The membrane can't tell what the cell needs
- The idea
- Predict
- See it
- Why
- Check
The idea
“The membrane lets in the molecules the cell needs, like glucose, and keeps out the ones it doesn't.”
Cells take in glucose all day, so it's easy to picture the membrane as a gate that knows its job. The lipid only knows chemistry.
The short version
The mistake: The membrane lets in the molecules the cell needs, like glucose, and keeps out the ones it doesn't.
What's actually true: The lipid bilayer lets small nonpolar molecules through and holds back ions and large polar ones, whatever the cell needs.
- The middle of the membrane is a hydrophobic layer of fatty acid tails. A molecule crosses easily on its own only if it can dissolve there.
- Nonpolar molecules like O2, CO2 and steroid hormones pass right through. Ions and larger polar molecules like glucose barely cross.
- The cell chooses what comes in with transport proteins, like glucose carriers and K+ channels.
On the exam: Small and nonpolar crosses the bilayer on its own. Ions and big polar molecules need a protein.