Which cargo needs a vesicle
- The idea
- Predict
- See it
- Why
- Check
The idea
“A big molecule just needs a wide enough channel, and a small one could ride in a vesicle.”
You might picture a channel opening wide for a protein, or a vesicle carrying anything. Both pictures leave out size.
The short version
The mistake: A big molecule just needs a wide enough channel, and a small one could ride in a vesicle.
What's actually true: Small nonpolar molecules cross the bilayer. Ions and small polar ones mostly use proteins. Large cargo rides in vesicles.
- Small nonpolar molecules like O2, CO2 and steroids diffuse straight through the bilayer.
- Ions and polar molecules like glucose barely cross the lipid core, so they use channels or carriers. Even water moves mostly through aquaporins.
- Proteins, particles and whole cells are too big for any channel or carrier. They cross in vesicles, in by endocytosis and out by exocytosis. Both use energy.
- Neurons are one exception. They store small neurotransmitters in vesicles to release them in a burst.
On the exam: Big cargo rides in vesicles. Small nonpolar molecules cross the lipids. Polar or charged ones need a protein.