The type of molecules that pass most readily through a cell membrane are nonpolar molecules, such as water, oxygen, carbon dioxide and fatty substrates. The process by which these molecules are transported across cell membranes is known as simple diffusion.
The cellular plasma membrane is a selectively permeable layer that only allows certain particles to pass through it while blocking other substances from entering the cell. Its primary function is to maintain cellular integrity and protect the cell from its external environment.
The primary constituents of the cell membrane are phospholipids and integral proteins. Phospholipids consist of two fatty acids, glycerol and a phosphate group. In the fluid-mosaic model, the two fatty acids form the tails and are nonpolar, while the phosphate group forms the head and is polar. This arrangement is commonly referred to as a "lipid bilayer," which enables hydrophobic molecules that readily react with the nonpolar end of the lipid bilayer to easily enter the cell.
Different molecules are transported across cellular membranes via several mechanisms. Membrane transports have two classifications: passive and active. The molecules that pass through the membrane via simple diffusion is an example of passive transport. Another type of passive transport is facilitated diffusion, where polar molecules and ions are transported across a membrane via the integral proteins.