Lipids
Lipids are a chemically diverse group defined by low solubility in water and high solubility in organic solvents. The exam tests fatty acid structure, triglyceride and phospholipid assembly, steroids, and essential fatty acids.
Core rules
- Fatty acids usually have even carbon numbers; saturated fatty acids have no C=C bonds, while unsaturated ones have one or more cis double bonds.
- Linoleic and linolenic acids are essential because humans cannot introduce double bonds at those positions.
- Triglycerides are esters of glycerol with three fatty acids; phospholipids have glycerol, two fatty acids and a phosphate-containing head group.
- Phospholipids are amphipathic and form bilayers; steroids have four fused rings and include cholesterol.
- Fats are solid triglycerides rich in saturated acids; oils are liquid and richer in unsaturated acids. Rancidity is oxidative or hydrolytic damage.
Worked example
Compare stearic acid (C18:0) and oleic acid (C18:1). Stearic acid has 18 carbons and no double bond, formula C18H36O2. Oleic acid has one C=C, so it has two fewer hydrogen atoms: C18H34O2. Each double bond removes two hydrogens relative to the saturated form. This is why unsaturated fatty acids have lower melting points and more kinks in the chain.
Traps that cost marks
Not all lipids contain glycerol; steroids do not. Cholesterol is not a triglyceride. Do not write that all unsaturated fats are unhealthy-essential fatty acids are required. Remember that a phospholipid has two fatty acid tails, not three.