All living things are composed of cells. Cells are the fundamental units of life. While there are organisms with only one cell in their body (e.g. amoeba), multicellular organisms such as humans consist of millions and millions of cells.
The outer layer or membrane of a cell provides a boundary between the internal and external environment of the cell. It controls the passage of substances into and out of the cell. This permits the cell to uptake nutrients and eliminate waste products.
According to Campbell and Reece (2002), cell membranes are approximately 8 nm thick. Cell membranes or plasma membranes display extraordinary features in their structure and functions. They are composed of a bilayer of phospholipids with proteins and cholesterol molecules associated with them. As the cell membrane is a matrix containing varieties of substances that can move within it, it is described as a ‘fluid mosaic’.
The substances found in the cell membrane determine its structure and functions. The cell membrane is made up of three main types of substances. They are lipids, proteins and carbohydrates.
Lipids
Lipids are insoluble in water and make up about half the weight of plasma membranes (Randall, 1997). Phospholipids and glycolipids are the prevalent lipids in the cell membrane.
Phospholipid is an amphipathic (containing a hydrophobic (water hating) and a hydrophilic (water loving) region) molecule . Phospholipids have a hydrophilic head, which is a phosphate group, and a hydrophobic tail containing a molecule of glycerol and two fatty acid chains.
Cholesterol is found incorporated between phospholipid molecules in the cell membranes of animal cells. Cholesterol makes the cell membrane rigid, by preventing interactions between fatty acid chains of phospholipids. The bent structure of cholesterol molecules immobilizes the neighbouring phospholipids, reducing the fluidity of the membrane but making it stronger. Depending on the type of cell, the amount of cholesterol in the cell membrane varies.
Glycolipids are lipids located on the surface of the cell membrane with a carbohydrate chain covalently bonded to them. Glycolipids play an important role in cell recognition. They act as markers or tags that help cells to differentiate between other cells in the body. They are able to recognise foreign cells. This feature is the basis for rejection of foreign cells as it appears in the immune system.
Proteins
Proteins in the cell membrane are divided into integral proteins and peripheral proteins. Proteins that are embedded in the lipid bilayer are called integral proteins. Peripheral proteins are those that are loosely bound to the surface of the cell membrane. Next to phospholipids, proteins account for most of the mass of cell membranes.
Proteins found in the cell membrane perform different functions. A single protein molecule may carry out multiple functions. According to the tasks they carry out, membrane proteins are categorized into different classes (e.g. structural proteins, transport proteins, receptor proteins). Campbell and Reece (2002), states that proteins determine most of the specific functions of the cell membrane. Some of these functions include transport of substances into the cell, communication of the cell with its outside environment, and cell recognition.
Carbohydrates
Glycolipids and glycoproteins (proteins with a carbohydrate unit attached to them via covalent bonding) are the substances that associate the cell membrane with carbohydrates. They are collectively known as glycocalyx (Clegg and Mackean, 1994). Glycoproteins and glycolipids are found on the surface of the cell membrane. Glycocalyces help in cell-to-cell recognition, cell protection and uptake of molecules into the cell.
References:
Campbell, N.A., & Reece, J.B. (2002). Biology. (6th ed. pp. 138-144). San Francisco: Pearson Education.
Clegg, C.J., & Mackean, D.G. (1994). Advanced Biology: Principles and Applications. (pp. 153-155). London: John Murray Ltd.
Randall, D., et al. (1997). Eckert Animal Physiology: Mechanisms and adaptations. (4th ed. pp. 94-97). New York: W.H. Freeman and Company.
