A protein can bind a molecule without moving it
- The idea
- Predict
- See it
- Why
- Check
The idea
“If a membrane protein grabs a molecule, it must be moving it across the membrane.”
Carrier proteins bind something and then move it, so that's the usual picture. Receptors bind too, and what they send in is a message.
The short version
The mistake: If a membrane protein grabs a molecule, it must be moving it across the membrane.
What's actually true: Many membrane proteins bind a molecule only to respond to it. A surface receptor relays a signal in, and its ligand stays out.
- A receptor has a binding site shaped for its ligand, the signal molecule. When the ligand binds, the receptor changes shape.
- That shape change reaches the part inside the cell. It switches on relay proteins or leads to a second messenger like cAMP.
- Carriers bind their cargo and move it through. Receptors and surface glycoproteins bind a molecule without carrying it through. So binding doesn't mean crossing.
On the exam: A bound molecule may still be outside. Ask where it ends up.