UNIT 3

Matter and Energy

in Living Systems
Lesson 1: Photosynthesis. . . . . 116

Lesson 2: Cellular Respiration . .128

Lesson 3: Modeling Matter and

Energy in Ecosystems . . . . . . . . 140

Lesson 4: Cycling of Matter and

Energy in Ecosystems . . . . . . . . 158

Thing Explainer: Tree . . . . . . . . 172

Unit Connections. . . . . . . . . . . . 176

Unit Review . . . . . . . . . . . . . . . . . 177

Unit Performance Task . . . . . . . 179

Image Credits: ©Dave Porter/Alamy Images

Kelp forests are important aquatic

ecosystems.

114 Unit 3 Matter and Energy in Living Systems

 FIGURE 1: These terrariums are self-sustaining environments.

Many terrariums are closed, self-sustaining systems. The organisms in these terrariums
are able to produce all of the materials needed for survival. Earth is also a closed
system. Very little matter is added to or lost from the Earth system.

Predict How do plants and animals grow if no new matter is added to the system?

DRIVING QUESTIONS

As you move through the unit, gather evidence to help you answer the following
questions. In your Evidence Notebook, record what you already know about these
topics and any questions you have about them.
1. What do plants need to survive? How do plants obtain energy?
2. How do animals obtain energy to grow?
3. How are energy and matter transferred through organisms and their
environment?
© Houghton Mifflin Harcourt • Image Credits: ©aon168/Shutterstock

UNIT PROJECT

Go online to download
Bottle Biome the Unit Project
Worksheet to help
plan your project.
How do energy and matter cycle through a closed system such as the Earth system?
How do the plants and animals survive? Make your own closed biological system
inside a bottle, and investigate how the plants and animals survive with no materials
being added to the system. Can you explain how the bottle represents Earth?

Unit 3 Matter and Energy in Living Systems 115

 3.1

Photosynthesis

Matter is recycled and

energy flows through CAN YOU EXPLAIN IT?
organisms and the
environment. The colonization of other planets was an idea once found only in science fiction stories.
Today, this idea is closer to becoming a realistic pursuit. One of the problems that must
be solved before the colonists leave Earth is this—where will the colonists get food?
One line of inquiry involves figuring out what it takes to grow plants in an environment
different from Earth.

Gather Evidence FIGURE 1: Astronauts from NASA and around the world have Explore Online
As you explore the lesson, been growing plants in space to learn how to someday grow them on other planets,
gather evidence to describe the such as Mars.
inputs and outputs of matter and
the transfer and transformation of
energy in photosynthesis.

Image Credits: (t) ©Pichugin Dmitry/Shutterstock; (b) ©NASA Kennedy Space Center

Predict Imagine you are colonizing another planet, and you want to grow plants
there as a food source. What do you need to bring, and what questions would you ask
about the planet in order to refine your list?

116 Unit 3 Matter and Energy in Living Systems

 EXPLORATION 1

Matter and Energy in Photosynthesis

Living systems take in energy and matter and convert them to forms they can use.
FIGURE 2: This panda is a
Plants, for example, are producers that capture light energy and convert it to chemical consumer that gets its
energy to carry out cell processes within the plant. The chemical energy takes the energy and nutrients from
form of chemical bonds in sugar molecules. When a consumer, such as a panda, eats eating leaves.
plant matter, it obtains this energy and other nutrients it needs for cell processes
and growth through the process of digestion. Any matter that cannot be digested is
excreted as waste.

Modeling Photosynthesis
Plants, algae, and some bacteria use a process called photosynthesis to capture and
transform light energy from the sun and store it in high-energy sugar molecules. Both
plant cells and animal cells use sugars made by photosynthesis as an energy source.
However, photosynthesis is not just important to organisms—it also helps regulate
Earth’s environment. Photosynthesis produces the oxygen we breathe, and it removes
carbon dioxide from Earth’s atmosphere.
Organisms are complex living systems. Organisms live and interact in ecosystems,
Explain Describe the
which are systems within the biosphere. All organisms play different roles in the
cycling of matter and the transfer of energy in their ecosystem. To better understand
transformation of energy
the relationship between organisms and the environment, scientists collect many as it is transferred from the sun to
different types of data. the panda.

FIGURE 3: This setup shows a plant in a closed system. Sensors are measuring carbon
dioxide and oxygen concentrations in the chamber. The gas concentrations are shown in
parts per thousand. Collaborate Discuss
with a partner why it
would be beneficial to human
survival to have plants on a planet
Gas Levels
where oxygen levels are low and
0.4

CO2 carbon dioxide levels are high.

(ppt) 0.2

0.0

212
O2
(ppt) 210

208

Time (min)
Image Credits: (t) ©DLILLC/Corbis

Gather Evidence Identify inputs and outputs for this system. How can the data
help scientists understand the relationship between plants and the environment?

Lesson 1 Photosynthesis 117

 Photosynthesis is important to life on Earth. Nearly all organisms on Earth depend
on this process. So understanding the relationship between organisms and
photosynthesis is critical. Using equipment to measure the rate of photosynthesis, for
example, is one way to study the impact that organisms have on the process. Using
models is another way to understand processes like photosynthesis. Scientists can
study the relationship between the inputs and outputs.

Energy and Matter

Model Draw a plant The process of photosynthesis can be modeled in various ways. For example, a
and label the inputs and chemical equation is one way to represent photosynthesis.
outputs of photosynthesis. enzymes
Where should the labels for 6CO2 + 6H2O →→→ 6O2 + C6H12O6
light
enzymes and light be placed?
This model shows the inputs and outputs as reactants and products. The multiple
arrows indicate that the process of photosynthesis has many steps. Light and
enzymes are placed over the arrows to indicate that they must be present for this
reaction to take place. In this equation, carbon dioxide and water are reactants,
and oxygen and glucose are products. Plant cells use glucose to form complex
carbohydrates such as starch and cellulose, which the plant uses for growth and
maintenance.

Light and Photosynthesis

Light is a form of energy known as electromagnetic radiation. Electromagnetic
radiation travels in waves of various wavelengths, as shown in Figure 4. Plants
absorb only visible light to use for photosynthesis. Even in the visible portion of the
electromagnetic spectrum, not all wavelengths are absorbed by plants. Visible light
consists of different wavelengths that correspond to different colors of light.
The Electromagnetic Spectrum
FIGURE 4: The Electromagnetic Spectrum

gamma ray x-ray ultraviolet infrared microwave radio wave

10 –14 10 –12 10 –10 10 –8 10 –6 10 –4 10 –2 100 102 104

visible

4 x 10 –7 5 x 10 –7 6 x 10 –7 7 x 10 –7

Analyze Think about light as a form of energy and answer the following questions:
What are microwaves used for? What are radio waves used for? What do you think might
happen if visible light were blocked from Earth? How would photosynthesis be impacted?

118 Unit 3 Matter and Energy in Living Systems

 In plant cells, light absorption and photosynthesis take place inside an organelle called
a chloroplast. Inside the inner membrane of the chloroplast are stacks of disc-shaped
sacs called thylakoids, which contain pigment molecules called chlorophyll.

FIGURE 5: The area inside the chloroplast is the stroma. The area inside the thylakoid sac
is called the lumen. The stages of photosynthesis occur across the thylakoid membrane
that separates the stroma and the lumen.
Explain Place these
chloroplast
systems in order from
outer membrane inner membrane largest to smallest, beginning with
Earth, and explain your reasoning:
leaf plant cell thylakoid
tree, biosphere, plant cell,
chloroplast, leaf

lumen stroma

Different types of chlorophyll absorb different wavelengths of light, transforming the

light energy into chemical energy through photosynthesis. Unabsorbed wavelengths
get reflected by the plant’s pigments, and our eyes detect these as the plant’s color.

FIGURE 6: Chlorophyll is a pigment molecule in chloroplasts. Plants have two main types
of chlorophyll, called chlorophyll a and chlorophyll b.
Analyze Which colors
of light are absorbed,
and which colors are reflected by
most plants?

Explore Online

Engineering Hands-On Lab

Investigating
Choosing a Light Source
Image Credits: (b) ©Ed Reschke/Peter Arnold/Getty Images

Light Sources and

Photosynthesis Design an
Scientists and engineers may study the inputs and outputs of a system as part experiment to investigate the
of optimizing the system. For example, different light sources can affect the rate effect of different light sources on
of photosynthesis in a plant system. Different light sources emit light with a the rate of photosynthesis.
variety of wavelengths. Light emitting diodes, or LEDs, can be designed to only
give off certain colors, such as red, blue, or green, which correspond to different
wavelengths of visible light. Applying specific light sources to plants is one way
to optimize the rate of photosynthesis.

Lesson 1 Photosynthesis 119

 Comparing Producers
Most, but not all, organisms rely directly or indirectly on sunlight and photosynthesis.
Places such as deep oceans and dark caves have thriving populations despite
never receiving any sunlight. The very hot water found near cracks in the ocean
floor, called hydrothermal vents, is one such environment. These vents release
chemical compounds such as hydrogen sulfide (H2S) that serve as an energy source.
Hydrothermal vents support a dense ecosystem made up of organisms completely
dependent on the chemicals coming out of the sea floor.

FIGURE 7: Chemosynthetic microbes live on or below the sea floor, and inside the bodies
of other vent animals. Tubeworms grow in clumps around the vents.

Image Credits: (l) ©University of Washington/National Oceanic And Atmospheric Administration (NOAA)/OAR/OER; (inset) ©Ralph White/Corbis
Chemosynthesis is the process of using chemical energy to make sugars from carbon
dioxide for energy storage. Like plants that rely on photosynthesis, chemosynthetic
organisms make their own food, but the raw materials differ.
The producers that live around hydrothermal vents carry out a process represented
by the following chemical equation. The process produces the carbohydrates these
producers need for energy.
6CO2 + 24H2S + 6O2 → C6H12O6 + 24S + 18H2O

Model Make a graphic organizer to compare the inputs and outputs for
chemosynthesis and for photosynthesis.

Analyze Think back to the question about growing plants on another planet and
answer the following questions:
1. What inputs would you need to provide in order for plants to carry out photosynthesis?
2. What are the outputs from plants that are needed for human survival?
3. How would producers that carry out chemosynthesis differ from photosynthetic
producers as a possible food source?

120 Unit 3 Matter and Energy in Living Systems

EXPLORATION 2

Transforming Light Energy into Chemical Energy

So far you have seen that plants transform energy from sunlight into chemical energy
stored in the chemical bonds of sugar molecules. But, how does this transformation of
energy happen? Chloroplasts in cells are like solar-powered chemical factories. They
transfer light energy to energy-carrying molecules called ATP and NADPH. Cells use
these molecules as energy currency for cell processes. In plant cells, they are used to
convert carbon dioxide into sugars.
Predict How do you think
FIGURE 8: Two energy-carrying molecules are used in photosynthesis. ATP stores energy
ADP +bond, and
in a phosphate-phosphate P NADPH carries high-energy
ATP electrons. plant cells transfer energy
from sunlight to the energy-
carrying molecules ATP and NADPH?
ADP + P ATP

NADP+ + H+ + 2 e- NADPH

NADP+ + H+ + 2 e- NADPH
Stages of Photosynthesis
Photosynthesis can be broken into two stages – the light-dependent reactions and
the light-independent reactions. The light-dependent reactions take place within and
across the membrane of the thylakoids, which are stacked inside the chloroplast. The
light-independent reactions take place in the stroma, the area outside the thylakoids.

Gather Evidence
FIGURE 9: The two stages of photosynthesis, light-dependent reactions and light-
independent reactions, occur in the chloroplast. Examine the diagram of the
chloroplast. How does alternating
H 2O CO2
between light-dependent and
light-independent reactions help
the cell conserve energy and
+
light energy NADP matter? Cite evidence from the
ADP diagram to support your answer.

light- light-
dependent independent
reactions reactions
ATP
NADPH
chloroplast
energy-carrying molecules

O2 sugar

Analyze Identify the inputs and outputs for both stages of photosynthesis. Specify
for both energy and matter.

Lesson 1 Photosynthesis 121

 The Light-Dependent Reactions
The light-dependent reactions are the photo part of photosynthesis. The main
functions of the light-dependent reactions are to capture and transfer energy. Light
energy is captured and transferred in the thylakoid membrane by two groups of
molecules called photosystem II and photosystem I. They are named for the order in
which they were discovered, not the order in which they occur.

Explore Online
FIGURE 10: The Light-Dependent Reactions

electron transport chain ATP synthesis

H+ H+ H+
H+ H+ H+
thylakoid H+ H+
lumen photosystem II photosystem I ATP synthase
H +
H+ H+
H 2O 2 H+ + ½ 02 H+
H+
e-
thylakoid e- e-
e- e-
membrane
e- e-

stroma
NADPH+ + H+ NADPH
H+
ADP ATP
light energy light energy
H+

The light-dependent reactions are summarized in the steps below.

1. Energy absorbed from sunlight—In photosystem II, chlorophyll and other pigment
molecules in the thylakoid membrane absorb energy from sunlight. The energy is
_
transferred to electrons (e ). These high-energy electrons leave the chlorophyll and
enter the electron transport chain, a series of proteins in the thylakoid membrane.
2. Water molecules split—Enzymes break down water molecules. Electrons from
+
water molecules replace the electrons that left the chlorophyll. Hydrogen ions (H )
remain inside the thylakoid, and oxygen is released as a waste product.
3. Hydrogen ions transported—Energized electrons move from protein to protein
in the electron transport chain. Their energy is used to pump hydrogen ions across
the thylakoid membrane. The result is a buildup of hydrogen ions inside the
Model Make a simple thylakoid, establishing a concentration gradient, which is a form of stored energy.
flow chart to show how The electrons move on to photosystem I.
energy is transferred from light to 4. Energy absorbed from sunlight—In photosystem I, chlorophyll and other
ATP in the light-dependent stage of pigment molecules in the thylakoid membrane absorb energy from sunlight.
photosynthesis. Energized electrons leave the pigment molecules.
5. NADPH produced—The energized electrons from photosystem I are added to
+
NADP to form NADPH, an energy-carrying molecule, by an enzyme.
6. Hydrogen ion diffusion—Hydrogen ions diffuse out of the thylakoid through the
ATP synthase protein channel. Diffusion of the hydrogen ions is powered by the
concentration gradient. ATP synthase uses energy from the concentration gradient
to make ATP by adding a phosphate group to ADP.

122 Unit 3 Matter and Energy in Living Systems

The Light-Independent Reactions
The second stage of photosynthesis uses energy from the light-dependent reactions
to make sugars. As the name for this stage implies, the light-independent reactions
do not need sunlight. These reactions can take place any time energy is available. The
energy sources for the light-independent reactions are the molecules ATP and NADPH
formed during the light-dependent reactions. This energy is needed for a series of
chemical reactions called the Calvin cycle, named for the scientist Melvin Calvin, who
discovered the process. The Calvin cycle is the synthesis part of photosynthesis. Its
chemical reactions use the energy carried by the ATP and NADPH produced by the
light-dependent reactions to make simple sugars.

Collaborate A common
FIGURE 11: The Light-Independent Reactions
misconception is that the
NADPH NADPH+ 2 bulk of a plant’s material comes
from soil or water. Explain where
C C C
the carbon in sugars actually comes
ADP
from, citing evidence from the
ATP Calvin cycle to support your answer.
C C C

C C C
C C C C C C
1
C C C C C C

C
from CO2 3

C C C C C C C C

ADP
ATP 4

The light-independent reactions are summarized in the steps below.

