Name
Class
CHAPTER 2
Date
Cycles in Nature
1 The Cycles of Matter
SECTION
National Science
Education Standards
BEFORE YOU READ
After you read this section, you should be able to answer
these questions:
LS 1c, 4b, 4c, 5a
• Why does matter need to be recycled?
• How are water, carbon, and nitrogen recycled?
Why Is Matter Recycled on Earth?
The matter in your body has been on Earth since the
planet formed billions of years ago. Matter on Earth is
limited, so it must be used over and over again. Each
kind of matter has its own cycles. In these cycles, matter
moves between the environment and living things.
STUDY TIP
Mnemonic As you read,
create a mnemonic device,
or memory trick, to help you
remember the parts of the
water cycle.
What Is the Water Cycle?
Without water there would be no life on Earth. All living
things are made mostly of water. Water carries other nutrients
to cells and carries wastes away from them. It also helps
living things regulate their temperatures. Like all matter,
water is limited on Earth. The water cycle lets living things
use water over and over.
In the environment, water moves between the oceans,
atmosphere, land, and living things. Eventually, all the
water taken in by organisms returns to the environment.
The movement of water is known as the water cycle. The
parts of the water cycle are explained in the figure below.
Precipitation is rain, snow, sleet, or hail
that falls from clouds to Earth's surface.
Most precipitation falls into the ocean.
It never touches the land.
Identify Describe to the
class all the things you and
your family do in a day that
use water. Can you think of
any ways you might be able
to use less water?
Condensation happens when
water vapor cools and changes
into drops of liquid water. The
water drops form clouds in the
atmosphere.
Evaporation happens
when liquid water on
Earth's surface changes
into water vapor. Energy
from the sun makes
water evaporate.
Groundwater is water
that flows under the
ground. Gravity can
make water that falls
on the land move into
rocks underground.
Say It
Runoff is water that
flows over the land into
streams and rivers. Most
of the water ends up
in the oceans.
TAKE A LOOK
1. Describe How do clouds
form?
Transpiration
happens when
plants give off
water vapor
from tiny holes in
their leaves.
Copyright © by Holt, Rinehart and Winston. All rights reserved.
Interactive Textbook
19
Cycles in Nature
Name
SECTION 1
Class
Date
The Cycles of Matter continued
What Is the Carbon Cycle?
STANDARDS CHECK
LS 4c For ecosystems, the major
source of energy is sunlight.
Energy entering ecosystems
as sunlight is transferred by
producers into chemical energy
through photosynthesis. That
energy passes from organism to
organism in food webs.
Besides water, the most common molecules in living
things are organic molecules. These are molecules that
contain carbon, such as sugar. Carbon moves between
the environment and living things in the carbon cycle.
2. Analyze Explain the
role of photosynthesis in
the carbon cycle.
Plants are producers. This means they make their
own food. They use water, carbon dioxide, and sunlight
to make sugar. This process is called photosynthesis.
Photosynthesis is the basis of the carbon cycle.
Animals are consumers. This means they have to
consume other organisms to get energy. Most animals get
the carbon and energy they need by eating plants. How
does this carbon return to the environment? It returns
when cells break down sugar molecules to release
energy. This process is called respiration.
PHOTOSYNTHESIS AND RESPIRATION
DECOMPOSITION AND COMBUSTION
Fungi and some bacteria get their energy by breaking
down wastes and dead organisms. This process is called
decomposition. When organisms decompose organic matter,
they return carbon dioxide and water to the environment.
When organic molecules, such as those in wood or
fossil fuels, are burned, it is called combustion.
Combustion releases the carbon stored in these organic
molecules back into the atmosphere.
The Carbon Cycle
Carbon dioxide
in the air
TAKE A LOOK
Respiration
3. Complete Carbon dioxide
in the air is used for
.
4. List What three processes
release carbon dioxide into
the environment?
Photosynthesis
Combustion
Carbon in
living things
Decomposition
Carbon in
soil and rock
Carbon in
fossil fuels
Copyright © by Holt, Rinehart and Winston. All rights reserved.
Interactive Textbook
20
Cycles in Nature
Name
Class
SECTION 1
Date
The Cycles of Matter continued
What Is the Nitrogen Cycle?
Nitrogen is also important to living things. Organisms
need nitrogen to build proteins and DNA for new cells. Like
water and carbon, nitrogen cycles through living things and
the environment. This is called the nitrogen cycle.
NITROGEN FIXATION
About 78% of Earth’s atmosphere is nitrogen gas. Most
organisms cannot use nitrogen gas directly. Bacteria in
soil can change nitrogen gas into forms that plants can
use. This is called nitrogen fixation. Other organisms
can get the nitrogen they need by eating plants or by eating organisms that eat plants.
Critical Thinking
5. Apply Concepts How is
nitrogen fixation important to
animals?
The Nitrogen Cycle
Nitrogen in the air
Lightning causes some
nitrogen fixation.
