They are a general group of non water soluble organic molecules that can be dissolved in non polar organic solvents, such as chloroform, ether and benzene. Tipically, lipids are energy storage molecules in the form of fatty acids and fat and when structural, as phospholipids, glucolipids and wax.
They are constructed from the simplest possible monomer. A carbon atom is covalently bonded to two hydrogen atoms forming a -CH2 – unit. When several of these monomers are linked together, a polymer called hydrocarbon is created. Hydrocarbons have two very important properties: 1) A lot of energy is stored in the C-C and C-H covalent bonds of the molecule and 2) They are very hydrophobic and will not dissolve in water to any great extent.
Fatty Acids.- They are not usually found in the cells as free forms. They are composed of a long chain of an even number of C atoms, between 14 and 22. They differ from each other on the number of carbons and in the fact that some of them could have double bonds in different positions in the chain. A fatty acid such as stearic acid, which has no double bonds it is said to be saturated. This is, all the posibilities of more bonds are already used. In other words, all the carbons in the chain present four bonds. In the other hand oleic acid which presents double bonds between carbon atoms in its chain, is said to be unsaturated, because its carbon atoms have the potential of bonding with more atoms.
Fat.- A fat molecule is formed by three molecules of a fatty acid linked to a glycerol molecule. Glycerol is an alcohol with three carbons with three hydroxyl groups (-OH). A fatty acid is a long hydrocarbon chain that finishes with an carboxyl group (-COOH). This chain is non polar and hydrophobic while the carboxyl group gives the molecule and acid property. As with polysaccharides, each bond between glycerol and the fatty acid is formed by the elimination of water (condensation, dehydration). These molecules are said to be neutral lipids because they do not have net positive or negative charges.
Phospholipids and Glucolipids.- As in fats, phospholipids and glucolipids are formed of fatty acid chains linked to a gylcerol molecule.
Phospholipids (phosphate lipids).- In phospholipids the third carbon atom of the glycerol is linked to a phosphate group as opposed to a fatty acid. Phosphate groups are highly hydrophilic since phospate is negatively charged while the fatty acid portion is highly hydrophobic. This kind of molecules are called amphipatic molecules. The relevance of this is that phospholipids are the main component of cell membranes.
Glucolipids (sugar lipids).- In glucolipids the third carbon atom of the glycerol is linked to a short carbohydrate chain. Depending on the particular glucolipid, this chain may contain in any position between one to fifteen monomers. The carbohydrate “head” is highly hydrophilic and the fatty acid “tails” are hydrophobic. In a water solution, glucolipids behave in the same way as phospholipids so they are also important components of cell membranes.
Waxes.- They are formed by joining together a single fatty acid molecule to another hydrocarbon molecule that has as part of its structure a hydroxyl (-OH) group at one end. The wax macromolecule, therefore, has two hydrophobic ends and is insoluble in water, which makes it an excellent waterproof material and constitutes its main role in living systems as coat that cover surfaces such as insect exoskeleton, or wax that covers feathers, skin etc.
Cholesterol and steroids.- Although not strictly polymers, steroids are usually classified along with lipids because of their strong hydrophobic character. All steroids are composed of the same basic four rings of carbon atoms. One steroid, cholesterol, acts in ways not fully undestood to change the properties of cell membranes and to regulate membrane fluidity. Other steroids, called hormones, act as messengers. They are released in tiny amounts by specialized glands and tissues and circulate in the fluids of multicellular organisms. Eventually, specific target cells recognize hormones, which become attached to or taken up by these cells and trigger changes in their metabolim or developmental fate. Sex hormones, for example, control the function of both male and female reproductive organs and ensure the correct formation of sex cells.