Introduction

The cell is the structural and functional unit of life. Bounded by a cell membrane, which maintains the homeostasis of the cell interior, it contains various membrane-bound compartments or organelles within, which subserve specialised functions. These membrane-bound organelles are characteristic of all eukaryotic cells, including those in humans.

Cell membrane

The cell membrane bounds all cells in the human body, forming a dynamic interface between the intracellular and extracellular environments.

It serves, or facilitates, the following functions:

1. • The maintenance of cell shape and structure. This is achieved by the presence of anchoring sites for cytoskeletal filaments and extracellular matrix components.

2. • A transport function. This is brought about by selective permeability to ions and macromolecules, allowing the maintenance of cytosolic ionic composition, osmotic pressure and pH (around 7.2–7.4).

3. • Intercellular communication, involving signal transduction, i.e. the detection of chemical signals (messengers) from other cells. These signals mediate nerve transmission, hormone release, muscle contraction and the stimulation of growth. This is the result of the binding of signalling molecules by transmembrane receptors.

4. • Intercellular adhesion. This is brought about by the fusion of the membrane with other cell membranes via specialised junctions.

5. • Directed cell movement.

Structure of cell membranes

The thickness of cell membranes ranges from 6–10 nm, typically being about 7.5 nm.

1. • One nanometre is equal to 10-9 metre.

Cell membranes are composed primarily of lipids and proteins. Lipids are the major components of membranes, including glycerophospholipids (phosphoglycerides), sphingolipids (sphingomyelin) and cholesterol. Cephalin (phosphatidylethanolamine) and lecithin (phosphatidylcholine) are the most common glycerophospholipids in membranes. Membrane lipids form self-sealing bilayers. They are amphipathic molecules, with hydrophobic and hydrophilic moieties. The hydrophobic groups, the long fatty acyl side chains, form the core, with the polar hydrophilic groups lining both surfaces.

Carbohydrates comprise 5%–10% of cell membranes. They consist of glycolipids and glycoproteins and formthe glycocalyx coat on the surface of the plasma membrane. This layer is responsible for the immunological characteristics of the cell and carries surface receptors that are involved in molecular recognition.

According to the fluid mosaic model of Singer and Nicolson cell membranes possess fluid structures, being considered as two-dimensional solutions of oriented globular proteins and lipids. They take the form of a continuous fluid but stable lipid bilayer, studded with an array of membrane-associated or membranespanning proteins.