Diffusion Across Membranes

Some particles are able to pass through the phospholipid bilayer.These molecules are non-polar and can diffuse through the membrane down their concentration gradient.Examples are oxygen and carbon dioxide which diffuse freely through the membrane.Ions and polar molecules cannot easily pass through the membrane.Water molecules are polar, but small and can only diffuse through the membrane at a very slow rate.Small polar molecules pass through the membrane more easily than large polar ones.As a result, the membrane is said to be partially permeable.The larger the surface area of the membrane and the thinner the membrane, the faster the rate of diffusion through the membrane for small or non-polar molecules.