Lipids are defined by their solubility. They are molecules that are insoluble in polar solvents (such as water), but will dissolve in non-polar solvents. Lipids are non-polar. Fats, oils, waxes and the steroids are all lipids. They function as energy storage molecules, as insulation and protection for internal organs, as lubricants and as hormones. One group, the phospholipids are the major structural elements of membranes.

Triglycerides (fats).

Triglycerides are composed of a glycerol backbone and three fatty acids. A fatty acid is a long chain of 12-24 carbon atoms with hydrogen atoms attached to them. At one end is a carboxyl or organic acid group. The acid molecule has the structure . The fatty acids are joined to the glycerol by a dehydration synthesis reaction. (If you have forgotten what this is, return to dehydration synthesis.)

As with the carbohydrates, this reaction requires a specific enzyme to make it occur. Fats have differing properties depending on whether their constituent fatty acids are saturated or unsaturated.
  1. Saturated fatty acids contain all of the hydrogens they can hold. The fatty acids in the animation above are all saturated with hydrogen. There are no carbon to carbon double bonds in saturated fatty acids. Saturated fatty acids are typical of animal fats and are believed to cause blockage of arteries which can lead to strokes and heart attacks.
  2. Unsaturated fatty acids do not contain all of the hydrogen possible. One or more carbon to carbon double bonds will be present in the carbon chain.Linoleic acid is an unsaturated fatty acid because there are two carbon-carbon double bonds in the carbon chain. Note that each carbon involved in the double bonding has only one bond left for bonding to hydrogen. Such molecules are not completely loaded with hydrogen so they are unsaturated.





The diagram shows a phospholipid. It is formed by replacing one of the terminal fatty acids in a triglyceride with a phosphate group. Because oxygen is so electronegative (see electronegativity) the oxygens in the phosphate make the region around them negative so that the phosphate part of the molecule is polar. Polar molecules are hydrophilic (water soluble), therefore, the phosphate head of the phospholipid. is water soluble. The fatty acid tails, however, remain nonpolar and hydrophobic (insoluble in water).






When a phospholipid is placed in water it usually forms a ball called a micelle. This happens because the hydrophobic parts of the phospholipid try to avoid the water and cluster together in the middle of the droplet. The polar phosphate heads, however, arrange themselves so that they are exposed to the positive hydrogen regions of water molecules. The micelle is the result.

Cell Membranes. 

The membranes that surround cells form from a double layer of phospholipids known as a bilayer. This is essentially a fat sandwich, with the fatty acids forming the filling and the phosphate heads forming the "bread". Since cells have much water in them and are surrounded by a watery medium, the phospholipid bilayer separates these two watery areas. The middle region of the membrane will be hydrophobic and the outside and inside will be hydrophilic.


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copyright June B. Steinberg, 2005