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Key Concepts • How does energy move

in ecosystems?• How is the movement of

energy in an ecosystem modeled?

Identify Important Words As you read this lesson, highlight all the words you do not understand. Then underline the part of the text that can help you learn what those words mean.

How does energy move in ecosystems?Think of an ecosystem. That ecosystem might seem

peaceful. If you visit, though, you will notice that it is full of movement. Birds squawk and beat their wings. Plants sway in the breeze. Insects buzz. Animals run over leaves.

An organism requires energy to move. Growth, reproduction, and other life functions require energy. Most of the energy for life on Earth comes from the Sun.

Energy does not cycle through ecosystems like matter does. Energy flows in one direction. In most cases, energy flow starts with the Sun. It moves from one organism to another organism. Many organisms get energy by eating other organisms. Organisms can change energy into different forms of energy. Not all the energy an organism gets is used for life processes. Some is released into the environment as thermal energy. Energy cannot be created or destroyed. Its form can change, though. This idea is called the law of conservation of energy.

What do you think? Read the two statements below and decide whether you agree or disagree with them. Place an A in the Before column if you agree with the statement or a D if you disagree. After you’ve read this lesson, reread the statements to see if you have changed your mind.

Before Statement After

5. The Sun is the source for all energy used by living things on Earth.

6. All living things get their energy from eating other living things.

Key Concept Check1. Explain How do the movements of matter and energy differ?

Matter and Energy in the EnvironmentEnergy in Ecosystems

LESSON 3

CHAPTER 12

Reading Essentials Matter and Energy in the Environment 201

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ProducersProducers make things. Living things that make their

own food from materials in their environments are called producers. Most of these producers use the process of photosynthesis (foh toh SIHN thuh sus). Some use the process of chemosynthesis (kee moh SIHN thuh sus).

Photosynthesis Plants, algae, and some bacteria use photosynthesis to make their food, as shown below. Photosynthesis is a series of chemical reactions that convert light energy, water, and carbon dioxide into the food energy molecule glucose and give off oxygen. This process is part of the carbon cycle that you read about earlier in this chapter.

Visual Check2. Identify What food do producers make in photosynthesis?

4. Classify To which group of consumers do most humans belong?

Reading Check3. Explain During chemosynthesis, what do producers use to make food?

Photosynthesis

Carbon dioxide

Light

energy

SugarsWater

Chemosynthesis Some producers use chemosynthesis to make food. Chemosynthesis is the process during which producers use chemical energy in matter rather than light energy and make food. Chemosynthesis can occur on the deep ocean floor. Hydrothermal vents are outlets for compounds that contain hydrogen and sulfur, as well as thermal energy from inside Earth. Chemosynthetic bacteria that live there use the chemical energy in the compounds and produce food.

ConsumersConsumers do not make their own energy-rich food. They

get their energy by consuming, or eating, other organisms.

Consumers can be classified by the type of food that they eat. The groups of consumers include herbivores, carnivores, omnivores, and detritivores.

Herbivores, Carnivores, and Omnivores Herbivores eat only producers. A deer eats only plants, so it is an herbivore. Carnivores eat other animals. Lions and wolves are carnivores. Omnivores eat both producers and other consumers. A bird that eats berries and insects is an omnivore.

202 Matter and Energy in the Environment Reading Essentials

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Visual Check6. Identify Circle each producer and draw a square around each consumer in the food chain.

Key Concept Check5. Explain How does a food chain model energy flow?

Use each side of your pyramid book to organize information about one of the ways energy flows in an ecosystem.

Food Web

Food

Cha

in

The Sun emitsenergy. The hawk obtains

energy by eating the snake.

Plants make energy-richfood using sunlight.

The snake obtains energy by eating the mouse.

The mouse obtains energy by eating the plant.

15

4

2

3

Detritivores Detritivores (dih TRI tuh vorz) get their energy by eating the remains of other organisms. Some detritivores, such as insects, eat dead organisms. Detritivores, such as bacteria and mushrooms, feed on and help decompose dead organisms. They are often called decomposers. They produce carbon dioxide that enters the air. Some decayed matter enters the soil. In this way, detritivores help recycle nutrients through ecosystems. They also keep dead organisms from piling up in ecosystems.

Modeling Energy in EcosystemsYou learned that energy does not cycle through ecosystems.

Instead, energy flows through ecosystems. Organisms store energy in their bodies as chemical energy and use some energy for life processes. When consumers eat these organisms, the energy is transferred to the consumers. However, some energy is changed to thermal energy in the process. Any extra thermal energy enters the environment. Decomposers transfer energy back to the environment when organisms die.

