Reading Notes for Chapter 17Reading Notes for Chapter 17Biological systems rely heavily upon the interactions of various biomolecules with water. While it is important for biomolecules to be soluble in water, it is equally important for some biomolecules (or some parts of biomolecules) to be insoluble in water. When a biomolecule is more soluble in nonpolar/organic solvents than water, it is classified as a lipid. Because they are not attracted to water, these substances are classified as hydrophobic.
Fatty acids are carboxylic acids that have long alkane or alkene "tails".
"Saturated" fatty acids have alkane tails... the carbons are "saturated" with hydrogens. Saturated fatty acid chains can pack very closely and effectively together. This makes them high melting and they are usually solids at room temperature.
"Unsaturated" fatty acids have one or more double bonds, with "cis" or "trans" configurations at each double bond. Because the double bonds put a slight "kink" in the chain, these chains cannot pack as closely/tightly as saturated fatty acid chains. This makes them melt at lower temperatures and can make them liquid at room temperature.
Esters formed from a fatty acid and a long-chain alcohol are usually classified as waxes.
"Soap" is usually the sodium or potassium salt of a fatty acid. Soaps work because they have a long hydrophobic tail and a polar, hydrophilic carboxylate "head".
Most common biological fats and oils are the fatty acid esters of 1,2,3-propantriol, commonly called glycerol. This gives rise to the name triglycerides.
Fats are triglycerides that are solid at 25°C, oils are triglycerides that are liquid at 25°C.
Like all esters, triglycerides can undergo hydrolysis to yield glycerol and free fatty acid molecules.
Unsaturated fatty acids and triglycerides can undergo "hydrogenation" to saturate the double bond, converting the alkene to an alkane. By controlling the number and configuration of double bonds, the melting point of the fat or oil can be carefully tuned and controlled.
Soaps work by forming micelles in which the hydrophobic tails of the soap molecules are tucked inside a shell of hydrophilic carboxylate groups. Greases (hydrophobic) are drawn into the inside of the micelle.
Lipids are also responsible for a variety of membranes in biological systems. At the interface between a hydrophilic (aqueous) phase and a hydrophobic phase, a monolayer of lipid molecules cab form.
Most membranes in biological systems are used to separate two aqueous phases from each other. In this case, lipid molecules form a bilayer which sandwiches the hydrophobic tails of the lipids between two layers of hydrophilic carboxylates. You can think of a bilayer membrane as sort of a flat micelle.
Steroids are lipids that cannot be saponified. Saponification is the hydrolysis of a triglyceride (or other fatty ester) to yield a soap.
Steroids typically have a conserved 4-ring system. This structure is found in cholesterols, steroidal hormones, and bile.
Although there are structural similarities, small changes in the structure lead to significantly different functions. For example, both estrogens ("female" hormones) and androgens ("male" hormones) are steroidal hormones, but they have significantly different functions.
NOTE: Although estrogens are commonly associated with female
characteristics and androgens are commonly associated with male
characteristics, both are present in both men and women in varying
amounts.
