Transport of substances across a biological membrane from an area of higher concentration to an area of lower concentration by means of a carrier molecule. Since the substances move along the direction of their concentration gradients, energy is not required.
For example, polar molecules and charged ions dissolved in water can not diffuse freely across cell membrane due to its hydrophobic lipids. They can only be transported across membranes by proteins forming transmembrane channels. Larger molecules are transported by transmembrane carrier proteins, such as permeases that change their conformation as the molecules are carried through, for example glucose or amino acids.