13.3 · Energy Flows One Way

From Ohio 5th Grade Science Power Workbook · Chapter 13 · Lesson 13.3.

Big idea: Energy enters an ecosystem as sunlight and passes from organism to organism in food. It moves in one direction and never comes back around.

Learning goals: Describe the path energy takes through an ecosystem

Explain why energy does not go around in a circle

Tell the difference between what happens to energy and what happens to nutrients

In as light, along in food, never back

Sunlight reaches the grass. The grass stores the energy in food. A rabbit eats the grass. A fox eats the rabbit.

At every step, the living thing uses some of the energy for its own processes. Some of that leaves its body as heat, warming the air around it.

Energy comes in as sunlight. It moves along in food. It is passed up the chain and never sent back. It travels in one direction.

Nutrients come back. Energy does not.

When a leaf falls, decomposers break it down. The nutrients in the leaf go back into the soil, and the tree can use them again.

The energy in the leaf is different. Decomposers use it for their own processes. Nothing sends it back to the tree or to the sun.

That is why an ecosystem keeps needing new sunlight. Without it, the stored energy runs down.

Follow one leaf

A fallen oak leaf holds both energy and nutrients. An earthworm eats it and uses the energy to live. The nutrients pass through the worm and back into the soil.

Next spring the oak takes those nutrients up again. The energy was used long ago and is not coming back.

Worked examples

Question: A student draws an ecosystem diagram with an arrow from the decomposers back to the sun and labels it 'energy returns'. What is wrong with the drawing?

Steps: Follow what the decomposers do with energy. They use it for their own life processes.

None of it travels back to the sun. The sun sends out new light every day; it does not receive energy from Earth's living things.

The arrow that does go back is for nutrients — from decomposers into the soil, not to the sun.

Answer: Energy does not return to the sun. Decomposers use the energy for their own processes. The student may be thinking of nutrients, which decomposers return to the soil.

Common mistakes

Energy is not a circle

Water goes around. Nutrients go around. So it feels natural that energy should go around too, from the sun to the plants to the animals and somehow back to the start.

It does not. Energy flows through an ecosystem in one direction: in as sunlight, along in food, used by each living thing on the way. Nothing carries it back to the sun or back to the producers. New sunlight has to keep arriving.

Check your thinking: If a diagram or an answer has energy going back to where it began, it is wrong. Check the arrows: every energy arrow should point away from the sun.

Ohio connection

What happens to Ohio's fall leaves

Every fall, Ohio's maples and oaks drop tons of leaves. By the next summer most of them are gone.

Earthworms, insects and fungi broke them down. The nutrients went back into the soil for the trees to use again. The energy in those leaves did not — the decomposers used it, and nothing sent it back.

Think: A gardener says leaf compost 'gives energy back to the plants'. Explain what compost really gives back.

Remember

Light in, food along, never back

Say it as a path: light in, food along, never back. One way. Nutrients are the ones that go around.

Practice tip

Remember: Watch the arrows.

Tip: A question may show a diagram and ask what is wrong or what a missing arrow should be. Energy arrows point one way, away from the sun. An arrow back to the sun or back to the plants is the wrong answer.

Written practice

Describe the path energy takes through a meadow ecosystem, starting where it enters. Use the words sunlight and food, and say whether the energy ever comes back to the start.

Sample response: Energy enters the meadow as sunlight. The grass uses it to make food. When a rabbit eats the grass, the energy passes along in the food, and it passes again when a fox eats the rabbit. Each animal uses some of the energy to live. The energy never comes back to the grass or the sun, so the meadow needs a steady supply of new sunlight.

Scoring guide: 1 point: energy enters as sunlight and is captured by producers, then passes from organism to organism in food.

1 point: energy does not return to the sun or the producers; it moves one way, so new sunlight is needed.

Ohio asks students to model energy moving through an ecosystem. The model has a start, a direction and an exit.