Explain How does the
1. Carbon dioxide added—A CO2 molecule is added to a 5-carbon molecule already Calvin cycle act as a bridge
in the cycle, yielding a 6-carbon molecule. between carbon in the atmosphere
2. Three-carbon molecules formed—The 6-carbon molecule splits, forming two and carbon-based molecules in the
3-carbon molecules. ATP and NADPH provide the energy to rearrange these food you eat?
3-carbon molecules into higher-energy molecules that also have 3 carbons each.
3. Three-carbon molecules exit—One high-energy 3-carbon molecule leaves the
cycle while the rest remain. One 6-carbon sugar molecule is formed from every two
3-carbon molecules that exit the cycle.
4. Three-carbon molecules recycled—Energy from ATP is used to change five
3-carbon molecules into three 5-carbon molecules, which stay in the Calvin cycle to
accept new CO2 molecules that enter the cycle.

Model Develop a model to illustrate how photosynthesis transforms light energy into
chemical energy. In your model, show how energy from sunlight is transformed to energy
stored in sugars, and identify the inputs and outputs for each stage of the process.

Lesson 1 Photosynthesis 123

 CONTINUE YOUR EXPLORATION

Guided Research

Variation in Photosynthesis

Not all plants carry out photosynthesis in exactly the same way. There are three
different pathways of photosynthesis that depend on the carbon-based compound
first produced when CO2 enters the light-independent reactions. Recall the light-
independent reactions, or Calvin cycle, use energy from ATP and NADPH to build
sugars from smaller molecules. Carbon enters the Calvin cycle as CO2 molecules, which
are rearranged during chemical reactions to form sugar. Early in the process, 3-carbon
molecules are formed and exit the cycle to form 6-carbon sugars. The formation of
Explain How are the 3-carbon molecules occurs in most plants, resulting in the name C3 plants. This is one
three pathways of pathway in which carbon is rearranged in plants. A second pathway results in 4-carbon
photosynthesis similar in terms molecules being formed early in the Calvin cycle. These plants are called C4 plants.
of carbon and the formation of Finally, a third pathway takes in CO2 and incorporates the carbon in organic acids
carbon-based molecules? called crassulacean acids, named after the plant types in which this process occurs.
Crassulacean plants include the succulent, or water-storing plants, such as cacti.
Nearly all land plants exchange gases through openings called stomata. Carbon
dioxide enters and oxygen exits through these openings. At the same time, water that
has been absorbed through the plant roots transpires, or is given off as water vapor
through the open stomata. So the stomata play an important role in regulating the
input of CO2 and the output of oxygen as part of photosynthesis, as well as overall
water loss.

FIGURE 12: Stomata are found on above-ground parts of plants, including the petals of
flowers, stems, and leaves.

Image Credits: ©Power and Syred/Science Source

The stomata do not stay open all the time. Instead, the stomata open and close in
response to homeostatic mechanisms in the plant. This helps the plant conserve water
when water availability is limited. In general, plants lose water fastest during intense
sunlight, especially when the temperature is warm, or when the air is dry, or in windy
conditions. The variations among C3, C4, and CAM plants are mainly based on plant
adaptations to different climates.

124 Unit 3 Matter and Energy in Living Systems

 Plants can be classified by the way their photosynthetic pathways are adapted Predict How would you
to environmental conditions. Most plants are C3 and C4 plants, which open their expect the abundance of C3
stomata during the day, losing most of the water taken up by their roots. But CAM plants to change as regions around
plants are adapted for life in extremely hot and arid climates. These plants generally the world become warmer and
keep their stomata closed during the day to reduce the amount of water that is lost drier?
in transpiration. The stomata often are open through the night, when it is cooler
and more humid. CAM plants fix CO2 at night, avoiding water loss by not opening
their stomata during the day. The CO2 is released during the day to be used in
photosynthetic reactions.

FIGURE 13: Three Pathways of Photosynthesis

C3 Plants C4 Plants CAM Plants

rice, wheat, oat, soybean, corn, nutgrass, and succulents, cacti, bromeliads,
cotton, most trees and tumbleweed and orchids
lawn grasses
Image Credits: (l) ©Image Source/Corbis; (c) ©I love photo/Shutterstock; (r) ©tonda/iStock/Getty Images Plus/Getty Images

stomata open in daytime stomata open in daytime stomata open at night

Language Arts Connection Carry out further research to learn more about these
variations in photosynthesis. Prepare a presentation to explain the differences between
C3, C4, and CAM plants. In your presentation, include information about how each type of plant
carries out photosynthesis, and how the differences help plants survive in different environments.
Use text, visuals, and interactive components to make the concepts in your presentation
engaging and easy to understand.

A multimedia presentation combines text, sounds, and images. A successful multimedia

presentation includes:
• a clear and consistent focus
• ideas that are presented clearly and logically
• graphics, text, music, video, and sounds that support key points
• an organization that is appropriate to its purpose and audience

INVESTIGATING LIGHT SOURCES THE COLOR OF PLANTS Go online to choose one of

AND PHOTOSYNTHESIS ON OTHER PLANETS these other paths.

Lesson 1 Photosynthesis 125

 EVALUATE

Lesson Self-Check

CAN YOU EXPLAIN IT?

As scientists and engineers plan for the next phase of space exploration—traveling
to and colonizing other planets, they must devise ways of meeting the needs of
humans. Today’s astronauts are studying how plants grow in space. Their results will
help scientists determine the best way to keep plants alive until they arrive at the new
planet. The next step in this process will be to determine how plants might grow in the
new planet’s environment.

FIGURE 14: Growing plants in space is important not only as a long-term food source,
but also as a connection to life on our home planet, Earth.

Explain Use what you have learned to further explain how plants could be grown
on other planets. Address the following in your explanation:
1. What inputs do plants need to carry out photosynthesis, and how might you
provide these on another planet?
2. What outputs do plants produce from photosynthesis, and how do these
benefit humans?
3. How do plants transfer energy from light to sugar molecules?
Image Credits: ©NASA Kennedy Space Center

4. What questions would you ask about the planet to refine your list of
necessary materials?

126 Unit 3 Matter and Energy in Living Systems

 CHECKPOINTS

Check Your Understanding 6. Draw a simple ecosystem made up of at least one

producer and one consumer. Add arrows and labels to
1. Which of these are the result of producers performing show how energy and matter flow from the sun to the
photosynthesis? Select all correct answers. producer and from the producer to the consumer.
a. Makes oxygen available for cellular respiration
b. Tranfers carbon dioxide back to the atmosphere 7. Draw a diagram showing the interaction between light
c. Transfers energy from sunlight to consumers and chlorophyll. The diagram should show how this
interaction results in the transfer of energy and electrons
d. Cycles carbon through the biosphere
through photosystem I and photosystem II.

2. Write the overall chemical equation for photosynthesis.

8. Is it true that all organisms on Earth depend on the sun as
Be sure to show the relationship of light and enzymes to
their energy source? Explain your answer.
the reaction.

3. Use the terms below to complete this paragraph:

NADPH, ATP, thylakoids, chlorophyll, chloroplasts, electrons MAKE YOUR OWN STUDY GUIDE
Light energy is absorbed by __ found in the membranes
of __, which are saclike structures inside __. The light
In your Evidence Notebook, design a study guide that
energy dislodges __, which are used to make __. Energy
from this process is used to make __. The electrons and supports the main idea from this lesson:
energy are used to make sugars, which the plant stores or Photosynthesis is a process used by most producers to
consumes for energy. transform light energy into stored chemical energy.
Remember to include the following information in your
4. Draw a Venn diagram to compare chemosynthesis to
study guide:
photosynthesis.
• Use examples that model main ideas.
• Record explanations for the phenomena you investigated.
FIGURE 15: The two stages of photosynthesis, light-dependent
• Use evidence to support your explanations. Your support can
reactions and light-independent reactions, occur in the
include drawings, data, graphs, laboratory conclusions, and
chloroplast.
other evidence recorded throughout the lesson.
Consider how the models for photosynthesis you have used in
this lesson can be used to explain changes in energy and matter.
Explain these changes in terms of energy flow and matter cycling
within and between systems.

5. Draw the diagram above, and add the following

labels to illustrate the transfer of matter and energy in
photosynthesis:
+
NADPH, NADP , sugars, light, ADP, O2, H2O, ATP, CO2

Lesson 1 Photosynthesis 127

 3.2

Cellular Respiration

Cars need energy to move.

A mixture of oxygen and
gasoline, when ignited by
a spark, produces a small,
controlled explosion in CAN YOU EXPLAIN IT?
the engine’s cylinder that
moves the axle. Both cars and your body need fuel. Most cars use gasoline as a fuel, which often
includes ethanol in addition to gasoline. For a car or a human body to use fuel, the
energy in the chemical bonds of the fuel must be released. A combustion reaction in a
car’s engine releases this energy. Your body uses a similar reaction in which it releases
the energy contained in the chemical bonds of the food you eat.

Gather Evidence FIGURE 1: The ignition of a mixture of ethanol and oxygen in the Explore Online
As you explore the lesson, air produces a combustion reaction, which releases energy in the forms of heat and light.
gather evidence that bonds are
broken and new bonds are formed
in the process of cellular respiration.

Image Credits: (t) ©Cultura/Echo/Getty Images; (b) ©Houghton Mifflin Harcourt

Predict How is the process of fuel combustion in a car engine similar to the way the
cells in your body release energy stored in fuel?

128 Unit 3 Matter and Energy in Living Systems

EXPLORATION 1

Matter and Energy in Cellular Respiration

Fuel is any material that reacts to release energy to be used for work. Not all fuels are Analyze How are
alike. They have many different chemical structures. glucose and ethanol similar
in structure and function? How do
HH FIGURE 2: Ethanol is a fuel made they differ?
HH C C OO HH from plant material, such as corn.
CC OO As a renewable energy source,
HH HH
HH it helps reduce petroleum use.
CC CC
OO HH HH HH HH
Glucose is a simple sugar that
HH OO OO HH
CC CC HH C C C C OO HH
living things use for energy.
HH OO HH HH HH

Glucose
Glucose Ethanol
Ethanol

Energy in Living Systems

Whether food for organisms or fuel for cars, almost all the energy on Earth has its
origins in the sun. In the process of photosynthesis, plants transform light energy from
the sun into chemical energy in the form of glucose. When an organism eats a plant,
any energy the plant has not used can be used by the consumer.
Ancient plants and animals that died decomposed and were buried under soil, rock,
and sometimes sea water. These organisms decomposed into organic materials that
contain unused stored energy. Over millions of years, heat and pressure transformed
these remains into the fossil fuels we use today. Chemical bonds must be broken for
the stored energy to be released. In cars, a combustion reaction provides the energy
needed to break these bonds and release energy. In cells, a similar process called
cellular respiration releases chemical energy from sugars and other carbon-based
molecules to make ATP when oxygen is present.
Exothermic Reaction

Exothermic Reaction
FIGURE 3: Activation energy is the
energy needed to start a chemical
activation energy
reaction. An exothermic reaction
releases more energy than it absorbs.
Cellular respiration is an exothermic
reaction.
Energy

reactants
difference in energy

products

Reaction progress

Gather Evidence Explain why cellular respiration is an exothermic reaction. Cite

evidence from the graph shown in Figure 3 to support your explanation.

Lesson 2 Cellular Respiration 129

 Hands-On Lab

Cellular Respiration and Exercise

Burning fuel through either combustion or cellular respiration requires oxygen. In each
process, bonds break and new bonds form. In this lab, you will use an indicator called
bromothymol blue to gather evidence to support a claim about the inputs and outputs
of cellular respiration. Bromothymol blue changes color in the presence of an acid.

Predict What evidence could there be to support the claim that during cellular
respiration, chemical bonds are broken and new bonds are formed?

SAFETY
Do not consume any of the materials used in this lab. Be careful not to breathe in
through the straw.

MATERIALS PROCEDURE
• bromothymol blue solution 1. Place the amount of bromothymol blue solution specified by your teacher in a cup
• cups or beakers (2) or beaker.
• straw 2. Get the timer ready. Slowly blow through the straw into the bromothymol blue
solution, and record how long it takes for the solution to change from blue to
• timer
yellow. Be sure not to inhale when the straw is in the solution.
3. Place the amount of bromothymol blue solution specified by your teacher in a
second cup or beaker.
4. Run in place for approximately one minute.
5. Get the timer ready again. Slowly blow through the straw into the bromothymol
blue solution, and record how long it takes for the solution to turn yellow.

ANALYZE
The water turned acidic when you blew into it because carbon dioxide in your breath
reacted with water to form carbonic acid.
1. How do your findings support the claim that bonds were broken and new bonds
were formed to produce the gas you breathed out?
2. When you exercised, what was different about the time it took the solution to
change color? Explain why this happened.

FIGURE 4:
Bromothymol blue
is an indicator that
Image Credits: ©Houghton Mifflin Harcourt

changes color in
the presence of
an acid.

130 Unit 3 Matter and Energy in Living Systems

The Process of Cellular Respiration
During cellular respiration, the breakdown of glucose and other carbon-based
molecules releases energy stored in their chemical bonds. The stored energy is
transferred to ATP, which we can think of as the cell’s “energy currency.” Energy in the
form of heat is also released in the process. The release of heat accounts for why the
body temperatures of mammals range from 36 to 39°C (97–103°F).
Cellular respiration is an aerobic process, which means that it requires oxygen to
take place. Some organisms can produce small amounts of ATP through anaerobic
processes, or processes that do not require oxygen. However, the presence of oxygen
allows cellular respiration to produce far more ATP from each glucose molecule. The
inputs and outputs of cellular respiration are shown in Figure 5.

FIGURE 5: Inputs and Explain What is the role

oxygen carbon dioxide of the organism in this
Outputs of Cellular
(O2) (CO2)
Respiration model of cellular respiration?
glucose water Explain your answer.
(C6H12O6) (H2O)

heat ATP

Energy and Matter The balanced chemical equation for cellular respiration is:
C6H12O6 + 6O2 → → → 6CO2 + 6H2O + heat + ATP

1. How does this equation represent the law of conservation of matter—that matter
cannot be created or destroyed?
2. How does this equation represent the law of conservation of energy—that energy
cannot be created or destroyed? Consider the role of photosynthesis in your answer.

