Membrane fluidity is important for:

• The fusion of membranes e.g. vesicles fusing with the plasma membrane.

• The diffusion of new membrane components within the membrane

• The diffusion of components involved in signaling/metabolic reactions.

The three factors that affect membrane fluidity are:

• Temperature

• Amount of cholesterol

• Type of fatty acids in phospholipids

A membrane containing a mix of saturated and unsaturated lipids will be more fluid than a membrane containing only saturated lipids. This is because the molecules can pack less closely together so there are fewer hydrophobic interactions between the molecules holding them in place.

Cholesterol helps maintain fluidity at extremes of temperature. At low temperatures it prevents the regular packing of phospholipids so prevents freezing to maintain fluidity. At high temperatures, it interacts with the non-polar phospholipid tails to maintain membrane structure and so prevents it from becoming too fluid.

In organisms that live at low temperatures:

• Phospholipids contain more unsaturated and/or shorter fatty acids to make the membrane more fluid.

• Membranes contain more cholesterol to help prevent the membrane freezing.

In organisms that live at high temperatures:

• Phospholipids contain more saturated and/or longer fatty acids to make the membrane less fluid.

• Membranes contain more cholesterol to help maintain membrane structure and reduce fluidity.

As temperature increases, the phospholipids gain more kinetic energy and move more. This increases the fluidity and therefore permeability of the membrane. At high temperatures, proteins become denatured and disrupt the membrane, further increasing the permeability of the membrane.

Organic solvents, such as ethanol, dissolve lipids and so damage cell membranes, increasing permeability.