Lipid Chemistry: Fats, Oils, Membranes, and Signaling Molecules

Fatty Acids

Fatty acids are long-chain carboxylic acids, typically containing 12-24 carbon atoms. Saturated fatty acids have no carbon-carbon double bonds and adopt extended, zigzag conformations that pack tightly together, producing solids at room temperature (like the fat in butter and lard). Stearic acid (18:0, meaning 18 carbons and 0 double bonds) and palmitic acid (16:0) are the most abundant saturated fatty acids in animal tissues.

Unsaturated fatty acids contain one or more carbon-carbon double bonds in the cis configuration. Each cis double bond introduces a roughly 30-degree bend in the chain that disrupts tight packing, lowering the melting point. Oleic acid (18:1, one double bond at position 9) is the most abundant fatty acid in olive oil. Linoleic acid (18:2) and linolenic acid (18:3) are essential fatty acids that humans cannot synthesize and must obtain from the diet. Omega-3 and omega-6 designations refer to the position of the first double bond counted from the methyl end of the chain.

Trans fatty acids, produced industrially by partial hydrogenation of vegetable oils, have a straight-chain geometry similar to saturated fats despite being unsaturated. This is because the trans configuration places substituents on opposite sides of the double bond, allowing the chain to extend rather than bend. Trans fats increase shelf life and provide desirable texture in processed foods but raise LDL cholesterol and lower HDL cholesterol, increasing cardiovascular disease risk. Many countries have restricted or banned the use of artificial trans fats in food.

Triglycerides: Energy Storage

Triglycerides (triacylglycerols) form when three fatty acid molecules esterify with the three hydroxyl groups of glycerol. The three fatty acids can be identical (simple triglyceride) or different (mixed triglyceride, which is more common in nature). Fats (solid at room temperature) contain predominantly saturated fatty acids, while oils (liquid at room temperature) contain predominantly unsaturated fatty acids.

Triglycerides are the most energy-dense biological molecules, yielding approximately 37 kJ per gram when completely oxidized, compared to 17 kJ per gram for carbohydrates and proteins. This efficiency arises because fatty acid carbons are highly reduced (bonded mainly to hydrogen), meaning they can undergo extensive oxidation. Adipose tissue stores triglycerides as an energy reserve, and animals migrating long distances (like birds) rely on fat stores because of their superior energy density and light weight.

Saponification is the base-catalyzed hydrolysis of triglycerides, producing glycerol and fatty acid salts (soaps). Soap molecules are amphiphilic: the long hydrocarbon chain is hydrophobic, while the carboxylate head is hydrophilic. In water, soap molecules self-assemble into micelles with the hydrophobic tails pointing inward and the hydrophilic heads facing the water, enabling them to solubilize grease and oils for removal during washing.

Phospholipids and Biological Membranes

Phospholipids have a glycerol backbone esterified with two fatty acids and a phosphate group that is further linked to a polar head group (such as choline, ethanolamine, serine, or inositol). This structure creates a strongly amphiphilic molecule with a hydrophobic tail region (the two fatty acid chains) and a hydrophilic head region (the phosphate and head group).

In water, phospholipids spontaneously self-assemble into lipid bilayers, with the hydrophobic tails facing each other in the interior and the hydrophilic heads facing the aqueous environment on both sides. This bilayer is the structural foundation of all biological membranes, creating a selectively permeable barrier that separates the inside of cells from the outside environment and partitions the cell interior into specialized compartments (organelles).

Membrane fluidity depends on the fatty acid composition of the phospholipids. Unsaturated fatty acids with cis double bonds increase fluidity by disrupting packing. Cholesterol, a steroid lipid embedded in animal cell membranes, modulates fluidity: at high temperatures it restricts movement, and at low temperatures it prevents tight packing, maintaining membrane fluidity across a range of conditions.

Steroids and Terpenes

Steroids share a characteristic four-ring carbon skeleton: three six-membered rings and one five-membered ring fused together. Cholesterol is the most abundant steroid in animal tissues, serving as both a membrane component and the biosynthetic precursor for all other steroid hormones. The body converts cholesterol into cortisol (stress hormone), aldosterone (salt balance), testosterone, estradiol, and progesterone (reproductive hormones), as well as bile acids (digestive surfactants) and vitamin D (calcium regulation).

Terpenes are a large class of lipids built from five-carbon isoprene units. Monoterpenes (10 carbons) include many plant fragrances: limonene (citrus), menthol (mint), and pinene (pine). Sesquiterpenes (15 carbons) include farnesol and the antimalarial drug artemisinin. Diterpenes (20 carbons) include retinol (vitamin A) and taxol (an anticancer drug). Triterpenes (30 carbons) include squalene, the biosynthetic precursor to cholesterol. The isoprene rule provides a powerful tool for understanding the biosynthesis and structural classification of natural products.