Ions and large polar molecules need a protein to cross
- The idea
- Predict
- See it
- Why
- Check
The idea
“Glucose and Na+ are small and dissolve in water, so they slip right through the lipids.”
Glucose and Na+ are tiny and dissolve easily in water. The inside of a membrane is more like oil.
The short version
The mistake: Glucose and Na+ are small and dissolve in water, so they slip right through the lipids.
What's actually true: The bilayer's core is nonpolar, so ions and large polar molecules like glucose need a channel or carrier protein to cross.
- A bilayer's middle is fatty acid tails. It's nonpolar, like a thin film of oil.
- Water clings to ions and polar molecules. Dragging an ion or a large polar molecule into the oily core costs too much energy, so almost none cross.
- A channel gives ions a water-filled pore. A carrier, like the glucose transporter, binds its molecule and changes shape to move it. Down a gradient, neither uses ATP.
- Size alone doesn't decide it. Nonpolar O2 and CO2 cross freely. Water, small but polar, leaks across slowly, and aquaporins speed it up.
On the exam: Ions and large polar molecules like glucose need a channel or carrier to cross.