cel! membranes(生物膜) Unfortunately scientists inferred from this that the permeability(通透性)and diffusion gradient of a substance determined its penetration of the p Most biologically important molecules are not lipid soluble and must penetrate the plasma membrane through proteinaceous transport systemsCell membranes(生物膜) • Unfortunately scientists inferred from this that the permeability（通透性） and diffusion gradient of a substance determined its penetration of the p.m. • Most biologically important molecules are not lipid soluble and must penetrate the plasma membrane through proteinaceous transport systems