Mitochondria
Cellular respiration takes place inside an organelle called the mitochondrion (plural Collaborate
mitochondria), shown in Figure 6. Mitochondria release the chemical energy required With a partner, cite
to make ATP. Both plant and animal cells contain mitochondria, because both plants evidence that supports the claim
and animals carry out cellular respiration. that mitochondria are the
“powerhouses of the cell.”
mitochondrion FIGURE 6: The
mitochondrion has
cristae an inner membrane
with many folds, called
cristae. The outer
membrane separates
the mitochondrion from
the rest of the cell. The
space between the two
intermembrane membranes is called the
space
matrix intermembrane space.
outer inner
membrane membrane

Lesson 2 Cellular Respiration 131

 Explore Online Cellular Respiration and Photosynthesis
Hands-On Lab
Almost all energy for living things comes from photosynthesis, either directly
Investigating or indirectly. Producers absorb light energy from the sun and transform it using
Photosynthesis and Cellular photosynthesis to a usable form of energy, or food. This energy is then passed from
Respiration Design an producers to consumers. Although only producers carry out photosynthesis, both
experiment to determine which producers and consumers carry out cellular respiration. Photosynthesis stores energy
organisms (pond snails or Elodea) from sunlight as chemical energy. In contrast, cellular respiration releases stored
produce carbon dioxide and which energy as ATP and heat.
use carbon dioxide.
FIGURE 7: A Comparison of Photosynthesis and Cellular Respiration

Photosynthesis light
energy
REACTANTS PRODUCTS

CO2 C6H12O6

H2O O2

Analyze What is the

relationship between
the inputs and outputs of
Cellular Respiration
photosynthesis and cellular
respiration? PRODUCTS REACTANTS

CO2 C6H12O6

H2O O2

heat ATP

Model Sort the following terms into those that occur during photosynthesis and those
that occur during cellular respiration. Then place the terms in the correct order.
• absorption of sunlight
• ATP production
• production of sugars
• breakdown of sugars

132 Unit 3 Matter and Energy in Living Systems

EXPLORATION 2

Using Chemical Energy

One way that organisms maintain homeostasis is through cellular respiration, which Analyze Identify the
releases energy to carry out cell processes and helps maintain body temperature. inputs and outputs of
Bonds in food molecules and oxygen molecules are broken and new molecules are glycolysis and the two stages of
formed that transfer energy in forms that the organism can use. Cellular respiration cellular respiration.
transfers chemical energy stored in the bonds of glucose and other molecules to ATP.

O2
FIGURE 8: The process of
electrons carried in NADH glycolysis occurs before the
two main stages of cellular
pyruvate
respiration: the Krebs cycle and
electrons carried the electron transport chain.
in NADH and
FADH2
glucose glycolysis electron
Krebs cycle
transport chain

mitochondrion

CO2 H 2O
ATP ATP ATP

Glycolysis and the Stages of Cellular Respiration

Cellular respiration occurs in mitochondria. Before it can take place, however, glucose
must be broken down into compounds the mitochondria can use. This process occurs
in the cytoplasm of the cell. Glycolysis, shown in Figure 9, is an anaerobic process that
uses a series of enzyme-catalyzed reactions to break glucose into two three-carbon
molecules, called pyruvate. Mitochondria use the pyruvate molecules to fuel cellular
respiration.

FIGURE 9: Glycolysis

ATP ADP ADP ATP

c c c c c c
c c c c c c
c c c c c c
glucose 2 pyruvate
NAD +
NADH

Gather Evidence Summarize evidence that bonds are broken and new bonds are
formed in glycolysis.

Lesson 2 Cellular Respiration 133

 Krebs Cycle
The Krebs cycle, sometimes called the citric acid cycle, is the first set of reactions in
cellular respiration. The function of the Krebs cycle is to complete the breakdown
of glucose started in glycolysis and fuel the production of ATP. This is done by
transferring high-energy electrons to the electron transport chain.

FIGURE 10: The Krebs Cycle

1 c c c pyruvate
NAD+

CO2
NADH
c
2 c CoA
Coenzyme A
CoA
NADH
NAD+ c citric acid
c c c c c 3
FADH2 CO2

FAD NAD+
4
6 c c c c NADH
4-carbon
compound c c c c c
5-carbon
ADP compound

5
ATP CO2
NADH NAD+

The Krebs cycle is summarized in the steps below.

1. Pyruvate is broken down A 3-carbon pyruvate molecule is split into a 2-carbon
molecule and a carbon dioxide molecule, which is given off as waste. High-energy
+
electrons are transferred to NAD , forming a molecule of NADH. The NADH moves
to the second stage of cellular respiration, the electron transport chain.
2. Coenzyme A is added A molecule called coenzyme A bonds to the 2-carbon
Analyze How is the
molecule, forming an intermediate molecule.
Krebs cycle a bridge
3. Citric acid is formed The 2-carbon part of the intermediate molecule is added to a
between the energy in sugars and
4-carbon molecule to form the 6-carbon molecule called citric acid.
energy-carrying molecules?
4. Citric acid is broken down The citric acid molecule is broken down by an enzyme,
and a 5-carbon molecule is formed. A molecule of NADH is made, which moves out
of the Krebs cycle. A molecule of carbon dioxide is given off as a waste product.
5. Five-carbon molecule is broken down The 5-carbon molecule is broken down by
an enzyme. A 4-carbon molecule, a molecule of NADH, and one ATP are formed.
Carbon dioxide is given off as a waste product.
6. Four-carbon molecule is rearranged Enzymes rearrange the 4-carbon molecule,
releasing high-energy electrons. Molecules of NADH and FADH2, another electron
carrier, are made. They leave the Krebs cycle, and the 4-carbon molecule remains.

Explain During the hands-on lab, which product of the Krebs cycle caused the
bromothymol blue solution to change color?

134 Unit 3 Matter and Energy in Living Systems

The Krebs cycle is a continual series of reactions. All the carbon atoms in glucose
eventually end up as carbon dioxide, a waste product expelled from the cell. The
role of the electron carriers NADH and FADH2 is to transfer electrons to the electron
transport chain in the next stage of respiration. The transferred electrons will fuel the
formation of ATP.

The Electron Transport Chain

The second stage of cellular respiration, the electron transport chain, uses proteins Model Make a simple
embedded in the inner membrane of the mitochondrion. It is similar to the electron flow chart to summarize the
transport chain stage of photosynthesis. The energy carried by the NADH and FADH2 energy transfer from energy-
molecules produced in the Krebs cycle is used to make ATP. A number of enzymes are
carrying molecules to ATP.
involved in this process.

FIGURE 11: The Electron Transport Chain Explore Online

electron transport chain ATP synthesis

H+ H+ H+
H+ H+ H+
intermembrane H+
space H+ H+ ATP synthase
H+ H+

H+ H+
H+
e-
inner e-
membrane e- e-

H+ FADH2 FAD H+ H+
matrix
NADH NAD+ 2 H+ + ½ O 2 H2 O
ADP ATP

H+

The electron transport chain is summarized in the steps below. Collaborate With a
1. Electrons are transferred Proteins inside the inner membrane of the partner, discuss how the
mitochondrion take high-energy electrons from NADH and FADH2. electron transport chain depends
2. Hydrogen ions are transported High-energy electrons travel from protein to
on the Krebs cycle. Consider the role
protein in the electron transport chain. The proteins use energy from the electrons of energy in your discussion.
to pump hydrogen ions across the inner membrane to produce a gradient, just as
in photosynthesis. The hydrogen ions build up in the intermembrane space.
3. ATP is produced Like in photosynthesis, the flow of hydrogen ions is used to make
ATP. Hydrogen ions diffuse through a protein channel in the inner membrane of
the mitochondrion. The channel is part of the ATP synthase enzyme. ATP synthase
adds phosphate groups to ADP to make ATP molecules.
4. Water is formed Oxygen picks up electrons and hydrogen ions to form water. The
water molecules are given off as a waste product.
Together, glycolysis and cellular respiration produce up to 38 ATP molecules for every
glucose molecule.

Lesson 2 Cellular Respiration 135

 Fermentation
The cells in your body cannot store large amounts of oxygen for cellular respiration.
The amount of oxygen that is provided by breathing is enough for your cells during
normal activities. When you are doing high levels of activity, such as playing a game of
basketball as shown in Figure 12, your body cannot bring in enough oxygen for your
cells, even though you breathe faster. How do your cells function without oxygen to
keep cellular respiration going?
The production of ATP without oxygen continues through the anaerobic processes of
glycolysis and fermentation. Fermentation does not make ATP, but it allows glycolysis
+
to continue. Fermentation removes electrons from NADH molecules and recycles NAD
molecules for glycolysis. Why is this process important? Because glycolysis, just like
cellular respiration, needs a molecule that picks up electrons. It needs molecules
+
of NAD .
The role of fermentation is simply to provide the process of glycolysis with a steady
+
supply of NAD . If you’ve ever felt your muscles “burn” during hard exercise, that is a
result of fermentation. Lactic acid is a waste product of fermentation that builds up in
muscle cells and causes that burning feeling. Once oxygen is available again, your cells
return to using cellular respiration. The lactic acid is quickly broken down and removed
from the cells.

Analyze What is the role FIGURE 12: During

of anaerobic respiration in strenuous or
organisms? What is the role in prolonged activity,
ecosystems? athletes may not be
able to sustain the
oxygen levels their
bodies need. If not
enough oxygen is
supplied to the cells,
anaerobic respiration
takes over.
FIGURE 13: Bifidobacteria live in

Image Credits: (r) ©MCMXCV/Larry Dale Gordon/ Image Bank/ Getty Images; (l) ©SCIMAT/Science Source
the digestive tracts of animals,
including humans.

Not all organisms rely on oxygen for respiration. Organisms that use anaerobic
respiration have an important role in an ecosystem, because they can live in places
where most other organisms cannot. For example, microorganisms, such as the
bifidobacteria shown in Figure 13, live in the digestive tracts of animals and help in
the process of digestion. They must get their ATP from anaerobic processes because
oxygen is not available.

Explain Summarize the evidence that you have gathered to explain how molecules
are rearranged and energy is transferred in the process of cellular respiration.
1. Cite evidence to support the claim that bonds are broken and new bonds are formed
in each stage of cellular respiration.
2. Explain how energy is transferred from the bonds of food molecules to
cellular processes.

136 Unit 3 Matter and Energy in Living Systems

 CONTINUE YOUR EXPLORATION

Hands-On Lab

Aerobic and Anaerobic Processes in Yeast

FIGURE 14: Yeast are single-
celled organisms that belong
The species used in this investigation, Saccharomyces cerevisiae, like other species to the group of organisms
of yeast, is a facultative anaerobe. It can break down sugars using either aerobic or called fungi.
anaerobic processes, depending on the presence of oxygen. When oxygen is not
present, yeast carry out ethanol fermentation. This process produces carbon dioxide
and ethanol, a type of alcohol.

Predict How will you know whether aerobic or anaerobic processes are occurring in the
bottle?

SAFETY
Obtain and wear goggles for this lab. Do not eat any materials used in this lab.

PROCEDURE MATERIALS
• active dry yeast (1 package)
1. Blow up the balloon a few times to stretch it.
• balance (optional)
2. Using the funnel, pour 150 mL of warm water into the bottle. Dry the funnel.
3. Using the dry funnel, add 1 packet of yeast to the water. Swirl the mixture gently. • balloon, round

4. Using the funnel, add 1 tablespoon (12 g) of sugar to the yeast solution, swirl, and • funnel
quickly cover the bottle with the balloon. Allow the mixture to react for 5 minutes. • graduated cylinder
5. After 5 minutes have passed, use the string, marker, and ruler to measure the
• marker
circumference of the balloon.
• ruler, metric
6. In a data table, record the circumference of the balloon, along with all of your
observations of what is happening in the bottle. Continue making and recording • string, 30 cm
observations every 5 minutes for the next 30 minutes.
• sugar, granulated
7. Dispose of waste according to your teacher’s instructions.
• tablespoon (optional)

• timer
ANALYZE • water, very warm (40°C)
1. Describe evidence, if any, that aerobic respiration took place in the bottle. • water bottle, plastic, 500 mL
2. How does matter cycle during aerobic respiration? Explain how the reactants are
• weighing boat (optional)
rearranged to form the products. What is the source of energy, how is the energy
transferred, and how is it used in the cell?
3. Describe evidence, if any, that fermentation took place in the bottle.
Image Credits: ©Andrew Syred/Science Source

4. How does matter cycle during fermentation? Explain how the reactants are
rearranged to form the products. What is the source of energy, how is the energy
transferred, and how is it used in the cell?

INVESTIGATING PHOTOSYNTHESIS Go online to choose one of

EVIDENCE FOR ENDOSYMBIOSIS
AND CELLULAR RESPIRATION these other paths.

Lesson 2 Cellular Respiration 137

 EVALUATE

Lesson Self-Check

CAN YOU EXPLAIN IT?

FIGURE 15: Because ethanol burns more cleanly than gasoline, Explore Online
it is added to gasoline to help reduce the emission of greenhouse gases produced by
combustion engines in cars. Like gasoline, ethanol contains energy in its chemical
bonds that can be released by the process of combustion.

Combustion and cellular respiration are both exothermic reactions that result in the
release of energy. The energy is released when chemical bonds that store energy are
broken. Combustion is a fast process that results in the production of energy in the
forms of heat and light.
CH3CH2OH + O2 → → → 2CO2 + 3H2O + heat
In contrast, cellular respiration is a slow process, with energy being released over a
series of several steps. This makes energy available for use whenever cells of the body
need it to carry out cellular activities.
C6H12O6 + 6O2 → → → 6CO2 + 6H2O + heat + ATP

Explain A scientist named Antoine Lavoisier demonstrated that cellular

respiration is a combustion process. Recall that car engines use a combustion
reaction to release energy. Construct an explanation for how the breakdown
of fuel in a car engine compares to the breakdown of fuel in your body’s cells.
Image Credits: ©Houghton Mifflin Harcourt

Answer the following questions:

1. Look carefully at the equations for both combustion and cellular respiration,
and compare the inputs and outputs. How can the different inputs result in the
same outputs based on what you know about chemical bonds and atoms?
2. What is missing from the process of combustion that makes it an imperfect
model for cellular respiration? Explain your answer.

138 Unit 3 Matter and Energy in Living Systems

 CHECKPOINTS

Check Your Understanding 6. Use the following terms to complete the statement:
ATP, cellular respiration, electron transport chain, glycolysis,
1. How does carbon flow between photosynthesis and Krebs cycle, photosynthesis
cellular respiration?
Living things require energy to grow and reproduce and
a. Photosynthesis produces carbon dioxide from glucose
to carry out different cell processes. Certain cells
generated by the process of cellular respiration.
can capture energy from the sun through the process
b. Cellular respiration produces carbon dioxide from of      . Through a series of reactions, that
glucose generated by the process of photosynthesis. energy is transferred to organisms. Through the
c. Photosynthesis produces carbon dioxide from ATP process of      , the energy currency of the
generated by the process of cellular respiration. cell,      , is produced. This is a three-part process,
d. Cellular respiration produces carbon dioxide from ATP beginning with       in the cell cytoplasm and
generated by the process of photosynthesis. proceeding within the mitochondrion with the
      and, finally, the      .
2. Which of the following are the main inputs, or reactants,
in cellular respiration? Select all correct answers. 7. How do you know that energy and matter are conserved
a. pyruvate during the process of cellular respiration? Explain.
b. glucose
c. carbon dioxide 8. Energy is transferred in several different ways during the
d. oxygen process of cellular respiration. Give two examples of ways
that energy is transferred during this process.
3. Which of the following are the main outputs, or products,
of cellular respiration? Select all correct answers. 9. Is oxygen necessary for the production of ATP in your
cells? Why or why not?
a. water
b. energy
10. How are photosynthesis and cellular respiration related?
c. oxygen
d. carbon dioxide
MAKE YOUR OWN STUDY GUIDE
4. Before cellular respiration, glucose must be broken down
by the process of
In your Evidence Notebook, design a study guide that
a. photosynthesis.
supports the main idea from this lesson:
b. glycolysis.
c. electron transport. Cellular respiration is a process that breaks down food molecules
d. fermentation. to release energy to fuel cellular processes in organisms.
Remember to include the following information in your
5. During which process is lactic acid formed when there study guide:
is not enough oxygen present for cellular respiration to • Use examples that model main ideas.
take place?
• Record explanations for the phenomena you investigated.
a. fermentation • Use evidence to support your explanations. Your support can
b. glycolysis include drawings, data, graphs, laboratory conclusions, and
c. Calvin cycle other evidence recorded throughout the lesson.
d. Krebs cycle
Consider how molecules are rearranged and energy is transferred
during the process of cellular respiration.

