Building directly on Lesson 17, learners connect the principles of kinetic and potential energy to all living things. Starting with the student's own body, the lesson traces the path of energy from sunlight to plants to animals to heat and ultimately out into space. Students discover that every food chain on Earth is really an energy chain, that life depends on a continuous supply of incoming solar energy, and that fossil fuels and renewable energy sources are both forms of stored or incoming sunlight.

Key Ideas

• Every living thing needs a continuous supply of energy to keep going. Without fresh energy coming in, life stops.
• Sunlight is kinetic energy entering Earth's system. Plants capture it through photosynthesis and store it as potential energy inside their bodies.
• Animals get energy by eating plants, either directly or through food chains. Every food chain on Earth traces back to a plant, and every plant traces back to the sun.
• When animals digest food, they release stored potential energy as kinetic energy that powers muscles, nerves, the brain, and every body process.
• Used energy becomes low-level heat. Heat flows toward cooler places, eventually leaving Earth into outer space. Life cannot recycle this energy, which is why it depends on fresh sunlight every day.
• Fossil fuels are stored solar energy from ancient plants and animals buried in the Earth millions of years ago. Burning them releases that ancient potential energy.
• Renewable energy sources such as solar cells, wind turbines, and water-powered generators are all different ways of capturing the same solar energy that is arriving today.

Vocabulary

• Photosynthesis: The process by which plants use sunlight, water, and carbon dioxide to build their bodies and store energy as chemical potential energy.
• Food Chain: The path that energy follows as it moves from plants to the animals that eat them and then to other animals further up the chain.
• Fossil Fuels: Coal, oil, and natural gas. These formed from ancient biological material buried in the Earth for millions of years. They hold potential energy originally captured from the sun.
• Renewable Energy: Energy from sources that are constantly replenished by the sun: solar cells, wind turbines, and hydroelectric generators.

Discussion Questions

• Where does the energy in your body come from? Can you trace it all the way back to the Sun?
• Why can't plants just make energy out of nothing? What are they actually doing with sunlight?
• Imagine there were no sunlight for a year. What do you think would happen to life on Earth?
• A plant gets eaten by a rabbit, and the rabbit gets eaten by a fox. Where did the energy that powers the fox's muscles originally come from?
• What happens to energy after an animal uses it to run, think, or move? Where does that energy go?
• Why does Earth need a constant supply of sunlight if energy can't simply be recycled forever?
• Fossil fuels contain energy from ancient living things. How is burning gasoline similar to releasing stored sunlight?
• What do solar panels, wind turbines, and hydroelectric dams have in common? How are they ultimately connected to the Sun?
• If you could invent a new way to capture energy from sunlight, wind, or water, what would it look like?
• Which seems more surprising to you: that plants can turn sunlight into stored energy, or that your body can turn food into the energy needed to think and move? Why?

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