chapter
33
Cell Processes
3
section ●
Energy for Life
Before You Read
What You’ll Learn
the differences
between producers and
consumers
■ that photosynthesis
and respiration store
and release energy
■ how cells get energy
■
Describe on the lines below why you think your body needs
food.
Read to Learn
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Trapping and Using Energy
Chemical energy is stored in food molecules. This
chemical energy is changed inside cells into other forms of
energy needed for life. In every cell, these changes involve
chemical reactions. In fact, all of an organism’s activities
involve chemical reactions. All the chemical reactions in an
organism make up metabolism.
The chemical reactions of metabolism need enzymes.
Enzymes cause changes, but the enzymes are not changed
during the reaction and can be used again. In the figure
below, an enzyme attaches to a large molecule and helps it
to change. At the end of the chemical reaction, the molecule
has changed into two smaller molecules, but the enzyme has
not changed.
Enzyme
Enzyme
Large
molecule
Locate Information Read
all the headings for this section
and circle any word you cannot
define. At the end of each
section, review the circled words
and underline the part of the
text that helps you define the
words.
Picture This
1.
Explain What happens
to the enzyme during the
chemical reaction?
Small
molecules
Reading Essentials
45
What happens during photosynthesis?
tab book, as shown below. Write
the names of the following
processes on the tabs:
photosynthesis, respiration, and
fermentation. Inside each tab,
describe the process.
Living things are divided into two groups—producers and
consumers—based on how they obtain their food.
Organisms that make their own food, such as plants, are
producers. Organisms that cannot make their own food
are consumers.
Plants and many other producers can convert light energy
into chemical energy. Producers use a process called
photosynthesis to change light energy from the Sun into
sugars, which can be used for food. Plants and other producers
that use photosynthesis are usually green because they
contain a green pigment called chlorophyll (KLOR uh fihl).
In plant cells, these pigments are found in chloroplasts.
Chlorophyll is used in photosynthesis to capture light
energy. Plants use chlorophyll to make sugar and oxygen
(O2) from the raw materials carbon dioxide (CO2), water
(H2O), and light energy. Plants get their raw materials from
the air, soil, and Sun. Some of the light energy is stored in
the chemical bonds that hold the sugar molecules together.
Enzymes also are needed for the reactions to occur. The
process of photosynthesis is shown in the figure below.
Review the chemical equation for photosynthesis to identify
the raw materials and the results of the chemical process.
O 2 and sugar
Light energy
H2O
and
CO2
Picture This
2.
Identify Circle the three
O2
things needed for photosynthesis to take place.
CO 2
Stored
food
Photosynthesis
0
C 6 H12 O 6 1 6O2
6CO 2 1 6H2 O 1 light energy
carbon water
chlorophyll sugar oxygen
dioxide
46
Cell Processes
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
D Describe Make a three●
How do plants store and use carbohydrates?
Plants make more sugar during photosynthesis than they
need for survival. The extra sugar is changed and then
stored as starches and other carbohydrates. Plants, such as
apple trees, use these carbohydrates for growth, for keeping
up cells, and for reproduction.
Why is photosynthesis important
to consumers?
3.
Explain How do plants
use carbohydrates?
Consumers get energy by eating producers and other
consumers. No matter what food you eat, photosynthesis
was involved directly or indirectly in its production. For
example, an apple tree uses photosynthesis to make apples.
When you eat an apple, the stored sugars help feed your
body. Some cheese comes from milk, which is produced by
cows that eat plants. The plants the cows eat are producers.
The cows and humans are consumers.
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
How do you use energy?
Imagine that you get up late for school. You dress quickly
and run three blocks to school. When you get to school, you
feel hot and are breathing fast. Why? Your muscle cells use a
lot of energy when you run. To get this energy, muscle cells
break down food. Some of the energy in the food is used
when you run and some of it becomes thermal energy,
which is why you feel warm or hot. Most cells need oxygen
to break down food. You are breathing fast because your
body was working to get oxygen to your muscles.