Lesson 2 Cellular Respiration 139

 3.3

Modeling Matter and

Energy in Ecosystems

CAN YOU EXPLAIN IT?

This green sea turtle takes
in energy and matter by Phytoplankton are single-celled organisms that live in aquatic environments.
eating sea grass. Many species of marine animals feed on phytoplankton as their main food source.
Phytoplankton are producers that use chlorophyll to carry out photosynthesis. Figure
1 shows a global map of the concentration of chlorophyll in the ocean. Greater
chlorophyll concentrations correlate to larger populations of phytoplankton.

FIGURE 1: Global Concentration of Chlorophyll a

Chlorophyll a concentration (mg/m3)

0.01 0.03 0.1 0.3 1 3 10

Gather Evidence

Image Credits: (t) ©Nature/UIG/Getty Images; (b) ©SeaWiFS Project/NASA Goddard Space Flight Center
As you explore the
lesson, gather evidence to explain
how energy and matter flow
through ecosystems.

Phytoplankton produce nearly half of all oxygen in the atmosphere and use a large
amount of carbon dioxide during photosynthesis. Scientists have discovered that the
global population of phytoplankton has been decreasing.

Predict How might a decrease in the phytoplankton population affect the global
flow of energy and matter?

140 Unit 3 Matter and Energy in Living Systems

 EXPLORATION 1

Introduction to Ecosystems

As its name suggests, an ecosystem is a system—it has boundaries, components,

FIGURE 2: A gray fox emerges
inputs, and outputs. Every living thing requires specific resources and conditions. The
from its den.
gray fox shown in Figure 2 requires certain types of food, shelter, temperatures, and
other factors to survive. Gray foxes live in dens located in underground burrows, under
rock crevices, or in caves. They eat plants, insects, and small mammals, such as mice
and rabbits. Many types of internal and external parasites live on and in gray foxes,
including ticks and tapeworms. Coyotes prey upon gray foxes, but the foxes can climb
trees to escape.

Analyze What types of living and nonliving things does a gray fox’s ecosystem include?

Ecosystem Structure
The Florida Everglades, illustrated in Figure 3, is an example of a complex ecosystem
that can be difficult to study as a whole. To understand the complicated relationships
that make up ecosystems, scientists break them down into smaller parts.

FIGURE 3: The Florida Everglades is an aquatic ecosystem that is found in the temperate
deciduous forest biome.
biosphere

biome Language Arts

Connection Before
they were listed as an endangered
species, alligators in the Florida
ecosystem
Everglades were hunted to near
community extinction. Use library and Internet
resources to find information and
write a report about how human
activities have affected organisms,
populations, and communities in
population the Florida Everglades ecosystem.
Image Credits: (t) ©Photos.com/Jupiterimages/Getty Images

individual

Ecologists can study ecosystems at different scales. They may study an individual
alligator to learn more about factors that affect that species. They may also study an
entire population of alligators. A population is a group of the same species that lives
in the same area. Multiple populations of different species form a community. In the
Everglades, an ecologist may study how a community of alligators, turtles, and birds in
a certain area Interacts with one another.

Lesson 3 Modeling Matter and Energy in Ecosystems 141

 An ecosystem includes all of the biotic, or living, and abiotic, or nonliving, components
in a given area. Energy and matter cycle through these various components during
processes such as photosynthesis, cellular respiration, and decomposition. Similar to
other systems, an ecosystem also has feedback mechanisms that keep it in equilibrium
and restore it to a balanced state when equilibrium is disrupted.

Explain What biotic and abiotic components are found in the ecosystem where you
live, and how do they interact?

Biomes and Biodiversity

A biome is a major regional or global distribution of organisms adapted to living
in that particular environment. Many different ecosystems make up a biome, and
changes in one ecosystem may significantly affect the entire biome. At the largest
scale, all life on Earth is part of the biosphere.
Biodiversity is a measure of the number of different species found within a specific
area. An area with a high level of biodiversity, such as a tropical rain forest, has a large
assortment of species living near one another. The amount of biodiversity found in
an area depends on many factors, including moisture and temperature. The complex
relationships in ecosystems mean that a change in one biotic or abiotic component
can have many effects, both small and large, on a number of different species.

FIGURE 4: World Biomes

Images; (c) ©Photodisc/Getty Images; (cr) ©Corbis; (bl) ©Comstock Images/Getty Images;
Image Credits: (tl) ©tonda/iStock/Getty Images Plus; (tc) ©Oleg Znamenskiy/Fotolia; (tr) ©Anh Luu/Shutterstock;
a Desert b Tropical grassland c Temperate grassland

d Tropical rain forest e Temperate deciduous forest f Temperate rain forest

(br) ©PetrJanJuracka/Shutterstock
©E+/oriredmouse/Getty
Credits:

g Taiga h Tundra
Image
(cl)

142 Unit 3 Matter and Energy in Living Systems

There are many different types of biomes. Scientists categorize biomes in several ways,
such as by the climate conditions and plant communities that thrive in them. This
makes each biome’s biodiversity different and unique. For the most part, the plants
and animals that live in one biome are not found in any other biome. Although biomes
can be categorized separately, they are still connected. Each of these broad biome
types can be further divided into more specific zones. For example, a prairie is a type of
temperate grassland. Frozen polar ice caps and high, snow- and ice-covered mountain
peaks are not considered biomes because they lack specific plant communities.

Model Using your knowledge of photosynthesis, cellular respiration, and ecosystem

structure, model how cutting down a tropical rain forest will affect surrounding biomes.
Consider how the loss of the rain forest will affect the rate of photosynthesis in the
area and how habitat loss will affect the rate of cellular respiration by animals in the
forest. Then, model how the change in amounts of CO2 and O2 could affect surrounding
ecosystems. What other ways might ecosystems be affected by such a loss?

Not all ecosystems are terrestrial, or land-based. About 71 percent of Earth’s surface
is covered with water, and it, too, is home to animal and plant life. These water-based
ecosystems are called aquatic ecosystems. There are two main categories of aquatic
ecosystems: salt water, or marine, and freshwater.

Collaborate Biodiversity tends to decrease the farther an ecosystem is located from

the equator. Discuss this pattern of biodiversity in terms of different biomes and
climate characteristics.

60°N

30°N

equator
0°

30°S

60°S

BIOMES

Desert Temperate rain forest

km 0 2,000 4,000
Tropical grassland Taiga

mi 0 2,000 4,000 Temperate grassland Tundra

Tropical rain forest
OTHER
Temperate deciduous forest
Polar ice

Lesson 3 Modeling Matter and Energy in Ecosystems 143

 FIGURE 5: Like terrestrial ecosystems, aquatic ecosystems vary widely in size, location, and
abiotic and biotic components.

1 lakes and ponds fresh water marine

2 rivers
3 wetlands
4 estuaries 1 5
5 kelp forests
6 coral reefs
3

2 6

Some types of aquatic ecosystems are shown in Figure 1 lakes and ponds
5. Marine 4 estuaries
ecosystems include
2 rivers
the open ocean, coral reefs, kelp forests, and estuaries. Oceans spread from 5 kelp forests
coastal
3 wetlands 6 coral
shallows to the great depths of deep-sea vents. Most coral reefs grow within tropical reefs
Analyze How could zones. Kelp forests exist in cold, nutrient-rich waters. Estuaries occur where freshwater
rising ocean temperatures and salt water mix together.
affect coral reef ecosystems?
Freshwater ecosystems include rivers, streams, lakes, ponds, and wetlands. Rivers and
streams are flowing freshwater, while lakes and ponds are standing bodies of water.
Wetlands are land that is saturated by surface water for at least part of the year.
Each of these ecosystems has unique groups of plants and animals that inhabit them.
The plants and animals that live in these ecosystems are often highly specialized.
Remember that aquatic plants utilize photosynthesis to convert sunlight into usable
energy. They can only grow to water depths where sunlight can penetrate.

Cause and Effect

FIGURE 6: Discarded plastics pollute Bicaz Lake in Romania. Analyzing Human Impacts

Human activities impact ecosystems, sometimes in severe

ways. We produce wastes, such as plastics, that are a
major source of pollution. Humans destroy habitats to
build cities, grow crops, and mine resources. Most of these
activities impair the air, water, soil, and biodiversity in
ecosystems. How do you impact your ecosystem?
Image Credits: (b) ©Alamy

Explain Describe how changing a biotic or abiotic factor can influence an entire biome.
Could changing biotic or abiotic factors be responsible for the decrease in phytoplankton
populations introduced at the beginning of this lesson? Explain.

144 Unit 3 Matter and Energy in Living Systems

 EXPLORATION 2

Energy and Matter Flow in Ecosystems

All organisms need a source of energy to survive. Energy is essential for metabolism,
FIGURE 7: Tropical rain forest.
which is all of the chemical processes that build up or break down materials in an
organism’s body.

Predict Describe two ways that energy and matter flow in the tropical rain forest
ecosystem shown in Figure 7.

Energy in Ecosystems
A terrarium, as shown in Figure 8, is a simple way to model the flow of energy in an
ecosystem. Life in an ecosystem requires an input of energy. The law of conservation
of energy states that energy cannot be created or destroyed. Energy changes form as it
flows within an ecosystem, but the amount of energy does not change.

FIGURE 8: Energy changes form as it flows through an ecosystem.

Explain How does energy
flow in this terrarium in
energy flow terms of photosynthesis and
cellular respiration?

light chemical
energy energy heat

Explore Online
Image Credits: (t) ©E+/oriredmouse/Getty Images; (b) ©FLPA/Alamy Stock Photo

Energy and Matter

Energy and Matter Flow Through Organisms FIGURE 9: A kingfisher dives

underwater to catch a fish.
The kingfisher and the fish shown in Figure 9 are components of an ecosystem.
Each organism has a role in the transfer of energy and matter within that ecosystem.
In addition to the kingfisher and the fish, plants, soil, and temperature also affect the
flow of energy and matter. As in a terrarium, energy and matter change form as they
cycle through this ecosystem, but they are not destroyed.

Model What is the relationship between energy and matter in the kingfisher?
Make a model that shows how matter and energy cycle through this ecosystem.

Lesson 3 Modeling Matter and Energy in Ecosystems 145

 An ecosystem is a complex web of interconnected biotic and abiotic components.
Changing one component in an ecosystem can affect many others. Imagine what
would happen if a chemical spill occurred at the lake the kingfisher depended upon
as a source of food. If the spill killed all the plants, this change would affect the insects
that ate the plants, the fish that ate the insects, and the kingfisher that ate the fish.
Thus one change can destabilize an entire ecosystem.
As part of the ecosystem, humans, like other species, rely on their environment for
survival. If residents of a local town also eat fish from this ecosystem, these changes
will negatively impact them. All species are affected by changes to the biotic and
abiotic factors in an ecosystem.

Image Credits: (bl) ©All Canada Photos/John E Marriott/Corbis; (tl) ©Patrick J. Alexander/U.S. Dept of Agriculture USDA Photography Center; (tc) ©Robert Harding World Imagery/James Hager/Getty Images; (tr) ©Getty Images
Food Chains
Feeding relationships are a major component of the structure and dynamics of
an ecosystem. Food chains and food webs are useful ways to model the complex
structure of an ecosystem to better understand how energy is transferred between
organisms. The simplest way to look at the transfer of food energy in an ecosystem
is through a food chain, as shown in Figure 10. A food chain is a sequence that links
species by their feeding relationships. This simple model follows the connection
between one producer and a single chain of consumers within an ecosystem.

FIGURE 10: Food chains help scientists understand the transfer of energy in an ecosystem.

Predict What might a Producer b Herbivore c Carnivore

happen in an ecosystem
if all the decomposers were Not all consumers are alike. Herbivores, such as desert cottontails, are organisms that
suddenly removed? eat only plants. Carnivores are organisms that eat only animals. Western diamondback
rattlesnakes are carnivores that eat desert cottontails. Omnivores are organisms that
eat both plants and animals. In a desert ecosystem, kangaroo rats are omnivores
that eat both seeds and insects. Detritivores are organisms that eat detritus, or dead
FIGURE 11: Decomposers break
organic matter. Earthworms are detritivores that feed on decaying organic matter
down dead organic matter,
in soil.
including plants and animals.
Decomposers are organisms that break down organic matter into simpler compounds.
These organisms include fungi, certain microbes in the soil, and earthworms.
Decomposers are important to the stability of an ecosystem because they return vital
nutrients back into the environment for other organisms to use.

Model Draw a food chain that includes organisms in the area where you live. Identify
the producer and consumers, and describe the flow of energy in the food chain.

146 Unit 3 Matter and Energy in Living Systems

 Trophic Levels
Trophic levels, shown in Figure 12, are the levels of nourishment in a food chain. The
first trophic level is occupied by the producer. The second level is occupied by the
primary consumer, usually an herbivore. The third and fourth levels contain secondary
and tertiary consumers, and so on, which can be omnivores or carnivores.

FIGURE 12: Each organism in a food chain occupies a different trophic level.
Image Credits: (l) ©Patrick J. Alexander/U.S. Dept of Agriculture USDA Photography Center; (cl) ©Robert Harding World Imagery/James Hager/Getty Images; (cr) ©Getty Images; (r) ©Michael Fitzsimmons/iStock/Getty Images Plus

a Producer b Primary consumer c Secondary consumer d Tertiary consumer

Explain Does energy transfer completely from one trophic level to another? Use
evidence from this lesson to support your answer.

Energy flows up the food chain from the bottom trophic level to the top. Food chains Collaborate Think
are limited in length because energy is lost as heat at each trophic level. Organisms about a typical meal
use the remaining energy to carry out life functions such as cellular respiration and you eat. With a partner, discuss
growth. In this way, less and less energy is available for the next organism in the chain. what trophic level you occupy
Eventually, there is not enough energy to support another trophic level. within that food chain.

Data Analysis

Population Size

A scientist sampled a small cross section of a grassland ecosystem. Her data for
each trophic level are shown in the table.

Primary Secondary Tertiary

Trophic Level Producers Consumers Consumers Consumers
Population Count 6,025,682 723,082 98,541 4

Analyze Answer the following questions in your Evidence Notebook:

1. How does the population size change at each trophic level in this sample?
2. What is the relationship between trophic level and population size?
3. Predict what would happen if a quaternary consumer were added to
this ecosystem.

Lesson 3 Modeling Matter and Energy in Ecosystems 147

 Food Webs
Food chains are not isolated units but are linked together in food webs. Each organism
in an ecosystem may feed on or be eaten by several other organisms and may be part
of many different food chains.

Gather Evidence FIGURE 13: A food web is made up of several different food chains.
How would the food web
be affected if the triggerfish were reef shark phytoplankton
removed from the ecosystem?
What about the algae?
sea turtle

parrotfish
jellyfish

sea sponge zooplankton

triggerfish

algae
shrimp

Model Expand the food chain of the area where you live to make a food web.

