9.1 Structure and classification of lipids

Lipid Types

Lipids are a structurally diverse class of biomolecules united by a shared property: they are poorly soluble in water. This hydrophobicity underlies their biological roles in energy storage, membrane architecture, and cell signaling. Understanding lipid structure at the molecular level explains why each class behaves differently in aqueous biological environments.

Fatty acids serve as the fundamental building blocks for most lipid classes. Their chain length, degree of saturation, and ability to form larger assemblies like micelles and bilayers directly determine the physical and functional properties of the lipids they compose.

Fatty Acids and Triglycerides

A fatty acid is a long hydrocarbon chain (typically 12–20 carbons) terminated by a carboxyl group ($-COOH$). The chain can be saturated, containing only $C-C$ single bonds, or unsaturated, containing one or more $C=C$ double bonds. This distinction has major consequences for molecular packing and physical state, covered in detail below.

Triglycerides (triacylglycerols) form when three fatty acids are esterified to a single glycerol backbone. The linkage is an ester bond between each fatty acid's carboxyl group and one of glycerol's three hydroxyl groups. Triglycerides are the primary long-term energy storage molecules in both animals (stored in adipose tissue) and plants (concentrated in seeds and fruit oils). Because all three hydroxyl groups on glycerol are esterified, triglycerides are nonpolar and highly hydrophobic, which is why they aggregate into lipid droplets rather than dissolving in the cytoplasm.

Phospholipids and Steroids

Phospholipids resemble triglycerides but with a key modification: one of the three fatty acid positions on glycerol is replaced by a phosphate group, which is often linked to a small polar molecule (choline, serine, ethanolamine, or inositol). This gives phospholipids an amphipathic structure: a hydrophilic phosphate head and two hydrophobic fatty acid tails. That dual character is exactly what makes them ideal for building cell membranes, where they spontaneously arrange into bilayers.

Steroids are structurally distinct from other lipids. Instead of long acyl chains, they share a fused four-ring carbon skeleton (three six-membered rings and one five-membered ring). Cholesterol is the most abundant steroid in animal cells and serves two roles: it modulates membrane fluidity, and it acts as the biosynthetic precursor for steroid hormones (testosterone, estradiol, cortisol) and bile salts. Small chemical modifications to the ring system or its side chains produce dramatically different biological activities.

Waxes

Waxes are esters formed between a long-chain fatty acid and a long-chain alcohol (both typically 14–36 carbons). The result is an extremely hydrophobic molecule with a high melting point. In plants, waxes coat leaf surfaces as the cuticle, reducing water loss. In animals, examples include beeswax and the cerumen (earwax) that protects the ear canal. Their water-resistant properties make them effective as protective barriers across biology.

Fatty Acid Structure

Saturation and Unsaturation

The degree of saturation in a fatty acid chain controls how tightly neighboring molecules can pack together, which in turn determines melting point and physical state.

• Saturated fatty acids have no $C=C$ double bonds. The hydrocarbon chain adopts a fully extended, straight conformation. These straight chains pack tightly via van der Waals interactions, producing higher melting points. Palmitic acid (16:0) and stearic acid (18:0) are common examples. Fats rich in saturated fatty acids, like butter, tend to be solid at room temperature.
• Unsaturated fatty acids contain one (monounsaturated, e.g., oleic acid, 18:1) or more (polyunsaturated, e.g., linoleic acid, 18:2) double bonds. In biological fatty acids, these double bonds are almost always in the cis configuration. A cis double bond introduces a roughly 30° kink in the chain, which disrupts tight packing between neighboring molecules. The result is weaker intermolecular interactions and a lower melting point. Oils rich in unsaturated fatty acids, like olive oil, are liquid at room temperature.

The shorthand notation (e.g., 18:2) lists the number of carbons first, then the number of double bonds. You'll see this notation frequently when comparing fatty acid composition.

Amphipathic Nature

Fatty acids are amphipathic: they contain both a hydrophilic region and a hydrophobic region within the same molecule.

• The carboxyl group ($-COOH$) is polar and can ionize at physiological pH to $-COO^-$, making it hydrophilic. It readily forms hydrogen bonds and electrostatic interactions with water.
• The hydrocarbon tail is nonpolar and hydrophobic. It cannot hydrogen-bond with water and is driven away from aqueous environments by the hydrophobic effect.

This amphipathic character is not just a structural curiosity. It is the thermodynamic driving force behind the self-assembly of fatty acids and phospholipids into organized structures like micelles and bilayers.

Lipid Assemblies

When amphipathic lipids are placed in water, they spontaneously organize to minimize unfavorable contacts between hydrophobic tails and water. The type of assembly that forms depends on the molecular geometry of the lipid.

Micelles

Micelles are small, roughly spherical aggregates. The hydrophilic heads face outward into the aqueous phase, while the hydrophobic tails cluster together in the interior, shielded from water.

Micelles form preferentially from lipids that have a single hydrocarbon tail (like free fatty acids and detergents) because the cone-like shape of these molecules favors a curved surface. Above a threshold concentration called the critical micelle concentration (CMC), individual molecules spontaneously aggregate into micelles.

In digestion, bile salt micelles are essential for solubilizing dietary lipids and fat-soluble vitamins (A, D, E, K) in the aqueous environment of the small intestine, making them accessible for absorption.

Lipid Bilayers

Phospholipids, with their two fatty acid tails, have a more cylindrical molecular shape. Rather than curving into a small sphere, they pack into flat lipid bilayers: two leaflets arranged tail-to-tail, with hydrophilic heads facing the aqueous environment on both sides.

This bilayer is the structural foundation of all biological membranes. It creates a hydrophobic interior roughly 30 Å thick that acts as a permeability barrier, separating the cell's interior from the external environment.

Several factors tune bilayer properties:

• Fatty acid saturation: More unsaturated tails increase membrane fluidity because kinked chains pack less tightly.
• Chain length: Shorter chains also increase fluidity by reducing van der Waals contacts.
• Cholesterol: At physiological concentrations, cholesterol inserts between phospholipids with its hydroxyl group near the head groups and its rigid ring system alongside the upper portion of the acyl chains. This broadens the phase transition, preventing the membrane from becoming too fluid at high temperatures or too rigid at low temperatures.
• Membrane proteins: Integral and peripheral proteins embedded in or associated with the bilayer carry out transport, signaling, and enzymatic functions.

The fluid mosaic model describes the membrane as a dynamic two-dimensional structure where both lipids and proteins can move laterally, giving the cell a flexible yet organized boundary.