10.6 Lipids: Structure & Classification

Lipids are one of the four major classes of biomolecules (alongside nucleic acids, proteins, and carbohydrates), but unlike the others, they are not defined by a repeating monomer-polymer structure. Instead, lipids are unified by a physical property: they are predominantly hydrophobic, meaning they are nonpolar or weakly polar and therefore insoluble (or only poorly soluble) in water, but readily soluble in nonpolar organic solvents. This hydrophobicity comes from lipids' high proportion of carbon-hydrogen bonds — long hydrocarbon chains or fused hydrocarbon ring systems — relative to polar or charged functional groups. The MCAT organizes lipids into three functional categories: storage lipids, structural lipids, and signal/cofactor lipids.

Storage Lipids

Storage lipids exist primarily to store chemical energy efficiently, since the fully reduced carbon-hydrogen bonds in fatty acid chains hold considerably more energy per gram than carbohydrates.

Triacylglycerols

A triacylglycerol (also called a triglyceride) consists of a glycerol backbone (a three-carbon molecule with three hydroxyl groups) esterified to three fatty acid chains, one at each hydroxyl position. Each glycerol-fatty acid linkage is an ester bond, formed by a condensation (dehydration) reaction between glycerol's -OH and the fatty acid's carboxylic acid group, releasing water. Triacylglycerols are the primary energy-storage form of lipids in adipose (fat) tissue, and because all three hydroxyl groups are esterified, the molecule has essentially no polar character left — it is almost entirely nonpolar and hydrophobic.

Free Fatty Acids and Saponification

A free fatty acid is a fatty acid not esterified to glycerol or any other backbone — just a long hydrocarbon chain terminating in a carboxylic acid group (-COOH). Free fatty acids can vary in chain length and in saturation: saturated fatty acids have no carbon-carbon double bonds (straight chains that pack tightly, raising melting point), while unsaturated fatty acids contain one or more carbon-carbon double bonds, typically in the cis configuration in nature, which introduces a kink that prevents tight packing and lowers melting point (this is why unsaturated fats are often liquid, and saturated fats solid, at room temperature).

Saponification is the base-catalyzed hydrolysis of the ester bonds in a triacylglycerol. Treating a triacylglycerol with a strong base (e.g., sodium hydroxide, NaOH) cleaves all three ester linkages, releasing free glycerol and three fatty acid salts (e.g., sodium carboxylates, RCOO-Na+) — commonly known as soap. This is the reverse of the esterification reaction that forms triacylglycerols, and it is a classic MCAT organic chemistry crossover topic (base-mediated ester hydrolysis).

Structural Lipids

Structural lipids form the physical architecture of cell membranes and other structural components.

Phospholipids and Phosphatids

A phospholipid (or phosphatid, more precisely a phosphoglyceride) has a glycerol backbone like a triacylglycerol, but only two of the three hydroxyl positions carry fatty acid chains; the third position carries a phosphate group, which is in turn typically linked to a small polar or charged head group (such as choline, ethanolamine, or serine). This gives phospholipids a distinctive two-part architecture: two nonpolar fatty-acid tails and one polar, often charged, phosphate-containing head. This dual polar/nonpolar character (amphipathicity) is the structural basis of biological membranes, discussed in detail below.

Sphingolipids

Sphingolipids are structurally similar in function to phospholipids (they are also major membrane components) but are built on a different backbone: sphingosine, an amino alcohol with a long hydrocarbon tail, rather than glycerol. A fatty acid is attached to sphingosine's amino group via an amide bond (forming a ceramide), and a polar head group (which may be a phosphate-linked group, as in sphingomyelin, or a sugar, as in glycosphingolipids like cerebrosides and gangliosides) attaches at the terminal hydroxyl. Sphingolipids are especially abundant in nerve cell membranes and the myelin sheath.

Waxes

A wax is an ester formed between a long-chain fatty acid and a long-chain fatty alcohol (rather than glycerol). Because both components are long hydrocarbon chains, waxes are extremely hydrophobic and are used biologically as protective, water-repellent coatings (e.g., on skin, fur, feathers, and plant leaf surfaces).

