Lipids, Fats & Steroids

Lipids are a chemically diverse class of biomolecules united by one property: they are hydrophobic (water-insoluble) and dissolve readily in non-polar organic solvents. They are not polymers built from a single monomer type — unlike proteins (amino acids) or nucleic acids (nucleotides). Instead “lipid” is a solubility classification, encompassing fats, oils, waxes, phospholipids, steroids, and fat-soluble vitamins.

Fatty acids are long-chain carboxylic acids, typically with an even number of carbons (4–24). The chain length and the number and position of double bonds determine the physical properties. A saturated fatty acid has no double bonds — its carbon chain is fully saturated with hydrogen. Because the chain is straight, saturated fatty acids pack tightly together, giving animal fats (lard, butter) their solid, waxy consistency at room temperature.

Unsaturated fatty acids have one or more C=C double bonds. The double bonds in naturally occurring unsaturated fatty acids are almost always cis (Z) configuration — this introduces a kink in the chain. The kinked chains cannot pack as tightly, so unsaturated fats are liquid at room temperature (oils). Polyunsaturated fatty acids (PUFAs) with multiple cis double bonds are especially fluid. Trans fats, however, have trans double bonds — the chain stays straight, packs like saturated fat, and has the same (or worse) cardiovascular effects.

A triacylglycerol (TAG, also called triglyceride) is three fatty acids esterified to the three hydroxyl groups of glycerol. It is the main form of energy storage in adipose tissue. Fats contain approximately 9 kcal/g — more than twice the energy density of carbohydrates or protein (4 kcal/g each) — because fat is highly reduced (more C–H bonds, more oxidisable). A person with 15 kg of body fat stores enough energy to walk to the moon and back (roughly).

When a fat (triacylglycerol) is heated with NaOH (or KOH), the ester bonds are hydrolysed in an irreversible reaction to give glycerol and three fatty acid sodium salts — soap. This is the same saponification reaction covered in Chapter 6, applied to fats. The sodium (or potassium) salts of long-chain fatty acids are soaps: they have a hydrophilic ionic head (–COO⁻ Na⁺) and a long hydrophobic tail, making them amphipathic and able to emulsify fats in water.

Phospholipids are the structural basis of all cell membranes. They differ from triacylglycerols in one crucial way: the third position on glycerol is occupied not by a fatty acid but by a phosphate group linked to a polar “head group.” The most common head group is choline (making phosphatidylcholine, also called lecithin). This gives the phospholipid two distinct regions: a non-polar, hydrophobic tail (two fatty acid chains) and a polar, hydrophilic head (phosphate + head group).

In water, phospholipids spontaneously organise into a bilayer — two sheets of phospholipids with the hydrophobic tails facing each other (away from water) and the hydrophilic heads facing the aqueous environment on both sides. This is the basic structure of every plasma membrane. The bilayer is fluid (phospholipids can move laterally), and the degree of fluidity is regulated by cholesterol content and the proportion of unsaturated fatty acids.

Steroids are lipids built from a characteristic four-ring hydrocarbon backbone — three six-membered rings (A, B, C) fused to one five-membered ring (D). This rigid, flat tetracyclic structure is unlike any other lipid class. Cholesterol is the most important steroid in humans: it is a component of cell membranes (regulates fluidity by plugging between phospholipid tails), the precursor of all steroid hormones (via the mevalonate pathway), and the precursor of bile acids and vitamin D.

Cholesterol has a hydroxyl group at C-3, a double bond at C-5/C-6, an 8-carbon hydrocarbon side chain at C-17, and methyl groups at C-10 and C-13. It is transported in blood as lipoprotein complexes (LDL, HDL, VLDL) because free cholesterol is insoluble in blood. LDL delivers cholesterol to tissues (high LDL → atherosclerosis); HDL removes cholesterol from tissues back to the liver for excretion (high HDL = protective).

Waxes are esters of long-chain fatty acids with long-chain alcohols (rather than glycerol). They are very non-polar and water-repellent — the waterproofing on leaves, the coating on apple skin, the wax of beeswax (myricyl palmitate), and the ear wax (cerumen). They are metabolically inert compared to fats.

The fat-soluble vitamins (A, D, E, K) are stored in adipose tissue and the liver, unlike water-soluble vitamins which are excreted in urine. This storage ability means they can accumulate to toxic levels with excess supplementation — hypervitaminosis A and D are real clinical concerns. Fat malabsorption (cystic fibrosis, coeliac disease, cholestasis) leads to deficiency of all four simultaneously.