Neighbors can signal without secreting anything
- The idea
- Predict
- See it
- Why
- Check
The idea
“A cell signaling a neighbor must release a molecule that drifts over to a receptor on it.”
It's easy to picture every local signal drifting through the fluid. Cells that touch can pass one without releasing anything.
The short version
The mistake: A cell signaling a neighbor must release a molecule that drifts over to a receptor on it.
What's actually true: Touching cells can also signal directly, through gap junctions or plasmodesmata, or with membrane proteins that bind on contact.
- Paracrine signals, like growth factors, are secreted and diffuse to nearby cells. That's one route.
- Gap junctions in animals and plasmodesmata in plants join neighbors' cytoplasm. Ions and small molecules like cAMP pass through. Plasmodesmata pass some proteins too.
- In contact-dependent signaling, a signal anchored in one cell's membrane binds a receptor on a cell it touches. Antigen-presenting cells signal helper T cells this way.
- If the fluid alone can't trigger a response, or closing junctions stops it, the signal wasn't secreted.
On the exam: Before you call a signal secreted, ask whether blocking contact or a junction stops it.