Signal and Cofactor Lipids

This category covers lipids whose primary biological role is signaling, regulation, or acting as enzymatic cofactors, rather than storage or structure.

Fat-Soluble Vitamins

Vitamins A, D, E, and K are hydrophobic and therefore classified as lipids. Because they are fat-soluble, they are absorbed alongside dietary fat, transported in lipoproteins, and can accumulate to toxic levels in fatty tissue if overconsumed (unlike water-soluble vitamins, which are readily excreted in urine). Vitamin A supports vision and gene regulation, vitamin D regulates calcium homeostasis, vitamin E acts as an antioxidant, and vitamin K is required for blood-clotting factor synthesis.

Steroids

Steroids share a core structure of four fused hydrocarbon rings (three six-membered and one five-membered), derived biosynthetically from cholesterol. Cholesterol itself is a structural component of cell membranes (modulating membrane fluidity) and the biosynthetic precursor to steroid hormones, including the sex hormones (testosterone, estrogen, progesterone) and corticosteroids (e.g., cortisol, aldosterone). Despite the shared ring scaffold, small differences in attached functional groups give steroids very different, highly specific signaling roles, generally acting through intracellular or nuclear receptors because their hydrophobicity allows them to diffuse directly across the plasma membrane.

Prostaglandins

Prostaglandins are local signaling lipids derived from arachidonic acid, a 20-carbon polyunsaturated fatty acid. Unlike steroid hormones, which can travel through the bloodstream to distant targets, prostaglandins act locally (autocrine or paracrine signaling), near the cells that produce them. They mediate diverse physiological processes including inflammation, pain sensitization, fever, platelet aggregation, and smooth muscle contraction — which is why nonsteroidal anti-inflammatory drugs (NSAIDs), which inhibit the cyclooxygenase (COX) enzymes that produce prostaglandins from arachidonic acid, reduce pain, fever, and inflammation.

Amphipathic Structure and Membrane Formation

The single most important structural concept for MCAT lipid content is amphipathicity: a molecule that has both a hydrophilic (polar/charged, water-loving) region and a hydrophobic (nonpolar, water-fearing) region within the same molecule. Phospholipids are the prototypical amphipathic biomolecule — a polar phosphate-containing head group attached to two nonpolar fatty-acid tails.

When phospholipids are placed in water, this dual character makes it thermodynamically unfavorable for them to exist as isolated, randomly dissolved molecules: the hydrophobic tails would be forced into contact with water, ordering surrounding water molecules into unfavorable solvation cages (the same hydrophobic effect discussed in the previous section on protein folding). Instead, phospholipids spontaneously self-assemble into a lipid bilayer: two layers of phospholipids arranged tail-to-tail, with the hydrophobic fatty-acid tails buried in the interior (shielded from water) and the hydrophilic phosphate head groups facing outward on both the inner and outer surfaces, in contact with the aqueous environment on either side of the membrane. This spontaneous self-assembly, driven by the same entropy-favoring hydrophobic effect that stabilizes folded proteins, is the physical basis of every biological cell membrane. A single layer of amphipathic phospholipids can also form a micelle (a small spherical aggregate with tails inward and heads outward, no aqueous interior) when there is no need to separate two aqueous compartments — the arrangement that emulsifies dietary fat during digestion via bile salts.

Major lipid classes on the MCAT:

• Fatty acids: long hydrocarbon chains with a carboxylic acid head; saturated vs unsaturated chain packing
• Triacylglycerols: three fatty acids esterified to glycerol; primary long-term energy storage
• Phospholipids: amphipathic membrane building blocks with two tails and a polar phosphate head
• Sphingolipids: sphingosine backbone; important in myelin and membrane signaling
• Steroids: four fused rings (gonane nucleus); cholesterol and steroid hormones
• Terpenes/terpenoids: isoprene-derived lipids; precursors to steroids and fat-soluble vitamins