A level Biology exam revision resources written by A level Examiners


The plasma membrane is the type of membrane that is found surrounding all cells and many organelles. Often it is called the cell surface membrane, though the term plasma membrane is technically more correct since organelles are surrounded by membranes as well as whole cells.

The plasma membrane is not a strong structure so it does not provide much mechanical strength or support. Its key role is that it controls entry and exit of substances into and out of cells and organelles.

The membrane is constructed of phospholipid molecules, with protein (and carbohydrate molecules) arranged within the phospholipid layers. The general construction is a fluid mosaic structure.

A portion of a plasma membrane is shown in figure 1 below.

Figure 1 : Plasma Membrane
Plasma Membrane

Figure 1 shows that the general construction of the membrane is that of a 'sea of phospholipids with protein icebergs'. It is important to realise that the phospholipids and proteins are not bonded to each other so are free to move about - this is why the membrane is fluid.

Figure 2: Phospholipids

Figure 2 shows the polar nature of phospholipid. When in aqueous environments phospholipids naturally form a bilayer. The hydrophilic phosphate heads facing outwards into the aqueous medium and the hydrophobic fatty acid tails face each other in the middle of the bilayer, as they are repelled by the water of the medium, but attracted to each other.

It is the hydrophobic region in the middle of the bilayer that acts as a barrier to all but the smallest water-soluble molecule.

The proteins present in the membrane are mostly large proteins that span the membrane, with surfaces exposed on both sides. There are also some smaller proteins that sit buried, or attached to just one side of the membrane. Many of the larger proteins are responsible for the transport of materials across the membrane. Some of the protein molecules are membrane bound enzymes.

Some protein molecules have carbohydrate molecules attached (glycoproteins) these molecules often act as antigen or as hormone receptor molecules. The carbohydrates can be attached to phospholipids forming glycolipids.

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