The ability of a membrane to be crossed by some substances rather than others defines its permeability.
In one solution are the solvent (dispersing liquid medium) and the solute (dissolved particle). Membranes are classified according to permeability into 4 types:
The) Permeable: allows the passage of solvent and solute;
B) Waterproof: does not allow solvent or solute to pass through;
ç) Semipermeable: allows solvent but not solute to pass through;
d) Selectively permeable: allows the passage of solvent and some types of solute.
This last classification fits the plasma membrane.
The random passage of particles always occurs from one place of higher concentration to another of lower concentration (in favor of concentration gradient). This is until the particle distribution is uniform. Once equilibrium is reached, the exchange of substances between two media becomes proportional.
The passage of substances through cell membranes involves several mechanisms, among which we can mention:
Sodium and Potassium Pump
Endocytosis and Exocytosis