Follow the energy, one step at a time

Full explanation

Learning goal: Explain how sunlight enters a typical ecosystem through producers, how photosynthesis changes energy into a usable stored form, and how feeding transfers that energy. This lesson targets Ohio 5.LS.2 and its amplification. Plan 50–60 minutes after the food-web lesson. Use arrows, colored pencils, and paper plant and animal cards.

Organisms need energy for the processes that keep them alive. Growing, repairing structures, transporting materials, and moving all involve energy. A resting animal still uses energy, and a plant also uses energy for life processes. Energy is not needed only when an organism visibly moves.

For ecosystems, the major source of energy is sunlight. Producers capture some light energy through photosynthesis and transform it into chemical energy stored in sugars. They use carbon dioxide and water to make these sugars. Minerals from soil help plants grow, but soil is not the main source of the energy stored in their food. Plants make food; their roots do not simply take ready-made food energy from the soil.

A producer can use energy from its sugars for its own life processes or store some in its tissues. When a consumer eats plant material, energy stored in that material can pass to the consumer. When another consumer eats that animal, some stored energy can pass again. Food-web arrows therefore represent energy transfers as well as feeding relationships.

Organisms use energy at each stage, and energy is dispersed to the surroundings as heat. Energy is not destroyed, but not all the energy at one stage remains in food available to the next consumer. This lesson does not assign a fixed transfer percentage: the supplied model has no measurements supporting one. Matter such as nutrients can be recycled through decomposition. The energy pathway is different; decomposers do not send food energy back to the sun for reuse.

Additional worked examples

Example 1: Trace energy to an owl. Use sunlight → grass → mouse → owl. First, light reaches grass. Second, photosynthesis transforms captured light energy into energy stored in sugars. Third, a mouse eats grass seeds and obtains some of that stored energy. Fourth, an owl eats the mouse and obtains energy from its tissues. The owl does not photosynthesize, and the grass does not create energy from nothing.

Example 2: Explain growth without visible movement. A seedling grows a new leaf while rooted in place. The growth still requires energy. Once leaves are functioning, the plant can capture light and store energy in sugars, then use that energy in its processes. A newly germinating seed can initially use stored material in the seed. “It does not walk, so it needs no energy” confuses visible movement with all the processes of life.

Example 3: Compare energy and materials. A fungus breaks down dead grass. The fungus obtains energy from material that was once part of the grass. Some nutrients become available in the environment again and may be taken up by plants. Draw a nutrient-return arrow separately from energy-transfer arrows. Returning nutrients to soil does not make soil a replacement for sunlight as the major energy source in this model.

Investigation discussion: Two similar seedlings receive equal water and suitable soil, but one receives much less light. Reduced light may reduce the energy captured through photosynthesis. To investigate, keep plant type, starting size, water, container, and temperature as similar as possible. Measure growth over time and repeat with several plants. One plant pair cannot establish a universal rule, and plants can temporarily use stored energy.

Independent practice and scoring

Guided explanation: Point to each arrow in sunlight → pond plant → snail → fish. Name where energy changes form and where feeding transfers stored energy. Photosynthesis in the plant is the light-to-stored-chemical-energy step; the snail and fish obtain energy by eating.

Independent tasks: (1) Draw and explain a three-organism food pathway beginning with sunlight. (2) Correct “Plants get their food energy from soil.” (3) Explain why an animal uses energy while resting. (4) Explain why decomposers recycling nutrients does not mean energy returns to the sun.

Sample responses: A plant captures sunlight and stores energy in sugars; a plant-eating animal and its predator receive stored energy through feeding. Plants use sunlight to make sugars, while soil supplies water and minerals. Resting animals still maintain and repair their bodies. Decomposers transfer and use energy from dead material; nutrients can return to the environment, while energy disperses as heat. Award 2 points per task: accurate science plus a connected explanation, up to 8.

Support: Offer the words capture, transform, store, transfer, and use, then ask students to put them into a causal explanation. Extension: Compare two paths through a food web and identify the evidence needed to compare their actual energy transfers. Do not infer quantities from the number of arrows alone.