A food web models the complex network of feeding relationships between trophic
levels within an ecosystem. A food web represents the flow of energy within and
sometimes beyond the ecosystem. The stability of any food web depends on the
presence of producers, as they form the base of the food web. In the case of a marine
ecosystem such as a coral reef, algae and phytoplankton are two of the producers that
play this important role.

Explain Use the evidence you have gathered in this lesson to answer the following
questions:
1. Scientists use both food chains and food webs to model energy and matter transfer in
an ecosystem. Describe the pros and cons of using a food chain or a food web.
2. In the phytoplankton example from the beginning of the lesson, how will the decrease
in phytoplankton affect the ecosystem’s food web?

148 Unit 3 Matter and Energy in Living Systems

 EXPLORATION 3

Energy and Matter Distribution in Ecosystems

Ecosystems get their energy from sunlight. Producers use energy from sunlight to
make food. Herbivores eat the plants but burn some energy in the process. The energy
is given off as heat, which escapes into space. Carnivores then eat the herbivores but
Image Credits: (tl) ©Patrick J. Alexander/U.S. Dept of Agriculture USDA Photography Center; (tcl) ©Robert Harding World Imagery/James Hager/Getty Images; (tcr) ©Getty Images; (tr) ©Michael Fitzsimmons/iStock/Getty Images Plus

again, a portion of the energy is converted to heat, leaving it unavailable for use by the
organism. Each level in the food chain obtains much less energy than the level below
it. Fortunately, the sun provides a constant flow of energy into the system and allows
life to continue.

FIGURE 14: Energy and matter transfer between trophic levels, but some energy is lost as heat.

heat heat heat

Reduction of Available Energy

When a consumer eats food, the energy it contains undergoes a transformation.
Some energy is used for cellular respiration, which provides energy for movement
and maintenance of the organism. Some is converted to new biomass, or growth. Of
the remaining energy, some is released to the environment as heat, and the rest is
excreted as waste, as illustrated in Figure 15. Although energy changes to different
forms in this process, the total amount of energy remains unchanged or is conserved.
Analyze How does the
FIGURE 15: As trophic level increases, the amount of available energy is reduced because
amount of energy at each
some is converted to heat or excreted as waste. trophic level compare? Use
evidence to support your reasoning.

apex
consumers
heat/cellular
respiration

quaternary consumers

tertiary consumers decomposers

secondary consumers

primary consumers

light producers
energy

Lesson 3 Modeling Matter and Energy in Ecosystems 149

 Data Analysis

Energy Calculations
SAMPLE PROBLEM Energy can be measured using calories (cal), kilocalories (kcal), and joules (J).
A caterpillar consumes 1000 J of energy from the plant it eats. However, the
caterpillar cannot digest all the plant matter, so 500 J of energy are lost as bodily
waste. Additionally, 320 J of energy are converted to heat or used for metabolism.
What percentage of energy remains for the caterpillar to use for biomass, or
growth?

FIGURE 16: A large amount of the energy from

energy a caterpillar consumes food 1000 J
is converted to heat via cellular
respiration or excreted as waste. growth
(new biomass)

waste 500 J cellular respiration 320 J

ANALYZE To determine the amount of energy left for the caterpillar to use, subtract
the amount converted to heat and excreted as waste from the total amount
consumed:
1000 J – 500 J – 320 J = 180 J
The caterpillar has 180 J left over to convert into biomass.

SOLVE To determine the percentage of energy that is usable, divide the amount of
available energy by the total amount of energy and multiply by 100 percent:
180 J
_____
1000 J
× 100% = 18%
So 18 percent of the total energy consumed by the caterpillar is available for

Image Credits: (t) ©Photolibrary/Matt Meadows/Getty Images; (b) ©Sebastien Cote/iStockPhoto.com

growth, and 82 percent of the energy is converted to other forms. Only a small
percentage of the energy in the food was converted to new biomass.

P R AC T I C E P R O B L E M
FIGURE 17: The energy a chipmunk energy
consumes is also largely converted to from food
heat or excreted as waste. growth
(new biomass)

waste cellular respiration

The chipmunk consumes 1000 J of energy from food, loses 177 J as waste, and
loses 784 J to cellular respiration.
1. How many joules of energy are available to convert into new biomass?
2. What percentage of the total energy was available to become new growth?
3. What percentage of the total energy consumed was converted to unusable
forms via cellular respiration, heat, and waste?
4. Make a model that supports the idea that energy is conserved. Use evidence
from this example to support your claim.
150 Unit 3 Matter and Energy in Living Systems
Pyramid Models
The same pattern of energy and biomass distribution at the organism level also occurs Gather Evidence
at the ecosystem level. Biomass is a measure of the total dry mass of organisms in a What information do
given ecosystem at the time of measurement. scientists need in order to
Pyramid models are useful for showing the productivity of an ecosystem and can determine how much energy is
illustrate the distribution of energy, biomass, and number of organisms. Productivity converted into biomass at different
is the percentage of energy entering the ecosystem that is incorporated into biomass trophic levels?
at a particular trophic level. Modeling ecosystem productivity with a pyramid allows
scientists to compare the distribution of energy, biomass, or numbers of organisms
between trophic levels.

Energy Pyramids
Trophic efficiency is the percentage of energy transferred from one trophic level to the
next. Remember that energy transfer from one organism to another is not efficient.

An energy pyramid models the transfer of energy beginning with producers and
working up the food chain to the top-level consumer. The pyramid illustrates how
available energy is distributed among trophic levels in an ecosystem. A typical
energy pyramid has a very large section at the base for producers, and sections
become progressively smaller above. Because energy is converted to heat lost to the
environment at each level of the pyramid, the more levels there are in the ecosystem,
the greater the loss of energy. The energy used by producers far exceeds the energy
used by the consumers they support.

In the simplified energy pyramid shown in Figure 18, energy flows from one trophic
level to the next. In this example, only 10 percent of energy produced is transferred to
the next trophic level. Notice that only 0.1 percent of the energy in the producer level
transfers to the tertiary consumer level.

FIGURE 18: An idealized energy pyramid of a grasslands ecosystem.

Data Analysis
tertiary
10 J 10% According to this model, if
consumers
the producer level contained
5000 J of energy, how many
secondary
100 J 10% joules of energy would be present
consumers
at the tertiary consumer level?
Using this information, can you
primary
consumers 1000 J 10% explain why the energy pyramid
is shaped the way it is?

producers 10 000 J 1%

1 000 000 J of sunlight

Lesson 3 Modeling Matter and Energy in Ecosystems 151

 The simplified pyramid in Figure 18 shows a trophic efficiency of 10 percent for each
link in the food chain. A simplified pyramid like this can help scientists make models
and hypotheses. In reality, the energy transfer between trophic levels, or the trophic
efficiency, can range from 5 to 20 percent, depending on the type of ecosystem.
Producers convert only about 1 percent of the energy available from sunlight into
usable energy. This is because not all of the sunlight hits the leaves of a plant, not
all wavelengths of light are absorbed, and photosynthesis and cellular respiration in
plants require large quantities of energy.

Gather Evidence Why is there a limit to the number of trophic levels in an

ecosystem? Is energy conserved in an ecosystem?

Biomass Pyramid
A biomass pyramid, such as the one shown in Figure 19, compares the biomass
at different trophic levels within an ecosystem. It illustrates the mass of producers
needed to support primary consumers, the mass of primary consumers required to
support secondary consumers, and so on. Biomass is measured as the total mass per
unit of area. The biomass measurement includes living organisms and dead organic
matter. As organisms die and decompose, the nutrients and matter in their bodies are
cycled back through the biomass pyramid by decomposers.

FIGURE 19: A biomass pyramid depicts the total dry mass of organisms found at each
2
trophic level. In this example the biomass is measured as g/m .

tertiary
consumers 75 g/m2

secondary
consumers 150 g/m2

primary
consumers 675 g/m2

producers 2000 g/m2

The amount of energy and biomass decreases in a biomass pyramid as you move up
the trophic levels. In an energy pyramid, the percentage of energy transferred from
one trophic level to the next is approximately the same at every level. In a biomass
pyramid, the percentage of biomass transferred to the next trophic level depends on
the types of organisms present in each trophic level and the level of consumption and
the availability of that biomass for consumption. For example, leaf biomass is more
available and useful for herbivores than wood.

Model Create a model that demonstrates the relationship between biomass and
energy in an ecosystem.

152 Unit 3 Matter and Energy in Living Systems

Pyramid of Numbers
A pyramid of numbers shows how many individual organisms are present at each
trophic level in an ecosystem. Two examples of a pyramid of numbers are shown in
Figure 20. This type of pyramid is effective in showing the vast number of producers
required to support even a few top-level consumers. Ecosystems vary in the number
and types of organisms in each level. These organisms also vary in their rates of growth
and reproduction, as well as in the amount of biomass each species needs to sustain
life and growth. A trophic level that contains organisms that reproduce and grow
rapidly often has less biomass at any given time than one in which reproduction and
growth rates are slow. The size of the organisms also plays a role in the shape of the
various pyramids. The larger the individual organism, the fewer that are needed to
support the next trophic level.
Number of individuals per 0.1 hectare Analyze According to the
FIGURE 20: A pyramid of numbers models how many organisms are at each trophic level.
grassland pyramid, how
grassland (summer) many grass plants would be needed
to support 12 birds?
1 bird

90 000 predatory insects

200 000 herbivorous insects

1 500 000 grass plants

temperate forest (summer)

5 birds

120 000 predatory insects

150 000 herbivorous insects

200 trees

Think about why a pyramid of numbers or a biomass pyramid might appear in an

upside-down or diamond formation. A single tree in a rain forest would be greatly
outnumbered by the primary and secondary consumers, such as insects and birds, that
live on the tree. The upper layers of the pyramid of numbers would be larger than the
bottom layer representing the single tree. If a secondary or tertiary consumer, such as
a condor, were added to the top of the pyramid, a diamond shape would result.

Explain Compare and contrast the different ways to model energy and matter flow in
an ecosystem. If you were a scientist studying an ecosystem, explain how you would use
each type of pyramid and what information you could gain from each one.

Lesson 3 Modeling Matter and Energy in Ecosystems 153

 CONTINUE YOUR EXPLORATION

Hands-on Activity

Biomagnification

MATERIALS Harmful chemicals enter aquatic ecosystems from the runoff of silt, pesticides,
• beads, large (16) and fertilizers. These chemicals enter the food chain and build up in the bodies of
organisms through a process known as biomagnification. Scientists study this process
• beaker, 500 mL
by measuring the amount of chemicals in each trophic level in parts per million.
• marker
FIGURE 21:
• tape, masking agriculture
Biomagnification
• paper cups (4 small, 2 medium, in an aquatic
1 large) ecosystem. pesticides
herring gull eggs
• pencil, sharpened 124 ppm

• salt
food chain

lake trout smelt zooplankton phytoplankton

4.83 ppm 1.04 ppm 0.123 ppm 0.025 ppm
ppm = parts per million

Predict How will the PROCEDURE

beads, or pollutants, 1. Label the small cups “Smelt,” the medium cups “Trout,” and the large cup “Gull.”
transfer between the cups? How is With just the pencil tip, punch one or two small holes in the bottom of each cup,
this a model of biomagnification? and cover them with tape.
How are contaminants magnified 2. Fill each of the cups halfway with salt. Add four beads to each small cup.
up the food chain? 3. Hold each of the small cups over the beaker and remove the tape. Allow the salt to
flow through the holes into the beaker.
4. Pour the remaining contents of two small cups into one medium cup. Pour the
contents of the other two small cups into the second medium cup. Repeat Step 3
with the medium-sized cups.
5. Pour the remaining contents of both medium cups into the large cup.

ANALYZE
1. What pattern did you notice for the transfer of pollutants between trophic levels?
2. Why would tertiary consumers have the highest concentrations of toxins?
3. How are humans affected by biomagnification? Use evidence from this activity to
explain why this is a concern.

WHAT DO IS YOUR DIET AQUATIC PRIMARY Go online to choose one of

ALLIGATORS EAT? ENERGY EFFICIENT? PRODUCTIVITY these other paths.

154 Unit 3 Matter and Energy in Living Systems

 EVALUATE

Lesson Self-Check

CAN YOU EXPLAIN IT?

FIGURE 22: Global Concentration of Chlorophyll a

Chlorophyll a concentration (mg/m3)

0.01 0.03 0.1 0.3 1 3 10

Phytoplankton are tiny photosynthetic organisms that live in marine environments.

They serve as the base for the aquatic ecosystem food web and are an integral
part in the energy and matter flow in aquatic ecosystems. Through their role in the
production of approximately half of Earth’s oxygen, phytoplankton are important to
terrestrial food webs and pyramids.

Explain Refer to your notes in your Evidence Notebook to explain how the flow
of energy and matter through an ecosystem is modeled. Using this information,
answer the following questions:
1. Explain the relationship between the phytoplankton population and
Image Credits: ©SeaWiFS Project/NASA Goddard Space Flight Center

chlorophyll concentration.
2. How can a decrease in the phytoplankton population affect life on Earth?
3. How might this change affect the flow of energy and matter in the biosphere?

Lesson 3 Modeling Matter and Energy in Ecosystems 155

 EVALUATE

CHECKPOINTS

Check Your Understanding 5. What is the relationship between a food chain and
trophic levels?
1. In a prairie ecosystem, which of the following a. A food chain demonstrates how the organisms at the
populations has the most stored energy for use by highest trophic levels have the most energy.
other organisms? b. Food chains illustrate the flow of energy from one
a. hawks trophic level to the next.
b. buffalo c. A food chain models the energy flow within a single
c. prairie dogs trophic level.
d. prairie grasses
6. A consumer eats 1500 J of food energy. The consumer
2. Which food chain correctly shows the direction that uses 15 percent of the food energy for new biomass
energy and matter flow through a forest ecosystem? and the rest for cellular respiration and waste. Use this
a. fruit—insect—sparrow—hawk information to answer the following questions:
b. hawk—fruit—insect—sparrow a. How many joules of food energy were converted into
new biomass?
c. insect—sparrow—hawk—fruit
b. How many joules of food energy are converted to heat
d. insect—hawk—fruit—sparrow
and excreted as waste?
3. Which of the following terms are in the correct order, c. What percentage of the food energy was converted to
from smallest to largest? heat and excreted as waste?
a. population, organism, community, ecosystem,
biome, Earth, biosphere FIGURE 23: Desert

b. organism, community, population, ecosystem,

biome, biosphere, Earth
c. organism, population, community, ecosystem,
biome, biosphere, Earth
d. ecosystem, organism, population, community,
biome, biosphere, Earth

4. Consider a pyramid model with a producer level, a

primary consumer level, a secondary consumer level,
and a tertiary consumer level. Which of the following
statements are correct?
a. The sun is the ultimate source of energy in an
ecosystem. 7. Why is a desert in North America, such as Arizona’s
Sonoran Desert (Figure 23), considered to be the same
b. Matter cycles and is generally conserved within or
biome as a desert in Africa? Image Credits: ©tonda/iStock/Getty Images Plus/Getty Images
among ecosystems.
c. Energy flows through ecosystems, but only a certain
8. What biotic and abiotic factors influence the flow of
amount of energy is transformed into biomass.
matter and energy in different biomes?
d. Energy flows through ecosystems, but some is lost to
the environment as heat.
9. Do you think it is possible for a biome to change from
e. Matter and energy are completely conserved and one type to another due to human activities? Explain a
transformed into biomass within an ecosystem. situation in which this might happen.

