Lipids: MDCAT Biology notes
Lipids MDCAT notes: fatty acids, ester bonds, acylglycerols, phospholipids, waxes, steroids, prostaglandins and why fats give the most energy.
What lipids are
Lipids are a varied group of organic compounds. They are insoluble in water but soluble in organic solvents such as ether, chloroform and benzene. Compared with carbohydrates, they contain a much higher proportion of C–H bonds and much less oxygen. That is why one gram of fat releases more than twice the energy of one gram of carbohydrate. Lipids give the most energy per gram of any biomolecule. When fatty acids are oxidized, each one produces many acetyl CoA units, and these feed the Krebs cycle to make large amounts of ATP.
Fatty acids
A fatty acid is a hydrocarbon chain with a carboxyl group (–COOH) at one end. Most natural fatty acids have an even number of carbons.
| Fatty acid | Carbons | Type |
|---|---|---|
| Butyric acid (butter) | 4 | Saturated |
| Palmitic acid (melting point 63.1 °C) | 16 | Saturated |
| Stearic acid | 18 | Saturated |
| Oleic acid | 18 | Unsaturated (one C=C) |
| Arachidonic acid | 20 | Unsaturated (four C=C) |
Saturated fatty acids have only single bonds and are usually solid at room temperature (animal fats). Unsaturated fatty acids have one or more double bonds and are usually liquid (plant oils). Melting point rises with chain length and falls with unsaturation. Arachidonic acid is the precursor of prostaglandins.
Acylglycerols (neutral fats)
Glycerol, an alcohol with three –OH groups, reacts with fatty acids by condensation to form an ester bond. One fatty acid gives a monoacylglycerol, two give a diacylglycerol, and three give a triacylglycerol (triglyceride). So an acylglycerol needs at least one fatty acid. Acylglycerols are the most abundant lipids in living things and the main energy store. Fat under the skin also insulates against heat loss.
The saponification number (the amount of alkali needed to saponify a fat) indicates the average molecular weight, or chain length, of its fatty acids.
Phospholipids
In a phospholipid, one fatty acid of a triglyceride is replaced by a phosphate group, often linked to a nitrogenous base such as choline or serine. The phosphate end is polar (hydrophilic) and the two fatty-acid tails are non-polar (hydrophobic). This dual nature lets phospholipids form the bilayer of cell membranes.
Waxes
A wax is an ester of a fatty acid with a long-chain alcohol, not glycerol. Waxes are water-proofing: they form the cuticle of leaves and fruits and coat feathers and fur.
Steroids and terpenoids
These lipids contain no fatty acids. Steroids have four fused carbon rings.
- Cholesterol ($\mathrm{C_{27}H_{46}O}$) is an animal sterol found in membranes, egg yolk, milk, meat and fish. It is absent from egg white and from plants.
- Steroid hormones are made from cholesterol: aldosterone, cortisone, and the sex hormones (testosterone, oestrogen, progesterone).
- Insulin, FSH, ADH and oxytocin are protein or peptide hormones. Adrenaline and thyroxine come from amino acids.
Common MDCAT traps
- Glycerol plus a fatty acid gives an ester bond, not an ether bond.
- Waxes use a long-chain alcohol, while acylglycerols use glycerol.
- Oleic acid is the unsaturated one; butyric, palmitic and stearic acids are saturated.
- Fats are energy-rich because of their high C–H proportion, not because they contain more oxygen.
- Steroids are the lipids without fatty acids.
Quick revision
- Butyric acid has 4 carbons.
- The polar head of a phospholipid is its phosphate group.
- Triglyceride = glycerol + 3 fatty acids.
- Aldosterone, cortisone and sex hormones are steroids.
- Arachidonic acid has 20 carbons and four double bonds.