The Cell Membrane

Function

Cell membranes are vital for the normal functioning of cells. Some of their main functions include:

• Control – the continuous barrier around cells and organelles allows enclosed chemical environments to be formed whose contents can be controlled
• Selectivity – the highly selectively permeable nature of the membrane allows ionic gradients to be maintained
• Communication – allows cells and organelles to respond to extra-cellular and extra-organelle signals
• Recognition – of signalling molecules, of adhesion proteins allowing mechanical connections to form, of host and foreign cells (important for immune responses)
• Signal generation – in response to a stimulus, e.g. creating a change in membrane potential that can be propagated

Different parts of a cell membrane may have different functions requiring their structure to be specialised for this. For example, in a neurone the cell membrane of the axon is specialised for electrical conduction whilst the dendrites are specialised for synapsing and the composition of the membrane reflects this.

Structure

A rough estimate of the dry weight of the components of the cell membrane is:

• 60% protein – e.g. channel proteins and carrier proteins
• 40% lipid – e.g. phospholipid molecules and cholesterol
• 1-10% carbohydrate – usually attached to proteins/lipids on the outside of the cell membrane, a coat of carbohydrate surrounding a cell is called the glycocalyx

Membrane Proteins

Proteins constitute the largest component of the cell membrane and are vital in almost every process within a cell. Some functions of membrane proteins include:

• Catalyse reactions – enzymes
• Allow selective movement of molecules – transporters, pumps and ion channels
• Allow the cell to respond to extracellular signals – receptors for hormones, local mediators and neurotransmitters
• Transduce energy from one form to another – specialised proteins

More active cells or organelles (e.g. mitochondria) tend to contain more proteins to match their function.

Membrane proteins can either be embedded within the bilayer (integral proteins) or attached to the surface of the membrane (peripheral proteins).

Phospholipids

By LadyofHats Mariana Ruiz [Public domain], via Wikimedia Commons

Fig 1
Structure of the cell membrane

Cholesterol

Cholesterol is a major structural component of the cell membrane. Each molecule contains a polar head, a planar steroid ring and a non-polar hydrocarbon tail.

It is essential for many functions in the cell and helps the membrane to maintain stability and fluidity with temperature changes.

Cholesterol is bound to neighbouring phospholipid molecules via hydrogen bonds. This limits the packing of phospholipid molecules at low temperatures (when kinetic energy is lowest), allowing a fluid phase to be maintained.

At high temperatures, cholesterol helps to stop the formation of crystalline structures and the rigid planar steroid ring prevents intrachain vibration making the membrane less fluid.

Carbohydrates

Carbohydrates in the plasma membrane are typically short, branched sugar chains (e.g. glucose, galactose and fructose) attached to proteins or lipids to form glycoproteins and glycolipids. These carbohydrate groups are found only on the extracellular surface, collectively forming a thin, hydrated, protective layer surrounding the cell called the glycocalyx.

Functions of the glycocalyx include:

• Cell recognition – distinct glycoprotein patterns allow cell identification (e.g. ABO blood group antigens system)
• Pathogen interaction – glycoproteins provide binding sites for signalling and immune recognition (e.g. MHC molecules)
• Cell-cell adhesion – many adhesion molecules use specific glycoprotein complexes, making them essential for adhesion and inflammatory responses
• Mechanical protection – the glycocalyx coating attracts water, cushioning the membrane and reducing mechanical stress