A lipid bilayer is selectively permeable: different substances cross its hydrophobic interior at very different rates.

The main barrier is the nonpolar core formed by hydrocarbon chains. A molecule entering that core must temporarily leave favorable interactions with water, so permeability depends strongly on molecular size, polarity and charge.

As a qualitative rule:

• small nonpolar molecules such as $\mathrm{O_2}$ and $\mathrm{CO_2}$ cross readily by dissolving in the bilayer and diffusing through it;
• small uncharged polar molecules can cross more slowly;
• large polar molecules cross poorly;
• ions such as $\mathrm{Na^+}$, $\mathrm{K^+}$ and $\mathrm{Cl^-}$ have extremely low permeability through the bare lipid interior because moving a charged particle away from the surrounding water molecules that stabilize it is energetically costly.

Water is small and can cross a lipid bilayer, but many biological membranes move water much faster through specialized channel proteins called aquaporins.

Selective does not mean sealed

The membrane need not be absolutely impermeable to preserve different internal and external compositions. What matters is that passive crossing is slow enough relative to the cell's transport and reaction processes.

A concentration difference can therefore persist when the relevant solute has low membrane permeability or when active processes continually restore the difference.

Proteins change the permeability landscape

The lipid bilayer provides the baseline barrier. Membrane proteins can create controlled pathways through that barrier. Channels provide hydrophilic pores, carriers bind selected solutes and pumps couple transport to an energy source.

This means membrane permeability is not determined by lipid chemistry alone. Cells can regulate which substances cross by changing the identity, abundance and state of membrane proteins.

Selective permeability is the physical basis for maintaining cellular composition: the membrane allows some substances to equilibrate relatively easily while making other exchanges dependent on specific transport mechanisms.