The passive movement of materials across cell membranes is the result of a) the permeability of the membrane to the diffusing substance and b) the driving force. For uncharged compounds, the concentration gradient is the driving force between the inside and the outside of the cell. In the case of ions, the driving force is a balance between the concentration gradient and the electrical potential difference between the inside and outside of the cell.
In general, biological membranes are far less permeable than an equivalent thickness of pure solvent. They are completely impermeable to many compounds. The combination of membrane permeability, driving force and the density and identity of active transport mechanisms determine the rate of movement of nutrients into and waste products out of cells. The rates of penetration through the membrane by various molecules are important in determining the chemical composition of the cell. A substance to which the membrane is impermeable can only be found in the cell if it is synthesized there or actively transported into the cytoplasm.
Here we will investigate the effects of lipid solubility and molecular weight (i.e. size) on the permeability of cell membranes. Our model system will be red blood cells. Since a membrane is largely phospholipid, the permeability of a substance (in the absence of active transport mechanisms) is largely determined by the lipid solubility and physical size of the molecule. Non-polar molecules readily dissolve in a membrane and readily pass; small molecules (particularly small uncharged molecules) readily pass.
There are mechanisms in place to increase the rate at which some molecules pass into cells. Membrane channels of various kinds help specific molecules pass through a membrane that would normally not pass. A membrane channel is a tube through the membrane formed by one or more intrinsic proteins. The outer surface of this protein channel is formed by amino...