156 Unit 3 Matter and Energy in Living Systems

 MAKE YOUR OWN STUDY GUIDE

10. A student thinks that populations higher in a food chain

are larger because they deplete the populations of I n your Evidence Notebook, design a study guide that
organisms lower in the chain. Using evidence from this supports the main ideas from this lesson:
lesson, explain why this student’s thinking is incorrect.
Life in an ecosystem requires a source of energy. The flow of
energy and matter in an ecosystem can be demonstrated by
FIGURE 24: Rabbits are herbivorous and hawks are carnivorous. food chains, food webs, and pyramid models.
Remember to include the following information in your
study guide:
• Use examples that model main ideas.
• Record explanations for the phenomena you investigated.
• Use evidence to support your explanations. Your support can
include drawings, data, graphs, laboratory conclusions, and
a Primary Consumer b Tertiary Consumer other evidence recorded throughout the lesson.
Consider how food chains, food webs, and pyramid models
11. Think about the trophic efficiency, or percentage
show the flow of energy and matter through trophic levels in an
of transferred energy between trophic levels, in an
ecosystem.
ecosystem. Why is an herbivorous diet more energy
efficient than a carnivorous diet? Use the example of
the rabbit and the hawk shown in Figure 24 to help
explain your answer.

12. An aquatic ecosystem contains 10,000 freshwater

shrimp, 1000 sunfish, 100 perch, 10 northern pike,
Image Credits: (l) ©Robert Harding World Imagery/James Hager/Getty Images; (r) ©Michael Fitzsimmons/iStock/Getty Images Plus

and 1 osprey. Draw a pyramid of numbers that

represents this ecosystem.

13. Describe how energy and matter flow, interact, and

change forms throughout the Earth system.

14. In your Evidence Notebook, make a model that explains

the relationship between river, estuary, and ocean
ecosystems. How do matter and energy flow within and
among these ecosystems?

Lesson 3 Modeling Matter and Energy in Ecosystems 157

 3.4

Cycling of Matter and

Energy in Ecosystems

CAN YOU SOLVE IT?

Oceans cycle energy
and matter through The curious-looking buildings shown in Figure 1 are part of a research facility located
the water cycle.
in Arizona called Biosphere 2. The tightly sealed glass and steel structure currently
serves as a place where scientists study Earth’s ecosystems to better understand
carbon and oxygen cycles, coral reef health, water recycling, and more.

Gather Evidence
FIGURE 1: Biosphere 2 is a research facility located in Arizona.
As you explore the lesson,
gather evidence to explain the
relationship between the cycling of
matter and the transfer of energy
through ecosystems.

Image Credits: (t) ©Onigiri studio/Shutterstock; (b) ©James Marshall/Corbis

On September 26, 1991, eight research scientists began a two-year adventure living
in Biosphere 2. The researchers, known as “biospherians,” were completely sealed off
from the outside environment to simulate living in a closed ecosystem. But the results
of the experiment were unexpected. The biospherians had to cope with inadequate
food, decreasing oxygen levels, and increasing carbon dioxide levels. The imbalances
resulted in many plants and animals dying, providing evidence that ecosystems are
much more complex and dynamic than originally thought.

Predict Why do you think researchers had problems with low oxygen levels and
increasing carbon dioxide levels in Biosphere 2? How would you solve this problem?

158 Unit 3 Matter and Energy in Living Systems

 EXPLORATION 1

Matter Cycles Through Ecosystems

Earth is an open system in terms of energy, as it gains energy from the sun. In contrast,
Earth is a closed system in terms of matter. All of the matter on Earth has more or less
been here for billions of years. Matter and energy cannot be created or destroyed, only
transformed into other forms.

Predict Matter and energy move through ecosystems between different organisms.
Where does this matter come from and how does it travel through an ecosystem or
through Earth’s spheres?

Energy and Matter in the Earth System

The Earth system includes all of the matter, energy, processes, and cycles within Earth’s
boundary with space. Energy from the sun drives the cycling of matter in Earth’s
spheres and in the many ecosystems within those spheres. Producers use only about
one percent of the sun’s energy that enters Earth’s atmosphere.

Math Connection

Solar Radiation FIGURE 2: Earth’s atmosphere absorbs and reflects energy.

When solar radiation enters Earth’s atmosphere, about

23 percent is absorbed in the atmosphere and about 48
percent is absorbed at the surface.

Collaborate With a partner discuss these questions: 23%

If energy is conserved, what percent of the solar energy
should be reflected back into space? How do you think
Earth’s ecosystems would be different if more or less solar 48%
radiation was reflected by the atmosphere?

Like energy, matter in the Earth system cycles within and among Earths’ spheres: the
atmosphere, geosphere, hydrosphere, and biosphere. A relatively small amount of
Image Credits: ©NASA Johnson Space Center

matter is lost into space from the very top of the atmosphere, but scientists generally
think of the Earth system as closed in terms of matter.
Matter also changes form as it cycles through the Earth system, but like energy,
it cannot be destroyed. For example, organisms metabolize food using chemical
reactions. These reactions break bonds and form new chemical bonds among the
same atoms to make new substances. The organism can use these new substances for
growth and cell processes. Some matter is excreted as waste, which is recycled in the
environment. The total amount of matter in the system remains unchanged.

Lesson 4 Cycling of Matter and Energy in Ecosystems 159

 Using food webs and pyramid models, you can see matter cycles through different
FIGURE 3: The Earth system is
trophic levels in an ecosystem. As one organism consumes another, that matter is
closed.
transferred into higher trophic levels. When organisms die, their matter is cycled
back through lower trophic levels. In this way, no new matter is created, but matter
continually moves through and between ecosystems, as illustrated in Figure 3.

The Water Cycle

Scientists model specific chemical cycles in order to better understand the cycling of
matter in the Earth system. The hydrologic cycle, also known as the water cycle, is the
circular pathway of water on Earth from the atmosphere, to the surface, below ground,
and back into the atmosphere.
As shown in Figure 4, within the hydrologic cycle, water moves by different processes
between reservoirs, such as oceans or lakes. Reservoirs are any location where cycling
matter is stored. Water molecules might be stored in a reservoir for a long period of
time, such as in a glacier, or for shorter periods of time, such as in a cloud. Evaporation
and precipitation are examples of processes that move water between reservoirs.

Analyze If the total FIGURE 4: The hydrologic cycle transfers water molecules Explore Online
amount of water on Earth between reservoirs.
does not change, why are there
concerns about global shortages of
fresh water?
condensation (clouds)

precipitation

transpiration
evaporation
evaporation

runoff

lake ocean

groundwater

percolation into soil

In the hydrologic cycle, heat from the sun causes water to evaporate from reservoirs
such as the ocean and to evaporate from plant leaves through transpiration. As water
rises into the atmosphere it cools and condenses into clouds. Water then falls back to
Earth in the form of precipitation, such as rain, snow, or hail. Precipitation seeps in the
ground or flows into streams or rivers. Water ends up in a reservoir where it is stored
until the process starts again.

Explain Choose two reservoirs in the diagram and, for each location, explain how
water cycles through the system.

160 Unit 3 Matter and Energy in Living Systems

Biogeochemical Cycles Explore Online

Hands-On Activity
Many elements are essential for the functioning and growth of organisms. These
elements include oxygen, hydrogen, carbon, nitrogen, and phosphorus. Just like water, Winter Water Chemistry
these elements cycle through the Earth system, ecosystems, and organisms. Model summer and winter lake
conditions to determine how
A biogeochemical cycle is the movement of a particular chemical through the
surface ice affects the water
biotic and abiotic components of an ecosystem. The sun and heat from within Earth
chemistry of a lake.
provide energy that drives these cycles. Like the hydrologic cycle, the matter in
biogeochemical cycles flows between reservoirs where it is stored for a period of time.
In contrast to the water cycle, bonds are broken, and atoms are rearranged into new
molecules in biogeochemical cycles. The main biogeochemical cycles are the oxygen
cycle, the carbon cycle, the nitrogen cycle, and the phosphorus cycle.

The Oxygen Cycle

Oxygen (O2) is released into the atmosphere as a product of photosynthesis. The
atmosphere serves as a reservoir for oxygen until it is taken in by an organism for use
in cellular respiration. Humans, and other organisms, also take in oxygen as part of
respiration, or breathing.

Collaborate With a
FIGURE 5: The oxygen cycle is driven by photosynthesis and cellular respiration.
partner, discuss how a
drought caused by a decrease
Photosynthesis Respiration in precipitation might affect
the oxygen cycle. Use evidence
O2 from previous lessons to support
your answer.

CO2

Some of the oxygen is incorporated into compounds that remain in the organism. Thus
the organism becomes a reservoir for the oxygen. Carbon dioxide (CO2) is released
back into the atmosphere as a byproduct of cellular respiration. Carbon dioxide is then
taken up by plants and used for photosynthesis, and oxygen is released back into the
atmosphere. Each cycle on Earth interacts with other cycles. For example, the water
cycle interacts with the oxygen cycle, because water is necessary for photosynthesis.

Gather Evidence In the Biosphere 2 project, oxygen concentration decreased over

time and carbon dioxide reached dangerous levels. Describe a possible solution to this
problem, and explain how it relates to processes in the oxygen cycle.

Lesson 4 Cycling of Matter and Energy in Ecosystems 161

 The Carbon Cycle
Gather Evidence Carbon is present in most chemical compounds that make up living things. Carbon
Without humans, could is also stored in abiotic components of the Earth system. For example, CO2 in the
carbon in these reservoirs be atmosphere, fossil fuels such as oil and coal, dead matter in the soil, and chemical
accessed? compounds in rocks are carbon reservoirs.

FIGURE 6: Processes such as photosynthesis and combustion drive the cycling of carbon. Explore Online

atmospheric CO2

photosynthesis
respiration
combustion
producers
exchange

consumers

decomposers
dissolved CO2

producers
burial
extraction
decomposers
consumers

fossil fuels
sedimentary rock sedimentation

Producers remove CO2 from the atmosphere through photosynthesis. Photosynthetic

organisms incorporate the carbon into carbohydrates to store in their tissues. When
consumers eat producers, they obtain the carbon, storing some of it in their tissues
and releasing some back into the atmosphere through cellular respiration. When the
Explore Online consumers die, decomposers break down the organic matter and release carbon back
into the atmosphere through cellular respiration. Carbon is also released into the soil.
Hands-On Activity
Some of the carbon in the organic matter may become fossilized. Under certain
Lungs of the Planet
conditions, the burial process stores that carbon in Earth’s crust where, over millions of
Why are rain forests called the years, it becomes fossil fuel. Since the 1800s, humans have extracted this carbon and
“Lungs of the Planet”? Investigate combusted it, releasing large amounts of carbon back into the atmosphere.
with Dr. Mike and Dr. Oberbauer to
Carbon dioxide diffuses into the ocean from the atmosphere. Oceans are carbon sinks
determine if this claim is valid by
that absorb and hold large amounts of carbon. Carbon enters the aquatic biotic cycle
measuring rates of photosynthesis
when algae and phytoplankton convert it during photosynthesis. Some dissolved
of rain forest plant life. CO2 is used in the processes of sedimentation and burial to form different types of
sedimentary rock. These processes are very slow, taking millions of years, but they
form extremely large carbon reservoirs.

Model Make a model illustrating the roles of photosynthesis and cellular respiration
in the cycling of carbon among Earth’s spheres. Be sure to include the inputs and
outputs for both processes in your model.

162 Unit 3 Matter and Energy in Living Systems

The Nitrogen Cycle
About 78% of Earth’s atmosphere is composed of nitrogen gas (N2). However, most
organisms are not able to use nitrogen in this form to build organic molecules. The
nitrogen must be fixed, or incorporated into other molecules that organisms can use.
Bacteria, which are involved in many steps of the nitrogen cycle, fix nitrogen into
ammonia, nitrite, nitrate, and other chemicals that organisms can use. As shown in
Figure 7, much of the nitrogen cycle takes place below ground.

FIGURE 7: The nitrogen cycle is made up of many processes that move nitrogen from the Explore Online
atmosphere to the biosphere and back again.

atmospheric nitrogen

nitrogen fixation
consumers

decomposers fertilizer factory

producers
soil

leaching
denitrification assimilation
ammonification
ammonia

nitrate nitrite ammonium

nitrification

Certain types of bacteria convert gaseous nitrogen into ammonia (NH3) through a
process called nitrogen fixation. Some of these bacteria are aerobic, which means Scale, Proportion,
they use oxygen. Other bacteria are anaerobic, which means they do not use oxygen. and Quantity
In aquatic ecosystems, this task is performed by a few types of cyanobacteria. Some Bacteria are microscopic organisms,
nitrogen-fixing bacteria on land live in small outgrowths, called nodules, on the roots but they are essential to life on
of plants such as beans and peas. Other nitrogen-fixing bacteria live freely in the soil. Earth. Using evidence from the
The ammonia released by these bacteria is transformed into ammonium (NH4+ ) by the nitrogen cycle, explain how the
addition of hydrogen ions found in acidic soil. Some ammonium is taken up by plants,
microscopic fixation of nitrogen can
but most is used by nitrifying bacteria as an energy source. These bacteria change
– have such a large impact on life.
ammonium into nitrate (NO3 ) through a process called nitrification.
Nitrates released by soil bacteria are taken up by plants through assimilation, which
converts them into organic compounds such as amino acids and proteins. Nitrogen
continues along the cycle as animals eat plant or animal matter. When decomposers
break down animal excretions or dead animal and plant matter, nitrogen is returned
to the soil as ammonium, in a process called ammonification. Denitrifying bacteria
use nitrate as an oxygen source, releasing nitrogen gas back into the atmosphere as a
waste product via denitrification.

Lesson 4 Cycling of Matter and Energy in Ecosystems 163

 Explore Online Nitrogen fixation can occur through biological processes carried out by special types
of bacteria, but it can also occur through industrial processes such as the production
Hands-On Lab
of fertilizer. Some nitrogen also enters the soil as a result of atmospheric fixation by
Nitrogen Fixation Investigate lightning. Energy from lightning breaks apart nitrogen molecules in the atmosphere.
the role of nitrogen-fixing bacteria Nitrogen recombines with oxygen in the air, forming nitrogen monoxide. The
by observing prepared slides of combination of nitrogen monoxide with rainwater forms nitrates, which are absorbed
legume root nodules. by the soil. Nitrates in the soil may be moved by water, eventually settling at the
bottom of lakes, swamps, and oceans in a process called leaching.

Analyze Organisms in a fish tank can become unhealthy if too much ammonium
from their waste builds up in the water. Explain why it is beneficial to add bacteria and
plants to a fish tank. Use evidence from the nitrogen cycle model to support your claim.

Energy and Matter

Rhizobia Bacteria

Gather Evidence FIGURE 8: Nitrogen-fixing bacteria live in a pea plant nodule.

