The majority of vital substances are transported through the cell membrane as solutes either in lipids or in water. Hydrophobic molecules such as fatty acids, steroid hormones, vitamin A, E, and D, may easily be transported into cells because they are soluble in lipids. Such facility is due to the structural nature of the membrane, which contains two layers of lipoprotein molecules and apolar chains in the inside of it, with its apolar ends reaching the external surface of the membrane. Most of the protein portion (i.e., the hydrophilic portion) of such molecules is located between the external and internal lipid surfaces. Therefore, the membrane of cells is a mosaic of fluid lipoprotein structure, with weak bonds between lipid molecules, which allow the easy passage of lipid-soluble substances into cells by passive diffusion, without energy expenditure. Conversely, the hydrophilic solutes, i.e., compounds soluble in...