ABSTRACT
This chapter describes some general principles of how small water-soluble molecules traverse cell membranes. The importance of membrane transport is reflected in the large number of genes in all organisms that code for transport proteins, which make up 15-30% of the membrane proteins in all cells. Some specialized mammalian cells devote up to two-thirds of their total metabolic energy consumption to membrane transport processes. Like synthetic lipid baitlayers, cell membranes allow water and nonpolar molecules to permeate by simple diffusion. Cell membranes, however, also have to allow the passage of various polar molecules, such as ions, sugars, amino acids, nucleotides, and many cell metabolites that cross synthetic lipid bilayers only very slowly. Plant cells and many bacteria are prevented from bursting by the semirigid cell wall that surrounds their plasma membrane. The inner membrane is the cell's plasma membrane. Bacteria with double membranes are called Gram-negative because they do not retain the dark blue dye used in Gram staining.
