Other than Phospholipids, plasma membranes are made up a number of other chemical components. These components are:
Cholesterol,
Glycoproteins,
Proteins, and
Glycolipids,
If you look at Diagram 2 you can the fluid mosaic model of membrane structure with the formation of these other components inside. Since the phospholipids are not chemically joined to each other or the proteins they can move freely around the membrane and is the reason why the membrane is sometimes referred to as a “phospholipid sea”.
In a plasma membrane there is approximately 1 cholesterol molecule per phospholipid. Cholesterol has the same properties as a phospholipid in amphipathic (being polar and non-polar) so it arranges itself polar end outwards and tail inwards. They are found between the phospholipids on each side of the membrane (see diagram 3).
Cholesterol packs in the phospholipids in the membrane and pulls the fatty acid tails together, restricting movement but not making them solid, which in turn the membrane it stability and rigidity. Cholesterol also prevents crystallization of hydrocarbons (the tails) prevent the membrane becoming solid. ()
Proteins make the membrane selectively permeable; some are fixed in place while others are moving about in the phospholipid. There are two main types of protein in a membrane, one is an integral protein and the other is a peripheral protein. An integral protein (also called an intrinsic protein) is a transmembrane protein, which passes through the membrane. The integral protein usually spans across the full membrane. (See diagram 2). (http://cellbio.utmb.edu)
Intrinsic proteins form pores or channels, in which ions and water molecules can pass through the membrane. These channels can either be carrier protein or ion channels.
Carrier proteins push the ion down the protein channel and across the membrane by changing their shape. This process is called facilitated diffusion.
Ion channels are gated. They are a protein or proteins joined together with a hole in the middle that can be open or closed to allow ions through. Each channel is specific the type or types of ions it lets through. (Boyle, Ingham, Senior, 1999).
Peripheral proteins are found on the membrane surface and are attached to integral proteins and are usually found on the cytoplasm side of the membrane. They are associated with cell shape and give structure and motility.
Proteins that pass through the membrane but not completely are called extrinsic.
Another constitute of membranes are polysaccharides. Simple sugars attach themselves to the extrinsic proteins or lipids to form Glycoproteins or Glycolipids, which are antigens. These polysaccharides stick out of the cell surface “like antennae” (Boyle, Ingham, Senior, pg 75) and are sites of recognition for the cell. They send out information to tell surrounding cells what they are so that they are not confused with invading bacteria. Another functions is that similar cells will know what the cell is and can bind with it to form tissues. They also act as the basis of the immune system as the Glycoproteins are antigens. (www.revisionnotes.co.uk)