Cells exchange many substances with their environments using simple diffusion through the lipid bilayer. The lipid bilayer is composed of two layers of phospholipids, with hydrophilic heads facing outwards and hydrophobic tails facing inwards. This structure allows for the selective permeability of the membrane, as hydrophobic molecules can easily pass through while hydrophilic molecules are blocked.
Simple diffusion through the lipid bilayer is not sufficient for all substances because some substances are too large or polar to pass through the hydrophobic interior of the bilayer. For example, ions such as sodium (Na+) and potassium (K+) cannot pass through the lipid bilayer due to their charge. Similarly, large molecules like glucose also cannot diffuse through the bilayer.