BIOLOGY Volume 1 - A Guide to General Biology - 2004

3. CHEMICAL COMPONENTS OF LIVING ORGANISMS

3.3. Lipids

Lipids are sometimes given a rather vague definition; it is commonly stated that they are Water-insoluble organic substances that can be extracted from Cells using organic Solvents such as ether, chloroform, and benzene. A more precise definition of this group of compounds is not feasible due to their immense chemical diversity; however, it can still be said that true lipids are esters formed As a result of a Condensation reaction between Fatty acids and an alcohol.

3.7. WHAT IS A condensation reaction?

3.3.1. Components of Lipids

Fatty acids

Fatty acids contain an acid group, —COOH (the carboxyl group), in their molecule. They are called "fatty" because some of the high-molecular-weight members of this series are constituents of fats. The general formula for fatty acids is R • COOH, where R is a hydrogen atom or an alkyl radical such as —CH₃, —C₂H₅, etc.; each successive member of this series differs from the previous one by a —CH₂ group. In lipids, the radical R is typically represented by a long chain of carbon atoms. Most fatty acids contain an even number of carbon atoms, ranging from 14 to 22 (most commonly 16 or 18). Fig. 3.16 illustrates The Structure of two of the most common fatty acids. Note the long chain of carbon and hydrogen atoms that makes up the hydrocarbon tail of the molecule. The hydrocarbon tails determine many of The properties of lipids, including their insolubility in water. Hydrocarbon tails are hydrophobic (from Greek hýdrōr — water; phóbos — fear).

Sometimes fatty acids contain one or more double bonds (C = C) (for example, in oleic acid; Fig. 3.16). In this case, the fatty acids, as well as the lipids containing them, are called unsaturated. Fatty acids and lipids whose molecules lack double bonds are called saturated. Unsaturated fatty acids melt at significantly lower temperatures than saturated ones. Oleic acid, the main component of olive oil, is liquid at normal temperatures (T_melt = 13.4 °C), whereas palmitic and stearic acids (T_melt = 63.1 °C and T_melt = 69.6 °C, respectively) remain solid at these temperatures.

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Fig. 3.16. Structure of two common fatty acids. A. Stearic acid (C₁₇H₃₅COOH) is a saturated fatty acid; palmitic acid (C₁₅H₃₁COOH) has a hydrocarbon tail shorter by two carbon atoms. B. Oleic acid (C₁₇H₃₃COOH) is an unsaturated fatty acid.

3.8. The cells of poikilothermic ("cold-blooded") animals typically have a higher content of unsaturated fatty acids than the cells of homeothermic ("warm-blooded") animals. How would you explain this?

Alcohols

Most lipids are triglycerides. They incorporate the alcohol glycerol (Fig. 3.17).