20 different amino acids that are
bonded to one another. Two or more linked amino acids form a unit
called a peptide. Proteins are large polypeptides. That is, they are made
up of a great many peptides and, therefore, a great many amino acids.
The links between amino acids in a peptide are called peptide bonds. It is these bonds that
must be broken to separate individual amino acids from proteins. Your body uses these
individual amino acids to build and repair body parts and for other life functions.
Enzymes Break Down Proteins and Peptides The breakdown of large
polypeptides begins in the stomach, where enzymes break down molecules of large
polypeptides into smaller polypeptides. From there, the molecules move into the small
intestine, where a new set of enzymes acts on them. Some of these enzymes are produced
by the pancreas. Other enzymes are produced by cells in the walls of the small intestine.
Together, these enzymes break the peptide bonds and transform these peptides into
individual amino acids.
2. Recognize In which two organs are large protein molecules broken
down into smaller molecules, resulting in amino acids the body can use?
_____________________________________________________________________
_____________________________________________________________________
How are lipids broken down?
Lipids are very large molecules that, in addition to carbon and hydrogen atoms, contain
oxygen atoms. These atoms are bonded to one another to form molecules of fats and
oils. Fats are solid lipids. Oils are liquid lipids. Your body stores energy in fats. Your
body also uses fats to insulate it against extremes in outside temperature and to build
membranes, such as those that encase and protect nerves. Foods rich in lipids include
nuts, butter, fatty meats, oily fish, bacon, and cheese.
Enzymes Break Down Lipids Many lipids are made of smaller molecules called
fatty acids and glycerol. The bonds between these molecules must be broken so that
your body can use lipids for life functions. The breakdown of lipids occurs in the small
intestine. There, enzymes made in the pancreas and small intestine break down large lipid
molecules into individual molecules of fatty acids and glycerol. The process is aided
by the addition of bile. Bile, which is produced by the liver, enters the small intestine
through a tube. Bile breaks up large globs of lipids into tiny droplets. The larger surface
area of the droplets allows the enzymes to break down the lipids more easily.
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Name _________________________ Class ______ Date _________
TEKS
Grade 7
Lesson 7.6C
3. Identify What two simple molecules make up many of
the complex lipids that you eat?
__________________________________________________________________
__________________________________________________________________
Lesson Check
1. Identify What is the general name for the substances that break down large food
molecules into smaller nutrient molecules?
__________________________________________________________________
2. Recognize In digestion, large complex molecules are broken down into smaller
simpler molecules for use by the body. Below, indicate the resulting smaller
molecules that are ultimately broken down from each large molecule.
Carbohydrates: a. ____________________________________________________
Proteins: b. ________________________________________________________
Lipids:
c. _________________________________________________________
d. ________________________________________________________
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Name _________________________ Class ______ Date _________
TEKS
Grade 7
Lesson 7.6C
3. Sequence Complete the sequence diagram to show the changes
that occur as a large complex carbohydrate molecule is broken
down
into a smaller simple carbohydrate molecule.
Large
polysaccharide
molecule
enzyme
s
b.
a.
enzyme
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Name _________________________ Class ______ Date _________
TEKS
Grade 7
Lesson 7.6C
4. Recognize What must happen to break down a protein into amino
acids?
__________________________________________________________________
__________________________________________________________________
5. Identify Proteins, carbohydrates, and lipids are broken down in different organs of
the digestive system. For each nutrient, put a check mark to indicate the organs in
which their digestion is begun, continued, or completed.
Nutrient
Mouth
Carbohydrates
√
Stomach
Proteins
LIpids
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Small Intestine