Plasma Membrane Structure.

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Question 2                Rob Hunt

Plasma Membrane Structure

The plasma membrane is found around the outside of the cell and separates the inside of the cell from the outside world. One of its main functions is to act as a selective barrier controlling the movement of substances in and out of the cell.

Other functions of a cell membrane are to:

  • Detect chemical messengers arriving at the cell surface
  • Link adjacent cells together by membrane junctions
  • Acts as receptor site for recognition

        

The cell membrane is made up of a phospholipid bilayer with proteins embedded and other components inside. A phospholipid is made up of a glycerol head part and two fatty acid tails, one saturated and one unsaturated. Diagram 1 shows the layout of the phospholipids in the membrane.

The phospholipids are set out in this way due to the polarity of the ends of the phospholipid. The head of the phospholipid is hydrophilic (loves water) and is said to be ‘polar’, whilst the fatty acid tails are hydrophobic (hates water) and is ‘non polar’. When put in water the tails try their best to move away from the water, they position themselves tail to tail to achieve this whilst the polar hydrophilic part move towards the polar water molecules. (Vander, Sherman, Luciano, l994)

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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 ...

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