What is respiration?
When you ran, your muscle cells were using the oxygen
for the process of respiration. During respiration, chemical
reactions break down food molecules into simpler substances
and release stored energy. Just as in photosynthesis, enzymes
are needed for the chemical reactions of respiration.
Respiration occurs in the cells of all living things. As you
are reading this page, millions of cells in your body are
breaking down food molecules and releasing energy. Two
waste products, carbon dioxide and water, are produced
during respiration. Your body gets rid of the carbon dioxide
and some of the water when you breathe out, or exhale.
4.
Draw Conclusions
Why does respiration occur
only in living things?
Reading Essentials
47
What is fermentation?
Even though you breathe harder when you run, your
muscle cells might not receive enough oxygen for
respiration. When this happens, a process in the muscle cells
known as fermentation releases some of the energy stored
in glucose (sugar) molecules.
Fermentation also releases energy and produces wastes.
The type of wastes produced depends on the type of cell.
They may be lactic acid, alcohol, and carbon dioxide.
Fermentation in your muscle cells changes simple molecules
into lactic acid while releasing energy, as shown in the figure
below. The presence of lactic acid is why your muscles
might feel stiff or sore after you have run to school.
Picture This
5.
Identify What are three
waste products created
during fermentation?
Yeast cells
Fermentation
Carbon dioxide
and alcohol
Muscle cells
Lactic acid
What products come from fermentation?
6.
Identify What waste
products come from the
fermentation of yeast cells?
48
Cell Processes
Some organisms, such as bacteria, carry out fermentation
and make lactic acid. Some of these organisms are used to
make yogurt and some cheeses. These organisms break
down a sugar in milk to release energy. The lactic acid
produced causes the milk to become more solid. Some of
the flavor in yogurt and cheese comes from this process.
Have you ever used yeast to make bread? Yeasts are
one-celled living organisms. Fermentation in yeast cells
breaks down the sugar in bread dough. The cells produce
alcohol and carbon dioxide as wastes. The carbon dioxide
waste is a gas that makes the bread dough rise. The alcohol
is lost as the bread bakes.
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Fermentation
How do photosynthesis and respiration
work together?
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Some producers make food through photosynthesis. All
living things release energy stored in food through
respiration or fermentation. If you think carefully about
photosynthesis and respiration, you will note that what is
produced by one process is used by the other process.
Photosynthesis and respiration are almost the opposite of
each other. Photosynthesis produces sugars and oxygen, and
respiration uses these products. The carbon dioxide and
water produced during respiration are used during
photosynthesis.
As you fill in the products in the figure below, review how
the products of one process are the wastes of the other
process. Photosynthesis and respiration cannot take place
without each other. And most life would not be possible
without these important chemical reactions.
Photosynthesis
(producers)
Picture This
7.
Illustrate In the figure
below, fill in the products
released by photosynthesis
and respiration.
Respiration
(all living things)
Reading Essentials
49
After You Read
Mini Glossary
photosynthesis: process that uses light energy, carbon
dioxide, and water to produce the sugars and oxygen
needed by all living things
respiration: chemical reaction that uses oxygen and breaks
down food molecules into simpler substances to release
their stored energy
fermentation: chemical reaction that releases energy
stored in glucose (sugar) molecules and produces carbon
dioxide, lactic acid, and alcohol as wastes
metabolism: all chemical reactions that take place in an
organism
2. Fill in the table below to identify what is needed by each chemical reaction and what is
produced by each chemical reaction.
Photosynthesis
What is needed?
Respiration
1.
1.
2.
2.
Fermentation
1. glucose molecules
3.
What is produced?
End of
Section
50
Cell Processes
1.
1.
1.
2.
2.
2.
3.
3.
Visit booka.msscience.com to access your textbook,
interactive games, and projects to help you learn more about
energy for life.
Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
1. Review the terms and their definitions in the Mini Glossary. Write a short paragraph that
describes how photosynthesis and respiration are related.