The cell membrane is a barrier to the entry and exit of substances. They are semi-permeable barriers, allowing some substances through but not others. It does this by having some small pores or channels.
The membrane is a thin sheet composed of a lipid (fat) bilayer called phospholipid. It consists of a water-soluble end (hydophilic), facing inwards to the cytoplasm and outwards to the extracellular fluid. The non-water-soluble (hydrophobic) ends face each other internally. The lipid bilayer is embedded with protein molecules. There are two types: Integral proteins, which span the membrane and peripheral proteins, which are attached to the integral proteins on the inside or outside of the membrane. Carbohydrate molecules are attached to the proteins and lipids on the side of the membrane facing the external environment. These complex molecules are called glycoproteins and glycolipids.
This model of the membrane is called the fluid mosaic model, proposed by Jonathon Singer and Garth Nicholson in 1972.
It shows the way that membranes allow materials to move across them.