Imagine for a moment that you stay after school one day to clean up the classroom. While cleaning, you move some plants away from the sunny windows. A week later, you remember to move the plants back. You notice something strange has happened. Instead of standing upright, the plants appear to be leaning toward the windows! Why? Plants need sunlight to survive. If a plant is moved away from sunlight, special cells in the plant help it turn back toward the Sun. The Sun’s energy allows plants to produce their own food. Plants then use this food energy to grow and reproduce. But not all organisms can make their own food. How do other organisms get their food energy? Do they get it from the Sun? Where do all food chains and food webs get their energy? All of the food energy that passes between organisms comes from the Sun. You might be wondering how this is possible. After all, humans can’t eat sunlight! Plants and other organisms that use sunlight first absorb it and then use that energy to make their own food. That energy passes to other organisms that eat plants. For example, grass uses sunlight to make food. A deer gets energy by eating the plants. After that, a wolf gets energy by eating the deer. The movement of food energy from one organism to another is called a food chain. Take a look at the food chain on the right. The arrows show how food energy is passed from one organism to the other. The plants use energy from the Sun to make their own food.

Suppose a dust storm blocked sunlight in your town for several weeks. What do you think would happen to the plants in the area? What would happen to the organisms that depend on the plants for food? Why?

You might think the arrows in a food chain show which organism is eating another organism. The arrows actually show how energy is moving through the food chain.

The mouse gets energy by eating plants. The hawk gets energy by eating the mouse. Organisms like mushrooms also help move energy through a food chain. In this food chain, they break down material from the hawk or other organisms once they die. Some of the material becomes part of the soil that is later used by plants. You’ll learn more about this process later in the lesson.

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How are plants and animals involved in the cycling of carbon dioxide and oxygen? Carbon dioxide and oxygen are two gases that are very important to life on Earth. Carbon dioxide is found in the air. The cells of organisms, including humans and other animals, also produce it. Carbon dioxide is released from the body when organisms breathe out or exhale. Plants need carbon dioxide to make their own food, and they release oxygen in the process.

The air we breathe is made up of more than just carbon dioxide and oxygen. About 78% of it is actually nitrogen! Oxygen is only about 21% of air. Carbon dioxide and other gases, such as argon, make up the rest (1%).

organisms: living things

cells: the basic units of living things

As the cheetah breathes, he exhales carbon dioxide, which provides fuel for the plants around him to produce their own food.

The cycle of carbon dioxide and oxygen on Earth is dependent on plants and animals. Plants provide the oxygen that animals and other living things need to survive. Animals and other living things provide the carbon dioxide that plants need to make their own food.

Oxygen is also found in the air. Organisms like plants and green algae make their own food. During this process, the organisms release oxygen into the air. Many living things get oxygen from the air when they breathe in or inhale. Oxygen helps the body’s cells function normally.

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What are the different parts of a food web? A group of overlapping or connected food chains is called a food web. A food web can be big or small. It can contain many different types of plants and animals or just a few. Whether a food web is big or small, the organisms fall into one of two categories: producers or consumers. • Producers: Producers are organisms that get their energy directly from the Sun. Their cells are able to turn sunlight into food through a process called photosynthesis. In photosynthesis, producers combine carbon dioxide, water, and sunlight to produce oxygen and sugar (their food). Other organisms get energy by eating producers. Have you ever eaten lettuce or any other vegetable? If so, you have eaten a producer! The lettuce plant converts sunlight into food your body uses as fuel. Producers are very important to life on Earth. Without them, other organisms would not survive. • Consumers: A bald eagle is an example of a consumer. It cannot directly use the Sun’s energy to make food. As a consumer, it has to eat or consume other organisms for energy. A consumer may eat producers (such as grass) or other consumers (such as a mouse or a bald eagle). Animals, fungi, and some bacteria are types of consumers. Consumers that eat only plants are called herbivores. Consumers that eat only animals are called carnivores. If a consumer eats both plants and animals, it is called an omnivore. Some consumers are called decomposers. Mushrooms are decomposers. This group of consumers eats only dead organisms. They break down the nutrients in the dead organisms and return them to the food web. They may eat dead producers or consumers. Suppose a tree dies in a forest. Bacteria and fungi like mushrooms consume the tree and return the nutrients in the tree to the soil. The grass in the forest absorbs those nutrients and uses them to grow.

carbon dioxide: a type of gas found in Earth’s atmosphere

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What is the relationship between plants and animals? Where is oxygen produced? Where is carbon dioxide produced? Plants and animals are linked to each other through the carbon dioxide and oxygen cycle. Plants produce oxygen, a gas that animals and other living things need. Animals and other living things produce carbon dioxide, a gas that plants need to make their own food. How are oxygen and carbon dioxide produced in plants and animals? Plants make their own food from sunlight in a process called photosynthesis. In photosynthesis, sunlight, water, and carbon dioxide are combined in special cells inside the plant. Glucose is one product of photosynthesis. Glucose is a sugar that plants can use for energy. Another product of photosynthesis is oxygen. Plants release oxygen as a waste product. Plants and other producers, such as green algae, are the source of the oxygen in the air we breathe.

