Membrane Structure

See Figs. 3.1 and 3.2, pg 36 in Timbrell)

• Biological membranes are based on lipid bilayers. A typical lipid in a bilayer is a phospholipid made up of a glycerol backbone esterified to two long-chain carboxylic acids (Fatty acids) and a phosphate group which is generally esterified to a second alcohol, or a sphingosine esterified to one fatty acid and a second alcohol:

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• PtdSer = phosphotidyl serine
• PtdEtn = phosphotidyl ethanolamine
• PtdCho = phosphotidyl choline
• PtdIns = phosphotidyl inositol

• In aqueous solutions these phospholipids tend to form two layers with the hydrocarbon tails facing and interacting with each other, forming a very non-polar, oil-like interior. The phosphate heads are then on the two opposing surfaces, in contact with the aqueous solutions on each side. This forms a very effective barrier to the transport of polar molecules. Non-polar molecules can, of course, pass freely though these membranes, but first they have to get through the aqueous solution. Between the two phases the cell interior is protected from high concentrations of most substances.




lipid bilayer

Let' now look at the proteins in membranes (mosaic part).

• The cell communicates with the outside via protein channels. These are generally quite specific in what can be transported. Thus only "desired" molecules and mimics can usually get into the cell.
• Proteins also serve as recognition/binding sites on membranes, along with carbohydrates (outside surface only).
• The various proteins "float" in the 2-D liquid of the membrane (retinal rod membrane has measured viscosity similar to "fine Italian olive oil").
• Membrane protein can be relatively fixed in position by being tied to an intracellular protein cytoskeleton.

 

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Proteins

Proteins serve as the machinery and much of the structure in living organisms - they do most of the jobs. [overhead - one million times magnification, protein Plate 7] Functionally we can categorize proteins into three broadly defined groups based on their functions:

Structural proteins.

Recognition proteins.

Enzymes.

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Last modified 27 January 2010

© RA Paselk 2001