What are the functions of the cell membrane?

The cell membrane separates the intracellular and extracellular environments. The lipid bilayer only allows small polar molecules (such as O2 or CO2), small hydrophobic molecules (fatty acids, steroid hormones) and water to cross. However charged ions and molecules, and large molecules (including proteins) cannot just diffuse across the membrane; they can only be transported across by other cellular mechanisms. They can be allowed to cross by a specific membrane protein. e.g. a glucose transporter protein makes the membrane selectively permeable to glucose. This means that molecules and ions can be at different concentrations inside and outside the cell, allowing gradients across the membrane (this is important in neurones for example). The cell membrane also helps to communicate extracellular signals into the cell, usually via a sequence of interactions between different proteins, known as ‘signalling pathway’. Specific enzymatic modification of some of the lipids in cell membranes can transmit a signal into a cell. Membrane proteins can be receptors for an extracellular signal such as a protein (growth factor) or hormone (insulin). Activation of a receptor in the membrane activates a signalling pathway inside the cell. Components of cell membranes allow cell-cell recognition; e.g. sperm and egg recognition in fertilisation. In prokaryotic cells, which do not have intracellular organelles, the cell membrane is an important environment for the necessary reactions of the cell, e.g. the electron transport chain.