Animals and other living things release carbon dioxide when they exhale. Where does the carbon dioxide come from? When organisms eat, their bodies break food down into very small parts. The process of breaking down food produces carbon dioxide and other substances. Too much carbon dioxide in the body can be poisonous, so organisms release it back into the air. Decomposers, such as mushrooms and bacteria, also release carbon dioxide when they break down dead material.

Plants produce more than just oxygen. They also produce carbon dioxide, just like other organisms. Plants break down their food, the glucose (or sugar) they make during photosynthesis, to fuel their cells just like animals do. The process of breaking down this sugar makes carbon dioxide, which the plants release into the air.

producers: living things that make their own food

decomposers: living things that break down dead organisms and waste

Take a look at the photographs below. Which photograph shows a way to support a healthy balance in the carbon dioxide and oxygen cycle? Explain your answer.

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What is the significance of the carbon dioxide and oxygen cycle to the survival of plants and animals? The carbon dioxide and oxygen cycle is critical to life on Earth. Humans, and most other organisms, need oxygen to survive. When we inhale, oxygen moves from our lungs into our blood. Oxygen travels through the blood to all the cells in the body. The cells use oxygen to complete important jobs. For example, you are using oxygen right now as you read this sentence. The muscles that control your eyes use oxygen. Without oxygen, you could not use any of your muscles. In fact, our cells die quickly if they do not receive oxygen. That is why it is so important to help someone who cannot breathe by providing them with oxygen. Plants and other organisms that perform photosynthesis rely on animals for carbon dioxide. Every time you exhale carbon dioxide, you are providing a plant with a building block it needs to make its own food. Looking to the Future: Carbon Dioxide and Global Warming As with any cycle, it is important that its parts stay balanced. Some scientists worry that human activities are adding too much carbon dioxide to the air. Humans burn certain materials called fuels in order to generate electricity. Burning fuels can add a lot of carbon dioxide to the air. Activities like cutting trees and clearing forests can also be harmful. These activities reduce the number of plants that can take in carbon dioxide from the air and give off oxygen. Scientists think that too much carbon dioxide in the air might be increasing Earth’s temperatures. This is a condition known as global warming and could cause problems by upsetting Earth’s natural balance. So what can we do? The first step may be to use fuels that do not produce carbon dioxide. Wind and solar power are two ways to generate electricity without burning fuels.

Solar panels like the ones on this house can capture energy from the Sun and use it to generate electricity.

Wind turbines are devices that capture the energy in wind and use it to produce electricity.

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Study the images in the table below, then decide if the organism in each image is a producer or a consumer. Write your answer in the first column of the chart. If the organism is a consumer, decide if it is a carnivore, herbivore, or omnivore. Write your answer in the second column of the chart. Finally, think about how each organism gets its food energy. Write your answer in the last column of the chart. The first two answers have been completed for you.

Organism Is the organism a producer or a consumer?

Is the organism a carnivore, herbivore or omnivore?

How does the o r g a n i s m g e t food energy?

Strawberry Bush

producer not a consumer

Deer Eating Grass Snake Eating a Frog Raccoon Eating Plants and Animals From a Trash Can

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The Carbon Dioxide and Oxygen Cycle at Home To explore the carbon dioxide and oxygen cycle, go on a photo safari of your neighborhood. Work with your child to find examples of organisms in your neighborhood that produce oxygen. Take a photograph of as many examples as you can find. If a camera is not available, have your child draw sketches and write brief descriptions. Look for organisms that perform photosynthesis, such as trees, houseplants, vines, moss, and grasses. Next take photographs of as many carbon dioxide producers as you can. To find examples of carbon dioxide producers, look for organisms that consume and break down food, such as humans, dogs, birds, or mushrooms. Remember, plants also produce carbon dioxide. When you have finished finding examples, you may wish to print the photographs. This will allow you to spread them out and draw connections between them to demonstrate how carbon dioxide and oxygen cycle through your neighborhood ecosystem. Here are some questions to discuss with your child:

• Did you find more examples of oxygen-producing organisms or carbon dioxide-producing organisms?

• How are plants a unique member of the carbon dioxide and oxygen cycle?

• How could you add more oxygen to your neighborhood ecosystem?

Squirrels are common in many ecosystems. They are a good example of a carbon dioxide producer and oxygen consumer.

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