Permeability is the condition of being capable of having materials flow into and out of a membrane. The permeability of a cell membrane is determined by how easily a molecule can diffuse across the membrane. Usually, only molecules that are fat-soluble can permeate across a cell membrane. Why is this the case?

1. Permeability is the condition of being capable of having materials flow into and out of a membrane. The permeability of a cell membrane is determined by how easily a molecule can diffuse across the membrane. Usually, only molecules that are fat-soluble can permeate across a cell membrane. Why is this the case?