Touch alone doesn't signal. It takes the right protein.
- The idea
- Predict
- See it
- Why
- Check
The idea
“If a cell touches other cells or gets crowded, that contact is the signal that sets off its response.”
It's called contact-dependent signaling, so the contact sounds like the message. The touch only brings the real signal close enough to bind.
The short version
The mistake: If a cell touches other cells or gets crowded, that contact is the signal that sets off its response.
What's actually true: A cell responds to contact only when its receptor binds a specific signal protein on the surface of the cell it touches.
- When cells signal by surface contact, the signal is a protein fixed to the sending cell's surface. It never leaves that cell, so the two cells have to touch.
- A receptor binds only a signal with the right shape. Proteins on other cells don't fit it, however tightly the cells are packed.
- So touching is needed but isn't enough. Cells that touch without the signal protein don't respond.
On the exam: Ask which protein on the other cell binds the receptor. Contact without it sends no signal.