Selective Permeability

The cell membrane is a phospholipid bilayer: two layers of phospholipids with their water-loving heads facing out and their fatty tails packed in the middle. That tail region is pure hydrocarbon, oily and uncharged. Oxygen is a small, nonpolar molecule, so it dissolves right into that greasy core and diffuses through without help. Sodium is tiny too, but it carries a full positive charge, and a charged ion sitting in an oily, uncharged environment is miserable. The core offers it no water molecules to interact with, so the ion is repelled and stays out unless a protein escorts it.

This is why size alone never predicts crossing speed. Carbon dioxide and glycerol are both small, but glycerol is polar enough that it crosses slowly compared to CO2. Meanwhile glucose, which is larger and very polar, cannot cross the bilayer at all on its own. What actually sorts molecules into fast, slow, or blocked is polarity and charge: nonpolar and very small uncharged polar molecules (like water in small amounts) slip through by simple diffusion, while ions, large polar molecules, and most charged species need a channel or carrier protein embedded in the membrane to get across.

This sets up the next idea directly: if the bilayer itself is choosy, the membrane proteins are what make controlled transport possible. Aquaporins are a clean example of a dedicated channel solving a diffusion problem. Water is polar and technically crosses the bilayer slowly on its own, but cells that need fast water movement, like kidney cells, install aquaporins to open a direct passive channel, turning a sluggish process into a fast one without spending any energy.