AP Biology / Cell Communication and Cell Cycle
Lesson
Hearing a signal and turning it into work
Communication pathways have a receiving door, a hallway, and an action.
Learning goals
- Describe reception, transduction, and response.
- Give an example of negative feedback stabilizing a system.
Explanation
A signaling cell releases a ligand. A receiving cell has a matching receptor. Binding changes the receptor’s shape, which starts a cascade inside—second messengers, phosphorylation, and other relays. The response might be a gene turned on, an ion channel opened, or an enzyme activated.
Negative feedback reverses a change and steadies a set point, as when high hormone levels later reduce their own release. Positive feedback pushes a change farther, useful in rare bursts such as a clotting cascade. Do not call every loop “balance.”
Key terms
- Ligand. A signaling molecule that binds a receptor.
- Transduction. The relay of a signal from receptor to cellular response, often with amplification.
Common mistakes
- Stopping after “the hormone enters the cell” for a receptor that sits on the membrane.
- Using positive and negative feedback as synonyms for good and bad.
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 can two cells treat the same signal differently?
Answer. They may have different receptors or different downstream proteins, so the response set is not the same.
The signal is information. The receiving machinery writes the meaning.
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