Lipid Identification and Chromatography

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Allan Harte       Paul Ravenscroft                                                                                                                  -  -AS Biology        

Lipid Identification and Chromatography



Lipids are the single class of biological macromolecules which do not contain polymers.  Molecules of carbohydrates and proteins for example can be made of sometimes thousands of monomers, each monomer linked to the next by a covalent bond.  However, a lipid is made of a very small number of parts in comparison, the smallest being made of only four smaller molecules: one glycerol and three fatty acids, each shown below.  The glycerol molecule is comprised of three carbon atoms, each of which is bonded to a hydroxyl group.  The fatty acids are hydrocarbons with a carboxyl group, and their carbon skeletons are saturated with hydrogen.  The glycerol will always remain the same in every lipid, but it is the number and size of the fatty acids that gives a lipid its variation.  A lipid is formed when the hydroxyl group of the carboxyl group and a hydrogen atom from the hydroxyl group of the glycerol dehydrate in a process known as esterification.  The bond that is formed is called and ester bond.  A completed lipid has three fatty acids attached to the glycerol, and so it is called a triglyceride, the most common form of lipid.

Because none of the atoms in the hydrocarbon chains are more electronegative than any other, a lipid is non-polar, giving it the well-known property of repelling water.  However, some lipids are unsaturated, meaning that there are some hydrogen atoms missing from the fatty acid chains, and so some double bonds are formed between the carbons.  This forms a kink in the chain, meaning that molecules of lipids cannot fit together as easily as when they are straight chains.  This differing formation creates the difference between solid butter and liquid oils, for example.  In the body, fats are used mainly for energy storage, its long-term food reserves being stored in adipose cells, which enlarge and shrink as fat is removed for use or deposited for storage.  However, fats can also be used for insulation and cushioning against external forces.

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Although all lipids are hydrophobic, some parts of some lipids known as phospholipids are hydrophilic, which proves very useful for cells in the body.  Phospholipids have only two fatty acids chains linked to a glycerol molecule instead of three, which leaves one hydroxyl group free to form a covalent bond with a phosphate group.  As the oxygen atoms in the phosphate group are more electronegative, they form a partial negative charge, and are in turn hydrophilic.  Phospholipid bilayers (two layers in opposite direction) are what make up cell membranes, the phosphate group part of the molecule being in contact ...

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