Legumes are often
planted and harvested as crops.
When this happens, the plants
are not left to decompose into
the soil. How does removing the
legumes from the ecosystem
affect the nitrogen cycle?

a Rhizobia bacteria (colored SEM) b Pea plant nodules

Image Credits: (l) ©Steve Gschmeissner/Science Source; (r) ©Dr. Jeremy Burgess/Science Source
Nitrogen-fixing bacteria live symbiotically, or in close relationship, with certain
types of plants, particularly those in the legume family. Rhizobia bacteria live in the
nodules on the roots of legumes, as shown in Figure 8. The plant provides essential
nutrients to the bacteria and, in return, the bacteria fix nitrogen into ammonia,
which the plant absorbs. Most of the ammonia made by the bacteria is kept by the
plant and very little is released into the soil until the plant dies. Then, decomposers
convert the ammonia molecules into other nitrogen compounds and release some
of that nitrogen back into the atmosphere as nitrogen gas.

The Phosphorus Cycle

Phosphorus is an important element for living things. It is a component of phosphate
groups in ATP, DNA, and phospholipids in cell membranes. Phosphorus occurs in
the form of phosphate salts found in ocean sediments and rocks. Geologic processes
expose these rocks, and water and wind break them down, making them available to
plants and animals.

164 Unit 3 Matter and Energy in Living Systems

 Explore Online
FIGURE 9: The phosphorus cycle interacts with the rock cycle through processes such as geologic
uplift and weathering.

phosphate mining phosphate rocks

weathering

fertilizer
soil leaching runoff

geological uplift
producers excretion and decomposition
consumers

dissolved phosphates

phosphate rocks
sedimentation

Collaborate
Analyze Which of Earth’s spheres is not part of the phosphorus cycle?
Discuss this question
with a partner: When the water at
As shown in Figure 9, weathering of phosphate rocks by rain releases phosphate Biosphere 2 became polluted with
compounds in soil and water. On land, plants can take up phosphate compounds from
too many nutrients, researchers
the soil and consumers gain phosphorus by eating the producers. Decomposers then
treated the water by running it
return phosphorus to the soil and water when they break down the organic matter
and wastes of the producers and consumers.
over mats of algae. Why did they
do this, and how does this action
Water can transport phosphorus to aquatic ecosystems through runoff and leaching. relate to the nitrogen and
Phosphorus compounds dissolve into phosphates where they can be taken up by phosphorus cycles?
algae and then consumed by other aquatic organisms. Some dissolved phosphates
settle at the bottom of oceans in a process called sedimentation, becoming phosphate
rocks over millions of years.
Certain geologic processes expose the phosphate rocks at the bottom of the ocean to
the atmosphere. The rocks then undergo weathering, releasing phosphate compounds
back into the ecosystem, and continuing the phosphorus cycle. Humans also introduce
phosphates into the ecosystem by mining them to make fertilizers and cleaners. Excess
phosphates from human activities can enter aquatic ecosystems through runoff and
leaching. Very little phosphate is naturally available in most bodies of water and any
increases can lead to significant changes in the ecosystem.

Explain How do the hydrologic cycle and the different biogeochemical cycles relate to
one another? How can a change in one cycle affect all of the other cycles?

Lesson 4 Cycling of Matter and Energy in Ecosystems 165

 EXPLORATION 2

Human Impact on Earth’s Cycles

Easter Island, located in the southeastern Pacific Ocean, was first inhabited between
FIGURE 10: Easter Island
400 CE and 700 CE. The human colony grew quickly over the next 1000 years, cutting
down the forests for lumber and for building boats. The forests were cleared faster
than they could grow back, and eventually the island was left with no trees. Without
trees, there was no wood for shelter or boats, the soil washed away, and habitat for
the island’s animal populations was lost. With no food and the island resources nearly
gone, the Easter Islanders disappeared. Today, a small population of people live on the
island. The stone monuments placed by the first inhabitants, shown in Figure 10, are a
major tourist attraction.

Predict What effect did the human population have on Easter Island? How did they
change the island’s natural cycling of matter and energy?

Air Pollution
Gather Evidence
As you read, record Without human activity, the cycling of carbon, phosphorus, and nitrogen in the Earth
evidence to support or refute the system would be in a relatively steady state. Each year humans add synthetic chemicals
idea that atoms are rearranged in and materials to Earth, and many of these chemicals cannot be integrated into normal
biogeochemical cycles. ecosystem functions. The harmful effect of these pollutants can be immediate or
delayed, but these effects may add up over time and can disrupt ecosystem functions.
FIGURE 11: Engine combustion The most common air pollution comes from the waste products produced by
contributes to air pollution. burning fossil fuels, such as gasoline and oil that contain carbon, nitrogen, and
phosphorus. Burning fossil fuels releases carbon dioxide, methane, nitrous oxide,

Image Credits: (t) ©Guido Cozzi/Atlantide Phototravel/Corbis; (b) ©Comstock Images/Jupiterimages/Getty Images
and other chemicals that pollute the air. Smog is a type of air pollution caused by the
interaction of sunlight with pollutants produced by fossil fuel emissions. The nitrogen
dioxide in smog reacts with oxygen to produce ozone, O3. The ozone produced by
reactions of nitrogen dioxide and oxygen tends to stay close to the ground, where
it can be harmful to human health and ecosystem functions. However, ozone also
exists naturally in the upper atmosphere. There, it acts as a shield protecting Earth’s
biosphere against harmful ultraviolet rays found in sunlight.

Algal Blooms
The production of fertilizers and detergents through industrial nitrogen fixation
and phosphate mining has increased greatly over the last few decades. When
these fertilizers are added to food crops or lawns, rain causes excess nitrogen and
phosphorus to run off into nearby streams or lakes. The addition of nitrogen to an
ecosystem alters the nutrient balance, which can lead to increases in producers such as
algae, causing what is known as an algal bloom.
Algal blooms affect the overall health of an ecosystem, and in the case of aquatic
ecosystems, deplete oxygen through a process called eutrophication. When algae die,
decomposers break down their bodies, consuming oxygen in the process. The lack of
oxygen harms aquatic organisms, and can even lead to major die-off events.

166 Unit 3 Matter and Energy in Living Systems

 Cause and Effect

FIGURE 12: Eutrophication Excess Fertilizer

harms aquatic ecosystems.
In many cities, residents over-fertilize their lawns. The excess nitrogen and
phosphorus are washed into lakes, streams, and ponds and can lead to
eutrophication, as shown in Figure 12. Some cities make efforts to educate
their citizens about how to test their soil so they apply just the right amount
of nutrients when fertilizing their lawns.

Model Make a model describing how over-fertilizing leads to eutrophication.

Then use your model to suggest one possible solution to this problem.

Climate Change
Carbon dioxide emissions released from the burning of fossil fuels have led to a
substantial increase in atmospheric CO2, as shown in Figure 13. The rate at which
carbon dioxide enters the atmosphere as a result of human activities is much faster
than the rate at which it is removed by other processes. Combusting fossil fuels and
clear-cutting forests are two examples of human activities that lead to increased
carbon dioxide levels in Earth’s atmosphere.

Analyze As carbon dioxide is added to the atmosphere, Atmospheric CO2 at Mauna Loa Observatory
more carbon dioxide also enters the ocean through FIGURE 13: Atmospheric carbon dioxide levels have risen
diffusion. Carbon dioxide reacts with water to produce substantially since 1960.
Atmospheric CO2 at Mauna Loa Observatory
carbonic acid, which lowers the pH of the water. What
400
effects do you think this might have on marine life?
380
Carbon dioxide is one of several greenhouse gases. These
Parts per million

gases act in a similar manner to a greenhouse for growing 360

plants: They allow sunlight to pass through and provide
energy for plant growth, but keep infrared radiation, or heat, 340
from escaping. Increasing the amount of carbon dioxide seasonal fluctuations
average value
in the atmosphere has been linked to increasing global 320
temperatures, which has a devastating effect on ecosystems.
Some species have already been observed moving into new 300
1960 1970 1980 1990 2000 2010
areas because the changes in the climate make it difficult Year
for these species to continue living in their natural range. Source: Scripps Institution of Oceanography, NOAA Earth System Research Laboratory
Increased carbon dioxide concentrations have also caused
the polar ice caps to shrink and sea levels to rise as a result.
Image Credits: ©Lance Rider/Shutterstock

Explain Many scientists worry that the influence humans have on the biogeochemical
and hydrologic cycles will cause lasting damage to Earth. Make a list of the activities you
perform in a day that may impact one of these cycles. Explain how you are interacting
with the cycle and how that could be affecting your local ecosystem. What can you do to
decrease your impact?

Lesson 4 Cycling of Matter and Energy in Ecosystems 167

 CONTINUE YOUR EXPLORATION

Guided Research

Evaluating Solutions to Human Impacts

Scientists and engineers are working

FIGURE 14: These wind turbines capture energy from the wind and convert it to
to develop solutions to human impacts
electricity. Wind energy is an alternative to energy from fossil fuels.
on the hydrologic and biogeochemical
cycles. Burning fossil fuels for energy
has one of the largest impacts on these
cycles. Finding alternatives to fossil
fuel energy is key to decreasing human
impact and making lasting changes.
Currently engineers and scientists
are investigating solar, wind, water,
biological, and geothermal energies
as potential alternatives to fossil fuels.
You may have already heard of wind
and solar farms. Scientists must ask
many questions when they consider
implementing new energy sources such
as these, including:
• Costs - Is the solution cost effective?
Can a similar solution be reached
in a less costly manner without
losing quality?
• Safety - Is the solution safe for
humans and other living things?
• Reliability - Is the solution going
to hold up over time in the given • Environmental impacts - How Language Arts Connection
conditions? Will it need large does the solution impact the Choose an alternative energy
amounts of upkeep to be maintained environment? Are there any source and research how it impacts the
over time? concerns about these impacts?
biogeochemical and hydrologic cycles or
• Aesthetics - Does the solution add • Meeting criteria - Does the solution how it reduces human impact on these
to or detract from the natural visual solve the problem and meet the cycles. Write a blog entry detailing your
beauty of the area? needs of those who will use the new research. Explain how the alternative
• Social and cultural impacts - How energy source? energy source will work for human
Image Credits: ©Malcolm Fife/Photodisc/Getty Images

does the solution impact human • Evidence to support the solution - populations in terms of its trade-offs, such
societies and cultures? Are there any How well does the evidence provided as cost, reliability, and impact on society
concerns about these impacts? support the claims that are being and the environment. Gather evidence
made about this solution and how it from multiple sources and describe specific
will work? evidence from each source.

NITROGEN WINTER WATER Go online to choose one of

LUNGS OF THE PLANET
FIXATION CHEMISTRY these other paths.

168 Unit 3 Matter and Energy in Living Systems

 EVALUATE

Lesson Self-Check

CAN YOU SOLVE IT?

FIGURE 15: Biosphere 2

The Biosphere 2 research center was originally built with five separate ecosystems: rain
forest, ocean, wetlands, grassland, and desert. Scientists thought that by replicating
Earth’s ecosystems they would be able to make a self-sustaining ecosystem in which
humans could live and grow their own food. Almost immediately, however, Biosphere
2 began suffering from a lack of oxygen and increased carbon dioxide concentrations.

Explain Refer to the notes in your Evidence Notebook to explain how matter
changes form as it flows within the Biosphere 2 system. Use this information to help
you answer the following questions:
1. How do matter and energy change form as they cycle through ecosystems and
Earth’s spheres?
2. Why do you think researchers had problems with low oxygen in Biosphere 2?
3. How would you solve this problem?

The Biosphere 2 experiment never recovered. The scientists built CO2 scrubbers to try
to remove excess carbon dioxide from the air and eventually had to pump in oxygen
to stay alive. The ecosystems inside Biosphere 2 suffered and never flourished as
Image Credits: ©James Marshall/Corbis

scientists had hoped they would. The original purpose of the experiment failed: A
group of people could not survive in a self-sustained system. However, scientists did
learn that Earth’s ecosystems are extremely complex and there is much the scientific
community has yet to learn. Today researchers use Biosphere 2 as a place to study
Earth’s ecosystems to better understand carbon and oxygen cycles, water recycling,
and more.

Lesson 4 Cycling of Matter and Energy in Ecosystems 169

 EVALUATE

CHECKPOINTS

Check Your Understanding 5. Complete the sentence by filling in the correct substance
in each blank.
1. The steps of the carbon cycle are described below.
In the carbon cycle, the role of photosynthesis is to take
Place the steps in the correct order.
in , and the role of cellular respiration is to
a. Animals and plants release carbon dioxide and give off .
water as a result of cellular respiration.
b. Carbon dioxide is released by plants and animals 6. Recently, some areas in the United States have seen
and moves into the biosphere. an increase in trees due to reforestation efforts. Draw a
c. Plants use water and carbon dioxide from the “before” and “after” model to show how the carbon cycle
atmosphere to make sugar and oxygen through might be altered after a large-scale reforestation effort.
the process of photosynthesis.
d. Animals and plants use sugar and oxygen for the 7. Draw a diagram of the water cycle, labeling each process.
process of cellular respiration. Add arrows and labels to show how energy drives the
e. Cellular respiration transforms sugar and oxygen cycle and is transferred through it.
into carbon dioxide and water.
FIGURE 16: Biosphere model
2. Which statement describes a difference between the
nitrogen and carbon cycles?
a. The carbon cycle involves only plants.
b. The nitrogen cycle requires a process called fixation
that is carried out by certain bacteria.
c. The carbon cycle requires that temperatures be
above 27 °C (80 °F).
d. The nitrogen cycle occurs entirely in the ocean.

3. What are the potential effects of introducing too much

nitrogen and phosphorus into an aquatic ecosystem?
Select all correct answers.
a. Fish populations would increase.
8. How is matter changing form in the biosphere model
b. Aquatic organisms would die off.
shown in Figure 16? How many different types of matter
c. Water would become clearer. cycles do you think are being shown in the model?
d. Algae would grow out of control.
e. Oxygen levels would increase. 9. There is evidence that the increasing carbon dioxide
levels in the atmosphere are affecting phytoplankton,
4. Which of the following things are common to all of which are tiny photosynthetic organisms in the ocean.
the biogeochemical cycles? Select all correct answers. Explain how the carbon cycle might be affected if
a. reservoirs and processes phytoplankton were to decrease in number.
b. an atmospheric component
c. photosynthesis and respiration
d. living things as a reservoir
e. the sun as a source of energy
f. can be affected by human activities

170 Unit 3 Matter and Energy in Living Systems

 f. Can be affected by human activities
MAKE YOUR OWN STUDY GUIDE

10. Decomposers are an important part of many

biogeochemical cycles. Some carry out aerobic In your Evidence Notebook, design a study guide that
respiration and some use anaerobic respiration as they supports the main ideas from this lesson:
break down organic matter. Explain why decomposers
Biogeochemical cycles are processes that move matter through
are so crucial for the cycling of matter in ecosystems. Cite
specific examples to support your answer.
and among Earth’s spheres. These cycles can be impacted by
human activity.

FIGURE 17: Rhizobia bacteria Remember to include the following information in your
study guide:
• Use examples that model main ideas.
• Record explanations for the phenomena you investigated.
• Use evidence to support your explanations. Your support can
include drawings, data, graphs, laboratory conclusions, and
other evidence recorded throughout the lesson.
Consider how matter and energy transform, but are not destroyed,
as they move through and among ecosystems and Earth’s spheres.

11. Explain the crucial role bacteria, such as those shown

in Figure 17, play in the nitrogen cycle. What would
happen to the nitrogen cycle if the bacteria were no
longer present?

12. How might Earth’s biogeochemical cycles help scientists

to understand the early history of life on Earth?

