VALREXA · Free student activity
How a peptide message reaches a cell
A short supplementary learning activity, not a medical exercise. No account or newsletter signup is needed. Write on the printed worksheet or in your own notes; this page does not collect answers. Your instructor will tell you whether to submit your work within your course.
Your learning goals
- Locate the receptor: Identify where a cell-surface receptor receives an external message.
- Trace the signal: Distinguish the messenger outside the cell from the relay inside it.
- Explain selectivity: Explain why exposure to the same messenger does not guarantee the same response.
Start with the idea
A cell-surface receptor can bind a signaling molecule outside the cell and transmit a signal across the membrane without that molecule entering the cell (OpenStax, cell-surface receptors). Intracellular signaling pathways can then produce a cellular response, and differences in the components present in different cells can affect that response (OpenStax, propagation of the signal).
Watch the narrated 90-second VALREXA cell-signaling video, or read its transcript on the same page. If internet access is unavailable, use the explanation above and the model below.
Draw the route
Draw a cell boundary and label these four items: peptide messenger P, a cell-surface receptor, an intracellular relay, and the cellular response. Add arrows showing the route of the message, and make clear which parts are outside and inside the cell.
If drawing is not accessible or practical, write an ordered description of the same route and label each part as outside, in the membrane or inside the cell. Either format is acceptable.
Test a simple model
These cells are fictional, not results from an experiment. In this model, a response requires all three conditions: P is present outside the cell, a functional receptor for P is present, and the required intracellular relay is working. There is no alternative pathway in the model.
| Cell | P present outside? | Functional receptor for P? | Required relay working? | Response: yes or no? |
|---|---|---|---|---|
| A | Yes | Yes | Yes | |
| B | Yes | No | Yes | |
| C | Yes | Yes | No | |
| D | No | Yes | Yes |
Compare A and C with a partner. What does the comparison tell you about the difference between receiving a message and carrying it forward?
Correct the explanation
A student says, “For a cell-surface receptor to work, the peptide must pass through the membrane and enter the nucleus.” Using the explanation and your drawing, rewrite that statement correctly in one or two sentences.
Exit question
Why is being exposed to a peptide messenger not enough, by itself, to guarantee a response in this model? Include the receptor and the relay in your answer.
Optional feedback
This is for improving the activity, not grading you. If your instructor shares feedback with VALREXA, omit your name, email address, student number and personal information.
Clarity: Circle one: 1 = very unclear / 2 / 3 / 4 / 5 = very clear.
One improvement: Which explanation, question or instruction should be clearer?
Original VALREXA worksheet, prepared November 11, 2026. The fictional model and questions were created for this activity; linked textbook passages and illustrations are not reproduced.
Textbook sources:
https://openstax.org/books/biology-2e/pages/9-1-signaling-molecules-and-cellular-receptors
https://openstax.org/books/biology-2e/pages/9-2-propagation-of-the-signal
Video and transcript: https://valrexa.com/originals.html#cell-signaling
Activity: https://valrexa.com/study/peptide-signaling-activity.html