Lesson
Energy leaks; nutrients come back around
Food webs are maps of eating. Cycles are maps of atoms.
Learning goals
- Explain why food chains are short in energy terms.
- Describe a human disruption of a nutrient cycle.
Explanation
Producers fix energy. Consumers take some of it by eating. At each step, much energy leaves as heat and waste, so few links can be supported. A pyramid of energy is a budget, not a popularity chart. Removing a keystone predator can unravel a community even if that predator was never the most numerous animal.
Carbon, nitrogen, and water cycle. Burning fossil carbon raises atmospheric CO2 faster than many sinks can absorb it. Fertilizer runoff can overfeed a waterway and trigger oxygen crashes. Those are matter stories. Do not mix them with “the sun running out,” which is not the classroom mechanism.
Key terms
- Trophic level. A feeding step in a food chain, such as producer or primary consumer.
- Keystone species. A species whose effect on community structure is large relative to its abundance.
Common mistakes
- Saying energy is recycled the way carbon is.
- Assuming the biggest animal is automatically the keystone.
Practice
Original Marlow Works items. Check the answer explanation after you try.
A cell’s surface-area-to-volume ratio is important because it affects:
Original Marlow Works item — not a College Board question.
Take your time—this is practice, not a test.
Answer explanation
Why is “energy cycles through an ecosystem” a sloppy sentence?
Answer. Energy flows from the sun (or another source) and leaves as heat; chemical nutrients cycle.
The two accounting systems are different, and exam explanations treat them that way.
Related resources
External links with reuse status. Marlow Works is independent and does not copy restricted exam or textbook material.
Official / link only
AP Biology course page
Official eight-unit framework, science practices, and lab resources.
College Board · All rights reserved · accessed 2026-10-01
Open sourceReference only — do not copy
OpenStax Biology 2e
Biology reference with commercial-use restrictions on the current publisher page.
OpenStax · CC BY-NC-SA 4.0 with additional restrictions · accessed 2026-10-01
Link only unless permission is obtained.
Open source