Transport across membranes: further key points Lipid soluble molecules Lipid soluble molecules such as glycerol and oestrogen can diffuse across membranes because the membranes contain phospholipids. Small uncharged molecules Small uncharged molecules like oxygen and carbon dioxide are able to diffuse through the phospholipid bilayer. . Selective transport Cells are able to be selective about the molecules/ions that are transported across their membranes by facilitated diffusion and active transport. The specific shape of carrier proteins only allows specific molecules or ions to pass. Differences in the number of carrier proteins can also affect the speed of transport. Ions Ions such as sodium (Na+) are small but carry a charge. They are repelled by the phospholipid bilayer and are unable to move by simple diffusion. They can only travel across membranes using carrier proteins or protein channels via facilitated diffusion or active transport. Simple diffusion vs facilitated diffusion Increasing the concentration, increases the rate of uptake by simple diffusion, by making the concentration gradient larger/steeper. In comparison, uptake is faster at low concentrations when using facilitated diffusion as the carrier proteins allow substances to pass through more easily. However, the rate of uptake reaches a maximum when all the carrier proteins are occupied. .