13. Develop a model that explains how energy from Earth

drives the biogeochemical cycles.
Image Credits: ©Steve Gschmeissner/Science Source

Lesson 4 Cycling of Matter and Energy in Ecosystems 171

 A BOOK EXPLAINING
COMPLEX IDEAS USING
ONLY THE 1,000 MOST
COMMON WORDS

TREE
A tree and the living and
not living things around it
You know that a tree is a complex living thing. Trees
also provide important habitats for a large variety of
other living things, a biotic community. These symbiotic
components make up the ecosystem in a tree. Here’s RANDALL MUNR
OE
XKCD.COM
an overview in simple terms.

172 Unit 3 Matter and Energy in Living Systems

GROWING UP LEAVES
Trees grow taller only by making Trees make power from
the ends of their branches longer. the Sun’s light using
The spot where a branch joins leaves. The green stuff
the main part of the tree is never in leaves eats light
lifted higher. (and the kind of air we
breathe out) and turns it
into power (and the kind
of air we breathe in).
POINTY CAT
This animal
walks around
slowly, climbing
trees and eating
leaves and sticks.
It’s covered in GRAY
sharp points that TREE-JUMPER
can stick in your These little
skin, so most animals sleep
QUIET NIGHT animals don’t in big round
CATCHER bother it. houses made
These birds fly very of sticks and
BIRD HOLES
quietly and have big leaves high up in
Some birds
eyes to catch animals on the branches.
make holes, but
the ground in the dark. a lot of them just
People think of them as use holes other
knowing a lot of things, birds make.
although that may just
be because they’re quiet
and have big eyes. DRINK HOLES
These were made by a
head-hitting bird looking for
tree blood to drink.
TREE-EATING LOUD JUMPERS
FLOWERS These two kinds of
This flower makes tiny animals make loud
holes in trees and noises and are known for HEAD-HITTING BIRD
steals food and jumping. One has bones. This kind of bird hits trees with
water from inside its head, making holes in the
them. If the flowers wood with its sharp mouth.
STORM BURN
get big, they can kill They make holes to find things
When flashes of to eat, and some also make
the branches they’re
power from storms holes to live in.
growing on, or even
hit a tree, they
kill the whole tree.
can burn a line in
When people stand the wood.
under this flower at
a party, other people
tell them to kiss.

Unit 3 Thing Explainer 173

 TREE

SKIN BURNER BROKEN BRANCH HOLE

These leaves have stuff on them that When a tree gets hurt,
makes your skin turn red. It gives you a like if a branch breaks
really bad feeling, like you need to rub off, the place where it got BIRD HOUSE
your skin with something sharp, but hurt grows differently, just
doing that only makes it worse. like when skin gets cut.
This leaf-flower grows in long lines Sometimes animals get in
across the ground or up trees. through these spots and
Sometimes it grows into the air like a make the hole bigger. FIRE HOLE
small tree of its own. Like many things, These holes are
its leaves come in groups of three. from fires long
DIRT BRANCHES ago. The leaves
Trees grow branches down and sticks on the
into the ground, like the ones ground burned,
in the air. The air branches and the wind blew
ANIMAL HILL get light from the Sun, while the fire against this
This is the dirt the walking the ground branches get side of the tree.
flies took out of the ground water and food from the The burned spot
while making their holes. dirt. They spread way out— grows in a different
often farther than the air way and can
DOOR branches—but usually not sometimes turn into
very deep. a large hole.

TINY
DOG

LONG-EAR
JUMPERS

WALKING FLIES LONG BITERS WITHOUT ARMS

LONG-HOLE
These tiny animals live in big OR LEGS (SLEEPING)
MAKERS
groups and make holes. Most of These long thin cold-blooded
them don’t have babies; each family animals don’t usually hang out
has one mother who makes all the together, and sometimes eat
new animals for the house. each other.
They usually don’t fly, and they’re During the winter, though, lots
not much like house flies. They’re of different kinds come together
in the same group with the kinds of and sleep all wrapped up
flies whose back end has a sharp together in big holes under the
point that can hurt you. ground where it’s warmer.

174 Unit 3 Matter and Energy in Living Systems

 SKIN
Go online for more
The outer skin of trees is where growing
about Thing Explainer.
happens and where they carry food up
and down. Cutting off a ring of skin all the TALL AND WIDE TREES
way around a tree will kill it. The same kind of tree can grow tall or wide. If there
Trees grow by adding new layers, and are other trees around, they’ll grow mostly up, each
grow differently in different parts of the one trying to get above the others to reach the Sun’s
year. If you cut open a tree, you can see light. If a tree is growing alone in a fi eld, it will spread
old layers, and count them to tell how many branches out to the sides so it can catch more light.
years old the tree is.

OLD METAL
When people use metal to stick signs
to trees, sometimes the tree grows
around the metal and eats it up.
Then, many years later, if someone
needs to cut down the tree, their saw
can hit the metal and send tiny sharp FIELD TURNING INTO FOREST
pieces fl ying everywhere. When people cut down a forest, sometimes they
leave a few trees—to make a cool shadow area, or
TREE-FOOD STEALER because the tree looks nice—and those trees will
Instead of growing dirt branches of their grow out into the new space.
own, these fl owers grow onto the dirt If the forest grows back, the new trees—fi ghting
branches of other trees and steal food with each other as they grow—will be tall and thin.
from them. If you fi nd a forest of tall thin trees with one wide
Some of these little fl owers don’t even tree with low branches in the middle, it might
have green leaves and can’t make their mean the forest you’re in was someone’s fi eld a
own food from light. hundred years ago.

FOREST ABOVE WATER

LITTLE
HOLE-
MAKERS

BIG HOLE-MAKERS
Image Credit: ©Alexander Mazurkevich/Shutterstock

DIRT-BRANCH LIFE
Most trees and fl owers have life growing on
their dirt branches. This life helps them talk
to the other trees and fl owers around them.
They can even use this life to share food or
attack each other. DIRT BRANCHES
If something tries to eat one tree, it can tell
other trees through messages carried by
this ground life, and the other trees can start
making bad water and other things to make
themselves harder to eat.

Unit 3 Thing Explainer 175

 UNIT CONNECTIONS

Engineering Connection FIGURE 1: Algae biofuel production.

Algae Biofuel Cars combust fossil fuels, releasing large amounts of carbon dioxide
into the atmosphere. Carbon dioxide gas is one of the leading contributors to
greenhouse gases and increasing global temperatures. To counteract this effect on
the environment, engineers have been researching alternative fuels, such as algae
biofuels, that do not release greenhouse gas emissions. Algae trap, transform, and
store solar energy as oil through the process of photosynthesis. The oil can then be
processed into biofuel.

Using library and Internet resources, research algae biofuels. Write a blog entry
explaining the potential uses of algae biofuels. What impact could biofuels have on
human-driven greenhouse gas emissions?

Social Studies Connection FIGURE 2: Black-footed ferrets released

into the wild.
BFFs: Black-Footed Ferrets As European settlers moved into the Great Plains, they
converted prairie land into farmland. These farmers and ranchers found the prairie
dogs that lived on that land a nuisance and killed them off in large numbers. The
black-footed ferrets (BFFs), which feed almost exclusively on prairie dogs, were

Image Credits: (t) ©David Maung/Bloomberg/Getty Images; (c) ©Kathryn Scott Osler/The Denver Post/Getty Images; (b)©Neo Martinez/Science Source
nearly eradicated in the process. BFFs are a key species in the ecosystem, and their
health is a primary indicator of the overall health of that ecosystem.

The BFF population has undergone a large captive breeding program and is being
reintroduced to the wild, with great success. Using library and Internet resources,
research the story of the BFF and what its reintroduction into the ecosystem means. Make a
pamphlet to document the history of the BFF, and explain any possible implications for local
ranchers and farmers.

Computer Science Connection FIGURE 3: A complex ecosystem

network developed using network
Computational Ecology In recent years, advancements in computer modeling modeling computer software.
software and processing speed have allowed scientists to study the complexity of
ecosystems in new depths. Historically, food webs had been presented as images
of producers and consumers connected by a web of energy arrows. However, with
new software, scientists can now model hundreds of interactions between species
and build a complete ecosystem network, as shown in Figure 3.

With a partner or a small group, review the ecosystem network shown here. What are
the pros and cons of making such a computer model of an ecosystem? Do you think a
human could analyze this network without a computer? On your own, make a list of questions Producers Apex Consumer
that, if you were a scientist, you would ask based on this model. Share your questions with
your partner or group. Did you have similar questions?

176 Unit 3 Matter and Energy in Living Systems

UNIT PRACTICE AND REVIEW

SYNTHESIZE THE UNIT DRIVING QUESTIONS

Look back to the Driving Questions from the opening section

In your Evidence Notebook, make a concept map, of this unit. In your Evidence Notebook, review and revise
graphic organizer, or outline using the Study Guides you your previous answers to those questions. Use the evidence
made for each lesson in this unit. Be sure to use evidence to you gathered and other observations you made throughout
support your claims. the unit to support your claims.

When synthesizing individual information, remember to follow

these general steps:
• Find the central idea of each piece of information.
• Think about the relationships between the central ideas.
• Combine the ideas to come up with a new understanding.

PRACTICE AND REVIEW

2. The cell is a system in which processes such as

FIGURE 4: Energy transferred to electrons moves through the
photosynthesis and cellular respiration take place so that
light-dependent reactions.
the cell can survive. Although photosynthesis only occurs
electron transport chain ATP synthesis
in certain producers, how does this process contribute to
the survival of other organisms, including humans?

H+ H+ H+
H+ H+
thylakoid
H+ 3. Describe the relationship between cellular respiration
H +
H+
lumen photosystem II photosystem I ATP synthase and photosynthesis in terms of energy and matter.
H+
H+ H+
H2 O 2 H+ + ½ O2 H+
H+
e- 4. In a pyramid of numbers, the highest-order organism
thylakoid e- e- has the smallest number of individuals in an ecological
e- e-
membrane
e- e- community. What might happen if the population of
stroma this organism increased significantly? In your Evidence
NADP+ + H+ NADPH Notebook, develop a model explaining the effect
H+
ADP ATP this increase would have on other members of the
light energy light energy
community.
H+

Use the information in Figure 5 to answer Question 5.

1. Solar panels capture energy from sunlight and convert it 5. If 90% of the energy is lost FIGURE 5: Energy in
to electricity. As light hits the silicon atoms in a solar cell, as heat between trophic trophic levels.
the energy is transferred to electrons. The electrons are levels, approximately
emitted from silicon atoms, and an electric field organizes how much energy is
the electrons into an electric current. Compare the way a available to the secondary
solar cell works to the way a chloroplast works to capture consumers in this energy
and transfer energy. pyramid?

50 000 J

Unit 3 Unit Closer 177

 UNIT PRACTICE AND REVIEW

FIGURE 6: Matter and energy cycle through the Earth system. FIGURE 7: During strenuous or prolonged activity, athletes
must sustain the oxygen levels their bodies need.

10. In your Evidence Notebook, make a model to explain

how the energy content of food molecules can be traced
back to the sun.

6. Make a model in your Evidence Notebook to show how 11. A forest fire began after a group of campers failed to
a biogeochemical cycle is connected to the transfer of extinguish their campfire completely. Forest fires release
matter and energy through a food chain. In your model, carbon, nitrogen, phosphorus, and sulfur that was
include media and text to convey the concepts of sequestered in the biomass of the trees back into the
producers, consumers, decomposers, and the cycling of atmosphere. In your Evidence Notebook, create a model
matter and energy. that shows how each of the carbon, phosphorus, and
nitrogen cycles in that area will be affected by the forest
fire. Then explain how the changes in the biogeochemical
7. An increase in energy can change the dynamics of a
cycle will affect the local ecosystem.
system. Explain how alterations to the carbon cycle result
in an increase in the amount of energy contained in the
Earth system. Discuss how this addition of energy would 12. Hydroelectricity is a form of renewable energy that
affect the cycling of matter in other biogeochemical cycles. involves building dams on rivers and streams. Upstream
of the dam, lakes are usually formed as the dam restricts
the flow of water. Downstream, the amount of water is
8. Why does the amount of energy in an ecosystem depend
usually reduced. How does this activity affect the water
on its producers?
cycle and the local ecosystems? Is hydroelectricity a
sustainable source of energy?
9. The nitrogen cycle relies on various organisms carrying
out very specific functions. One vital group is the
nitrogen-fixing bacteria. Which of the following explains UNIT PROJECT

Image Credits: (r) ©MCMXCV/Larry Dale Gordon/ Image Bank/ Getty Images
how the nitrogen cycle would be disrupted if there were a
sudden population explosion of nitrogen-fixing bacteria? Return to your unit project. Prepare your research and
a. A population explosion of nitrogen-fixing bacteria materials into a presentation to share with the class. In
would lead to a decrease in ammonium levels in the your final presentation, evaluate the strength of your
water. hypothesis, data, analysis, and conclusions.
b. A population explosion of nitrogen-fixing bacteria Remember these tips while evaluating:
would cause dissolved nitrogen levels in the water to
increase. • Look at the empirical evidence—evidence based
c. A population explosion of nitrogen-fixing bacteria on observations and data. Does the evidence
would cause dissolved oxygen and dissolved carbon support the explanation?
dioxide levels to decrease. • Consider if the explanation is logical. Does it
d. A population explosion of nitrogen-fixing bacteria contradict any evidence you have seen?
would cause ammonia levels to rise, which can be • Think of tests you could do to support and
detected by testing the ammonia levels in the water. contradict the ideas.

178 Unit 3 Matter and Energy in Living Systems

UNIT PERFORMANCE TASK

Analyzing Water Pollution

The small town of Lakeview is located on the shores of Piper
FIGURE 8: The fertilizer plant is upstream from Piper Lake.
Lake. The town relies on the lake for trout fishing, eagle
watching, and recreational activities. Recently, a fertilizer
plant, H.T.C. Fertilizers, was built upstream on Eagle River,
which feeds into Piper Lake. The town has noticed an increase Lakeview
H.T.C. Fertilizers
in algal blooms in the lake. They are concerned the fertilizer
plant is dumping too much nitrogen into the river and their
livelihood could be affected. Is the town right? Does the plant Eagle River
need to control the waste they put into the river?

Piper Lake
1. DEFINE THE PROBLEM
With your team, write a statement outlining the problem
you’ve been asked to solve. Record any questions you have
about the problem and the information you need to solve it.

2. CONDUCT RESEARCH
With your team, investigate the cause-and-effect relationship
between nitrogen, algae blooms, and fish populations. Could
the fertilizer plant be responsible for the changes the town is
experiencing?
CHECK YOUR WORK

3. ANALYZE DATA A complete presentation should include

On your own, analyze the problem you’ve defined along the following information:
with your research. Make a model to show how excess
nitrogen cycles through the aquatic ecosystem. Your model • a clearly defined problem with supporting
should also show any effects the nitrogen may have on questions that are answered in the final
the ecosystem using a food web, energy pyramid, biomass presentation
pyramid, or pyramid of numbers. • a model of the effect of the fertilizer
runoff
• an explanation based on your analysis
4. COMMUNICATE of the runoff and whether or not it is
Present your findings to the town and the fertilizer company adversely affecting the lake ecosystem
explaining whether or not the runoff from the fertilizer plant • images and data that further support your
is adversely affecting the lake ecosystem. Your presentation claims
should include images and data to support your claims.

Unit 3 Unit Closer 179