The specificity of membrane transport proteins was first indicated in the mid-1950s by studies in which single gene mutations were found to abolish the ability of bacteria to transport specific sugars across their plasma membrane. Each protein transports a particular class of molecule (such as ions, sugars, or amino acids) and often only certain molecular species of the class. These proteins occur in many forms and in all types of biological membranes. Special membrane transport proteins are responsible for transferring such solutes across cell membranes. 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. Like synthetic lipid bilayers, cell membranes allow water and nonpolar molecules to permeate by simple diffusion. There Are Two Main Classes of Membrane Transport Proteins: Carriers and Channels Thus, synthetic bilayers are 10 9 times more permeable to water than to even such small ions as Na + or K + ( Figure 11-2). By contrast, lipid bilayers are highly impermeable to charged molecules (ions), no matter how small: the charge and high degree of hydration of such molecules prevents them from entering the hydrocarbon phase of the bilayer. Small uncharged polar molecules, such as water or urea, also diffuse across a bilayer, albeit much more slowly ( Figure 11-1). Small nonpolar molecules, such as O 2 and CO 2, readily dissolve in lipid bilayers and therefore diffuse rapidly across them.
In general, the smaller the molecule and the more soluble it is in oil (the more hydrophobic, or nonpolar, it is), the more rapidly it will diffuse across a lipid bilayer. The rate at which it does so, however, varies enormously, depending partly on the size of the molecule, but mostly on its relative solubility in oil. Given enough time, virtually any molecule will diffuse across a protein-free lipid bilayer down its concentration gradient. Protein-free Lipid Bilayers Are Highly Impermeable to Ions