Nonpolar molecules, such as water, oxygen, carbon dioxide, and fatty substrates, move through a cell membrane the most easily. Simple diffusion is the method through which these molecules are transferred across cell membranes.
The plasma membrane of the cell is a selectively permeable layer that allows just some particles to flow through while preventing other substances from entering the cell. Its major job is to defend the cell from the outside world and maintain cellular integrity.
Phospholipids and integral proteins are the main components of the cell membrane. Two fatty acids, glycerol, and a phosphate group make up phospholipids. The two fatty acids form the tails and are nonpolar in the fluid-mosaic model, while the phosphate group forms the head and is polar. This arrangement is known as a “lipid bilayer,” and it allows hydrophobic substances to easily enter the cell by reacting with the nonpolar end of the lipid bilayer.
Several processes are used to transport molecules through cellular membranes. There are two types of membrane transports: passive and active. Passive transport is exemplified by molecules that flow through the membrane via simple diffusion. Facilitated diffusion is another type of passive transport in which polar molecules and ions are moved across a membrane by integral proteins.