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3.1: Food Chains: How Energy Gets to You

Difficulty Level: At Grade Created by: CK-12
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Where do you get the energy to live?

Grass grows. Dogs run and bark. Birds sing. Maybe you sing, too. You think and read, run and play, and go to school. But where do you think living things get the energy to do everything they do? Let's start with how energy works in your body. Everything you do is a chain reaction of events. Moving your finger, scratching an itch, and even reading this page is the result of a series of actions that begins in your brain.

“Each living thing is a spark of sunlight energy, a crystal bead in the net of life.”

Steve Van Matre, The Earth Speaks

Suppose you want to move your finger. The action starts with an idea-“I want to move my finger.”

Figure 2.1 The action starts in your brain.

Then your brain sends the message to move your finger. The message travels through your nervous system. When the message reaches your finger, tiny structures called filaments deep inside your muscles contract. The contractions move your finger.

What allows this chain of events to take place? Energy makes everything happen. Energy is what moved the message from the brain to the finger-and moved the finger. Your body gets energy from the food you eat. Food also provides minerals and vitamins that help your cells work properly. Your digestive system processes the food you eat and drink into tiny molecules sent to every cell in your body. Let's see how the breakfast you eat is broken down and transported from your mouth to the cells in your body.

Figure 2.2 The brain sends the message.

Figure 2.3 The message reaches your finger and the muscles contract.

Figure 2.4 Your body obtains energy from two main ingredients-food and oxygen.

  1. You start the day with a healthy breakfast-maybe a couple of eggs, a piece of toast, some fruit, and a glass of milk.
  2. The food travels into your stomach and intestines. There it is broken down into smaller and smaller bits such as sugars and other molecules your cells need.
  3. Blood traveling through your stomach and intestines picks up these molecules and takes them to your muscle cells. There they are absorbed.
  4. At the same time, your blood picks up oxygen from your lungs and delivers it to cells in your muscles. So oxygen and the molecules from food get to your cells.

Tiny bits of the food you ate break down into a simple sugar called glucose. In your cells, energy from the sugar is released when oxygen and glucose combine in a reaction. The reaction produces water, carbon dioxide, and energy in the form of ATP. This chemical process is happening all the time in your cells. The chemical process is called aerobic respiration (ayr-ROH-bik res-pur-AY-shun).

Did You Know?

Aerobic means “with oxygen.” Aerobic exercise refers to exercise in which your muscles are getting enough oxygen. When your muscle cells don't have enough oxygen, they get energy by a process that produces acidic wastes. These acidic wastes can give you a muscle cramp!

The process of aerobic respiration can be written like this.

Oxygen + Glucose Carbon dioxide + Water + Energy

Or it can be written like this.

6O2+C6H12O66 CO2+6H2O+36ATP

Did You Know?

The symbol for carbon dioxide is CO2.

The symbol for water is H2O.

So your cells obtain energy when sugar and oxygen react in a specific way. But you probably noticed the reaction produces carbon dioxide and water, too. You know what water is. And you know that water is very important to living things. But did you also know that two-thirds of your body is water? You exhale water as a vapor when you breathe. You can see the exhaled water vapor when it's chilly. You perspire excess water produced in your cells when you exercise. And you also get rid of excess water when you urinate.

Figure 2.5 About 23 of the human body is water! But don't be confused by the drawing. You are not like a glass that fills from the bottom to the top. Instead, the water is distributed throughout your body.

The other product from the reaction of sugar and oxygen is carbon dioxide (CO2). There is a lot of CO2 in the air you exhale. You and other animals need to get rid of CO2 because it is a waste product of aerobic respiration. However, plants need CO2 to live and they absorb it easily.

The energy released when sugar reacts with oxygen can't just bounce around. It has to be stored somewhere. The energy is stored in a chemical compound called ATP (or Adenosine Triphosphate). Your cells can make ATP molecules. When you eat, you provide sugar for your cells. When you breathe, you provide oxygen for your cells. In your cells, the sugar and oxygen combine and the energy released is stored in bonds that hold the atoms together in molecules of ATP. ATP is not energy itself. It is a storage place for energy. When energy is needed to do something, your cells break down ATP and the stored energy is used. For example, your cells break down ATP to get the energy to move a finger.

Did You Know?

You may be familiar with CO2 as the gas that puts the fizz in soft drinks.

Figure 2.6 A carbon dioxide molecule, CO2, is made up of one atom of carbon and two atoms of oxygen. A water molecule, H2O, is made up of one atom of hydrogen and two atoms of oxygen. Bonds hold the atoms together. And energy is stored in those bonds.

Figure 2.7 ATP is a molecule that stores energy in cells. Cells use the energy released when ATP breaks apart.

Figure 2.8 Trace the path by which the energy in food is transported from your mouth to your finger so that your finger can move.

Okay, now you know that your body needs food to produce energy. But where does the food you eat get the energy that it passes along to you? Let's think back to breakfast and those eggs. The eggs came from a chicken. The chicken needed energy to produce and lay the eggs. To get that energy, the chicken ate something, too. If it was a free-range chicken, it might have eaten a grasshopper that jumped into its barnyard. As the chicken digested the grasshopper, the chicken's circulatory system carried tiny food molecules and oxygen to the chicken's cells to make ATP for energy. This process is like the one taking place in your body that you read about earlier.

Think about the last time you were exercising so hard that you were panting and gasping for breath. Describe what it felt like in your mouth, your throat, your chest and lungs. Now describe what you imagine is happening to the oxygen and carbon dioxide as it moves in and out of your body.

Figure 2.9 Energy is passed from organism to organism in the form of food.

Did You Know?

