The main properties of molecules that determine how they may be transported across a membrane are solubility, size and charge.

Large molecules can’t cross the membrane, charged molecules also can’t, and naturally, lipid-repelling (or water-attracting) molecules can’t. Conversely, small molecules can cross the membrane barrier, alongside molecules with no charge (nonionised) as well as lipophilic (hydrophobic) molecules.

It’s important to understand the role of microvilli. These are elongations of plasma membrane which increase the surface area available for reaction or absorption.

Diffusion

Diffusion = the spread of particles from a region of higher concentration to a region of lower concentration, until the particles are…