2.5 Membrane Permeability

Selectively permeable

The membrane is selectively permeable — it lets some things through easily and blocks others. What crosses depends on the molecule's size and polarity and the membrane's hydrophobic core.

What crosses, what doesn't

Small nonpolar molecules (O₂, CO₂) and small uncharged polar molecules slip directly through the bilayer. Large polar molecules and ions cannot cross the hydrophobic core on their own — they need transport proteins. The hydrophobic interior repels charged particles.

Key terms

Selective permeability

The membrane's ability to let some substances cross while blocking others.

Hydrophobic core

The fatty-acid tail region that blocks charged and large polar molecules.

Crosses freely

Small nonpolar molecules (O₂, CO₂) and small uncharged polar molecules.

Needs help crossing

Ions and large polar molecules — they require transport proteins.

Why ions are blocked

Their charge is repelled by the nonpolar membrane interior.

Try an AP-style question

Which molecule would cross a pure phospholipid bilayer most easily?

1. O₂
2. Na⁺
3. Glucose
4. A small protein