The cell membrane separates the inside of a cell from the environment surrounding it. It helps to protect the cell, controlling what can enter and leave – it is semipermeable.
Phospholipid Bilayer
One of the main components of a cell membrane are phospholipids. These are made of two fatty acids, glycerol and a phosphate group. Phospholipids in a cell membrane are arranged in two layers called a phospholipid bilayer.
The phosphate group is hydrophilic (meaning water ‘loving’), and is often referred to as the head of a phospholipid.
The two fatty acids are referred to as the tails of a phospholipid and are hydrophobic (meaning water ‘hating’).
As the extracellular (outside) and intracellular (inside) spaces of a cell are both aqueous the phosphate heads face outwards, towards the aqueous environments.
The hydrophobic tails point inwards towards each other and away from the aqueous fluid that’s both outside and inside the cell.
This means there are two layers of phospholipids. The tails of both layers point towards the inside of the membrane. The heads of one layer point towards the cytoplasm and the heads of the other layer point towards the extracellular environment. This is called the phospholipid bilayer, as shown in the diagram below:
Components of The Cell Membrane
The cell membrane doesn’t just consist of phospholipids. The diagram below shows the structure of a small section of a cell membrane:
As you can see there are a number of other components that each play an important role.
Proteins are the second main component after phospholipids and can be found on the inner or outer surface, partially integrated into the membrane or extend all the way across the membrane.
Some proteins aid the transport of substances across the cell membrane (such as the channel protein in the diagram) while others are involved in communication with other cells.
Carbohydrates are also an important component of the cell membrane. They are found attached to either proteins or lipids and are on the surface of the extracellular side of the membrane. These molecules are called glycoproteins and glycolipids:
Glycoprotein – a protein attached to a carbohydrate
Glycolipid – a lipid attached to a carbohydrate
These molecules help cells to recognise each other and act as important markers for the immune system.
Another key component in the cell membrane is cholesterol. Cholesterol is another type of lipid and can be found in between the phospholipids. This increases the rigidity of the membrane.
The Fluid Mosaic Model
The fluid mosaic model is the current accepted model for the structure of the cell membrane,
In this model the membrane is fluid as the phospholipids are constantly moving. The other components of the membrane (such as the proteins) are scattered throughout the bilayer like a mosaic.
Due to the fluidity of the membrane the components can move around freely.
The fluidity can be changed by a number of factors. As mentioned above cholesterol can increase the rigidity of the membrane. The fluidity can also be affected by the proportion of saturated to unsaturated fatty acids in the phospholipids as well as by temperature.
Saturated fatty acids have no double bonds and are straighter than unsaturated fatty acids. The double bonds in unsaturated fatty acids causes kinks. These kinks prevent the phospholipids from packing tightly together and increase the fluidity.
At lower temperatures the phospholipids pack closely together, increasing the rigidity of the membrane. If the membrane contains a higher proportion of saturated fatty acids than unsaturated fatty acids the phospholipids will be able to pack much closer together and the fluidity will reduce further than if there were more unsaturated fatty acids.
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