Plant roots take up
nitrogen from soil.
Bacteria in soil
convert nitrogen
back to gas.
Animals get nitrogen
from plants.
Dead animals and plants
Decomposition
releases nitrogen
into soil.
Bacteria in soil and
plant roots perform
most nitrogen fixation.
TAKE A LOOK
6. Identify What process
releases nitrogen into the soil?
How Are the Cycles of Matter Connected?
Other forms of matter on Earth also cycle through the
environment. These include many minerals that living
cells need, such as calcium and phosphorus. When an
organism dies, every substance in its body will be recycled
in the environment or reused by other organisms.
All of the cycles of matter are connected. For example,
water carries some forms of carbon and nitrogen through the
environment. Many nutrients pass from soil to plants to animals and back. Living things play a part in each of the cycles.
READING CHECK
7. Explain What happens
to the substances in an
organism’s body when the
organism dies?
Copyright © by Holt, Rinehart and Winston. All rights reserved.
Interactive Textbook
21
Cycles in Nature
Name
Class
Date
Section 1 Review
NSES
LS 1c, 4b, 4c, 5a
SECTION VOCABULARY
combustion the burning of a substance
condensation the change of state from a gas to
a liquid
decomposition the breakdown of substances
into simpler molecular substances
evaporation the change of state from a liquid to
a gas
precipitation any form of water that falls to
Earth’s surface from the clouds
1. Identify In the water cycle, what makes water evaporate?
2. Summarize Draw arrows to show how carbon cycles through the environment
and living things.
Plants
Air
Animals
Decomposers
3. Explain Why does matter need to be recycled?
4. Explain Why is water so important to life on Earth?
5. Define What is nitrogen fixation?
6. Define What are organic molecules?
Copyright © by Holt, Rinehart and Winston. All rights reserved.
Interactive Textbook
22
Cycles in Nature
E
Environmental Science Answer Key continued
SECTION 2 ECOLOGICAL SUCCESSION
Review
1. Possible answers: food, water, sunlight,
2.
3.
4.
5.
1. 34%
2. the first species to live or grow in an area
3. Lichens don’t have roots. They get their
shade, shelter
Individuals will die or will have to move to a
new area. This will cause the population to
decrease.
No, some species may use more resources
than others. Some species may be more
affected than others by a limiting factor.
First row, left to right: parasitism, parasite (flea)
Second row, left to right: both, none
Third row, left to right: commensalism
Commensalism; the pollinator is not
harmed, but the plant still benefits.
4.
5.
6.
7.
Review
1. Abiotic factors are the nonliving parts of the
2.
Chapter 2 Cycles in Nature
SECTION 1 THE CYCLES OF MATTER
3.
1. Water vapor cools and changes into drops of
2.
3.
4.
5.
6.
7.
liquid water. The water drops form clouds.
Photosynthesis uses carbon dioxide from
the air. When the sugars that a plant makes
during photosynthesis are broken down, carbon returns to the environment.
photosynthesis
respiration, combustion, decomposition
Animals need to get nitrogen from plants or
other animals. Plants get their nitrogen from
nitrogen fixation.
decomposition
They are recycled in the environment or
reused by other organisms.
4.
5.
6.
7.
Review
1. energy from the sun
2. There should be arrows from air to plants
3.
4.
5.
6.
water from the air, so they do not need soil.
tall trees
in places where living things already exist
weeds
conifers
environment. They include water, light, and
space.
Primary succession is the change from bare
rock to a community of organisms. Secondary
succession is a change in a community where
other living things already exist.
In secondary succession, there is already
soil for new plants to use. In primary
succession, soil has to develop before
species other than lichens can grow.
There is little grass in a mature forest,
because the tall trees prevent the light from
reaching the ground. Nuts grow on many
kinds of trees. Therefore, there would be
more nut eaters than grass eaters.
Tall trees need deep soil. Pioneer species are
the first species to live or grow in an area.
There usually would not be soil in an area
where no living things had been before.
the variety of species that live in an area
pieces of rock that have been broken down
and remains of dead lichens
Chapter 3 The Earth’s
Ecosystems
to animals to decomposers to air, and from
plants to decomposers.
Matter on Earth is limited, so it needs to be
used over and over again.
Living things are made mostly of water.
Water carries nutrients to cells and carries
wastes away. Water also helps organisms
regulate their body temperatures.
Nitrogen fixation is the process in which
bacteria in soil change nitrogen gas into a
form that plants can use.
molecules that contain carbon
SECTION 1 LAND BIOMES
1. Biomes are made of many related
2.
3.
4.
5.
6.
ecosystems.
Africa, South America
plenty of rain, moderate temperatures
deciduous trees and shrubs
in cones
The evergreen conifers shade the forest
floor, but the deciduous trees of the temperate forest allow light to reach the ground.
Copyright © by Holt, Rinehart and Winston. All rights reserved.
Interactive Textbook Answer Key
30
Environmental Science