Scientists use models to study this flow of energy through an ecosystem. The model they use depends on how many organisms they are studying.

Food ChainsA food chain is a model that shows how energy flows in an ecosystem

through feeding relationships. In a food chain, arrows show the transfer of energy. A food chain is shown below.

Reading Essentials Matter and Energy in the Environment 203

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Visual Check8. Interpret Diagrams Draw a circle around the organism in the food web that receives energy from the greatest amount of producers and consumers.

Key Concept Check 7. Name two models used to show the transfer of energy in an ecosystem.

Orca

Leopard seal

Diatoms

Copepods

Squid

Krill

Fish

Great white shark

Food WebsImagine that you are working on a jigsaw puzzle of a

prairie. The food chain is just one piece of that puzzle. It shows just one small part of the prairie. A food chain is like one piece of an ecosystem jigsaw puzzle. It can help you study parts of an ecosystem. It does not show the whole picture, though.

Look again at the food chain. The mouse might also eat the seeds of other producers, such as corn or trees. The snake might eat other animals, such as frogs, crickets, or earthworms. The hawk might also eat other animals, such as mice, squirrels, rabbits, or fish. The feeding relationships in this ecosystem are complex.

Scientists use a food web to study these feeding relationships. A food web is a model of energy transfer that shows how food chains in a community are interconnected. A food web is many overlapping food chains. The food web below shows the complex feeding patterns in an ecosystem. Arrows show how energy flows.

204 Matter and Energy in the Environment Reading Essentials

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Energy PyramidsFood chains and food webs show how energy moves in

an ecosystem. They do not show how the amount of energy in an ecosystem changes, though. An energy pyramid is a model that scientists use to show the amount of energy available in each step of a food chain. The steps of an energy pyramid are called trophic (TROH fihk) levels.

Look at the energy pyramid below. Producers, such as plants, make up the bottom trophic level. Consumers, such as squirrels, that eat producers make up the next level. Consumers, such as hawks, that eat other consumers, make up the highest level.

Notice that as you move to a higher level, there is less energy available for consumers. Why? As you read earlier, organisms use some of the energy they get from food for life processes. During life processes, some energy is changed to thermal energy. The thermal energy is then transferred to the environment.

Visual Check10. Interpret Diagrams How does the amount of available energy change at each trophic level?

Trophic level 3(1 percent of

energy available)

Ava

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gy

dec

reas

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Trophic level 2(10 percent of

energy available)

Trophic level 1(100 percent of

energy available)

The first trophic level—producers—obtains energy from the Sun. They use 90 percent of the energy for their own life processes. Only 10 percent of the energy remains for the second trophic level—herbivores. Assume that each trophic level uses 90 percent of the energy it receives. Use the following steps to calculate how much energy remains for the next trophic level.First trophic level gets 100 units of energy. First trophic level uses 90 percent = 90 unitsEnergy remaining for second trophic level = 10 unitsSecond trophic level uses 90 percent = 9 units Energy remaining for third trophic level = 1 unit9. Use Percentages If the first trophic level receives 10,000 units of energy from the Sun, how much energy is available for the second trophic level?

Math Skills

Reading Essentials Matter and Energy in the Environment 205

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Mini Glossary

Reread the statements at the beginning of the lesson. Fill in the After column with an A if you agree with the statement or a D if you disagree. Did you change your mind?

What do you think

END OF LESSON

Log on to ConnectED.mcgraw-hill.com and access your textbook to find this lesson’s resources.

ConnectED

chemosynthesis (kee moh SIHN thuh sus): the process

during which producers use chemical energy in matter

rather than light energy to make food

energy pyramid: a model that scientists use to show the

amount of energy available in each step of a food chain

food chain: a model that shows how energy flows in an

ecosystem through feeding relationships

food web: a model of energy transfer that scientists use to show

how food chains in a community are interconnected

photosynthesis (foh toh SIHN thuh sus): a series of

chemical reactions that convert light energy, water, and

carbon dioxide into the food energy molecule glucose and

give off oxygen

1. Review the terms and their definitions in the Mini Glossary. Write a sentence that describes one way in which photosynthesis and chemosynthesis differ.

2. Fill in the graphic organizer below. Name and describe the three models that scientists use to show the flow of energy through an ecosystem. Clues have been provided.

complex feedingrelationships in an

ecosystem

Food Chain

206 Matter and Energy in the Environment Reading Essentials

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