Plants capture sunlight with colored pigments the most common of which is green chlorophyll. However, some algae and bacteria use pigments that are blue, red, brown, and even yellow!

So the chicken ate the grasshopper. But where did the grasshopper get the energy to live, grow, and hop into that barnyard? The grasshopper ate something that was full of energy, too, such as grass or leaves. As you can probably see, there is no simple answer to the question, “Where did you get the energy to move your finger?” The energy came from the egg that came from the chicken, which ate the grasshopper, which ate the grass. But we still don't have an answer to the question, because the story doesn't end with the grass. Grass can't grow without energy. Where did the grass get the energy that was passed on to the grasshopper?

If oxygen and sugar are the only things required to provide energy to move our muscles, why do we bother to eat other foods? Why do you think we eat other foods, such as potatoes, pasta, meat, ice cream, pizza, and spinach?


Grass is not like you, the chicken, or the grasshopper. Grass can't eat something to get the energy it needs to grow. Grass has to get the energy it needs in another way. Like other plants, grass captures energy from the sun by a process called photosynthesis (foh-toh-SIN-thuhsis). In photosynthesis, a plant uses sunlight, water, and carbon dioxide to produce sugar and oxygen. Now read that sentence again. Then read this sentence: Your cells use sugar and oxygen to produce energy, carbon dioxide, and water. How do you think those two processes relate to each other?

What Do You Think?

You might say that you have the energy to wiggle your finger because the sun shines. Read the quote from Steve Van Matre again: “Each living thing is a spark of sunlight energy, a crystal bead in the net of life.”

What do you think the author meant when he wrote that sentence?

Photosynthesis and Respiration Play Put on a play about photosynthesis and respiration. Brainstorm a list of roles. People will need to play the roles of the “Sun,” “Sugar,” and “Carbon Dioxide.” Think of other roles in the processes of photosynthesis and aerobic respiration. Write a script. Design and create signs, props, or costumes so that the audience knows what each person represents. Now act out the play!

Figure 2.10 Notice the connection between photosynthesis and respiration. Can you find a beginning or end in this cycle?

Photosynthesis uses energy to make sugar and oxygen. Aerobic respiration uses sugar and oxygen to make energy. Photosynthesis is the opposite of aerobic respiration. Plants produce what animals need. Animals produce what plants need. Follow this cycle shown in Figure 2.10.

  • During what parts of the day does respiration occur in a plant cell? Explain your reasoning.
  • Can plants photosynthesize in the dark? Explain your answer.
  • Give an example of energy used by a living organism that cannot be traced back to the sun.

We began this section by asking how you get the energy you need to move your finger. Now you know that the energy that moves the muscles in your finger passes through a long chain of organisms and events. You got the energy by eating food such as an egg that came from an organism, which ate another organism, which ate a plant that captured the sun's energy through photosynthesis. Ecologists call this series of events a food chain. A food chain is a description of the path by which energy gets from the sun to an animal so that it can move, grow, and reproduce. The food chain in Figure 2.11 is just one of the possible food chains that leads to you!

Figure 2.11 Trace the movement of energy through this food chain.

Activity 2-1: Draw a Food Chain


What is the source of energy in your breakfast? One way that you are connected to various parts of your environment is by the many different organisms that provide you with the energy in food. In this activity you analyze what organisms contributed to your breakfast by creating a food chain. You also create a food chain based on the breakfast of a bird of prey.


  • Colored marking pens, pencils, or crayons
  • Paper
  • Activity Report


Step 1 Make a list of all the things you ate for breakfast this morning. If you didn't eat much, use a big breakfast that you have eaten recently. Try to include something that came from an animal-such as bacon or milk. Write your list on your Activity Report.

Step 2 Draw yourself at the top of your paper eating breakfast. Pick a food item that came from an animal and draw it under your picture. Draw the plant or animal that was the source of the food under the breakfast item. Then determine where that plant or animal got its energy. Was it from another animal, a plant, or the sun? Draw its source of energy. Continue doing this until you trace the original source of energy in the food chain back to the sun.

Step 3 Follow Step 2 as you determine and draw the steps to the sun for each item that you ate for breakfast. (Hint: Take a look at Figure 2.11.)

Step 4 Look at Figure 2.12 to see one kind of food a Peregrine falcon and a Golden eagle eat. Follow Step 2 as you draw a food chain leading up to a bird of prey of your choice. You can use the birds in Figure 2.12 or another one you may know about such as an owl, pelican, or hawk. List the foods your chosen bird might eat. Then list, on your Activity Report, the foods each of the prey might eat.

Step 5 How many steps are in each of your food chains? Which food chain that you drew has the most steps? Write your answers on your Activity Report.

Long ago, when the land was beginning to be inhabited by living organisms, which do you think came first-animals or plants? Explain your answer.

Figure 2.12 Golden eagles (top left) and owls (top right) are birds of prey that capture and eat small animals.

Did You Know?

Golden eagles are relatives of the bald eagle. They live throughout much of the United States but are more common in the west. These eagles favor open habitats and feed on small mammals, snakes, and dead animals. They have been protected since 1962. Before they were protected, more than 20,000 eagles had been killed in the previous ten years by sheep ranchers. These ranchers mistakenly thought that the eagles attacked their livestock. Currently, golden eagles are threatened. Many are killed by power lines and by poisoned bait intended for coyotes.

Review Questions

  1. Where do cells get their energy?
  2. What process do plants use to capture the sun's energy?
  3. How are respiration and photosynthesis related?
  4. Where do plants get their energy? Where do animals get their energy?
  5. What is the original source of almost all energy used by living things?
  6. What is a food chain?

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Date Created:
Feb 23, 2012
Last Modified:
Aug 06, 2016
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