In this lesson, students gather evidence to understand that organisms in an ecosystem are tied together by their need for energy. In Part I, students read an interactive story that explains how the Sun's energy is captured by producers and passed along to other consumers in the food chain. Then they watch a video on decomposers, organisms that get their energy by feeding on dead organisms and the wastes of living things. They learn that decomposers break down dead organisms and wastes and release the nutrients they contain into the soil, where they are again available to the roots of plants (producers). In this way, decomposers play an important role in recycling nutrients and getting rid of waste.
In Part II, students explore an ocean ecosystem and construct a food chain to show how energy flows through this environment.
- Identify the producers, consumers, and decomposers in an ecosystem
- Draw a food chain to illustrate how energy flows through an ecosystem
- Describe how energy derived from the Sun is used by plants to produce sugars and is transferred within a food chain from producers to consumers
- Recognize that events that affect one species in an ecosystem will have either positive or negative effects on the other organisms in its food chain.
Grade Level: 3-5
- Four 45-minute blocks
- Handout: Where Do Plants Get Their Energy? Document
- Slice of bread
- Knife to cut bread
Before the Lesson
- Make copies of the handouts for each student.
Part I: Where Do Plants and Animals Get Their Energy?
1. Ask students what they had for lunch. List their responses on the board. Write the words plant and animal on the board. Ask students to sort the food items into these two categories. For example, a lunch consisting of a cheeseburger, fries, and milk would be sorted this way:
2. Ask students:
- Why do we need to eat?
- Where do cows get the energy they need to build muscle and produce milk?
- Where do plants get the energy they need to make leaves, like the lettuce we eat?
Listen to their ideas, and then explain that they are going to explore an Energy Flow Web activity that will answer these questions.
3. Assign each student a partner, and distribute copies of the Handout: Where Do Plants Get Their Energy? (PDF) handout. Have students explore the Energy Flow Web activity and use the information it contains to answer the questions on the handout.
4. Assess students' understanding of the Energy Flow Web activity by reviewing their answers to the questions on the handout:
- Where do plants get the energy they need to grow?
- What do plants use the sun's energy to manufacture?
- What do plants use most of their energy for?
- How much of the energy that the plant captures through photosynthesis ends up stored as starch in the kernel?
- For what does the cow use the energy from the corn?
- How much of the energy stored in the corn gets passed on to you in burgers?
- For what do you use the energy in the burgers?
- How would eating more plants help us better feed the many people in the world?
- What else besides energy do we get from plants and animals? When we eat them? (Answers: vitamins, minerals, and other nutrients needed to build body parts and keep the body running smoothly)
5. Students may have difficulty grasping the idea that only a small part (10%) of the energy captured or eaten at one step in the food chain is available to organisms at the next step in the food chain. To reinforce this idea, do the following demonstration:
6. Hold up a slice a bread and tell students that one slice of bread contains approximately 100 calories of energy. If they eat the bread, they will get all 100 calories of energy to use for moving, growing, and making heat.
Now cut the bread into 10 pieces. Explain that if a cow eats the bread instead, and students then eat a burger made from that cow's meat, they would get only 10 percent of the energy from that slice of bread. (Hold up one of the bread pieces.) That's because the cow uses 90 calories or 9/10 (90%) of the energy in the bread to move, grow, and make heat. Only 10 calories or 1/10 (10%) of the energy from the bread gets stored in the cow's meat and is available to students when they eat the hamburger.
7. Ask students:
- What happens to all the trash you throw away at lunch? Where does it go? What happens to it there?
- What happens to the energy stored in uneaten food and in dead plants and animals?
8. Have students watch the Decomposers video.
9. Discuss the following questions:
- What do decomposers eat?
- What do decomposers do with the energy they get from eating dead things and waste material from living things?
- What important role do decomposers play in our environment? (Be sure to point out the role decomposers play in returning nutrients back to the soil.)
- Based on what you learned in theEnergy Flow Web activity, what percentage of the energy stored in dead plants and animals do you think is available to the decomposers that eat them?
- What rule can we make about the percentage of energy that's passed on from one organism to another in a food chain?
10. Summarize by drawing a food chain that shows how energy in an ecosystem comes from the Sun and flows from producers to consumers to decomposers. For example:
Sun -> grass -> rabbit -> fox -> bacteria (decomposers) feeding on dead fox
Then review the following statements:
- Organisms need energy to move, grow, and keep warm.
- Energy comes from the Sun, gets captured by plants, and is then transferred from organism to organism.
- Energy is lost each step of the way as heat or energy needed for the chase.
Part II: More on Food Chains
11. Review the following terms: ecosystem, producers, consumers, and decomposers, food chain.
12. Have students watch the video Beneath the Waters of Cocos Island and record the names of the producers and consumers mentioned in the video.
13. Have students draw pictures of the living things mentioned in the video. On a bulletin board construct a huge food chain, using these organisms and others that might be found in the ocean, to show how energy flows through this ecosystem.
14. Ask students what effect might it have on the system if one of the producers, consumers, or decomposers disappeared? Discuss as a class.