Lipids are a broad class of biomolecules defined by their insolubility in water. They serve as energy stores, structural components of membranes, and precursors to hormones and signaling molecules. This section covers the major lipid classes you need to know: triglycerides, phospholipids, glycolipids, sterols, and waxes.

Structure and Composition of Triglycerides

Triglycerides are the most abundant lipids in your diet and the primary way your body stores energy. Each triglyceride is built from a glycerol backbone (a 3-carbon alcohol) with three fatty acids attached via ester bonds. Those ester bonds form through a condensation reaction between the hydroxyl groups on glycerol and the carboxyl group ( $-COOH$ ) on each fatty acid.

Fatty acids themselves are long hydrocarbon chains, typically 12–22 carbons long, with a carboxyl group at one end. The length of the chain and the presence or absence of double bonds between carbons determine the fatty acid's physical and chemical behavior.

Saturated and Unsaturated Fatty Acids

The distinction between saturated and unsaturated fatty acids comes down to double bonds in the hydrocarbon chain, and this single structural difference has major consequences for physical properties.

Saturated fatty acids have no carbon-carbon double bonds. Their chains pack tightly together, which is why saturated fats are solid at room temperature (butter, lard, coconut oil).
Monounsaturated fatty acids (MUFAs) have one double bond, which introduces a kink in the chain. That kink prevents tight packing, so these fats are liquid at room temperature (olive oil, avocados).
Polyunsaturated fatty acids (PUFAs) have two or more double bonds, creating even more kinks. These are also liquid at room temperature (soybean oil, corn oil, fatty fish like salmon).

The kink matters because it disrupts the orderly stacking of fatty acid chains. More kinks mean weaker intermolecular forces between chains, which lowers the melting point. That's why a bottle of olive oil flows freely while a stick of butter holds its shape.

One more detail worth knowing: unsaturated fatty acids can exist in cis or trans configurations. Naturally occurring unsaturated fats are almost always cis, which produces the characteristic bend. Trans fats have a straighter chain shape similar to saturated fats, which is why industrially produced trans fats raise similar health concerns.

Structure and Composition of Triglycerides, Unit 3: Biochemistry – Douglas College Human Anatomy & Physiology I (2nd ed.)

Phospholipids and Glycolipids

Structure and Function of Phospholipids

Phospholipids look similar to triglycerides but with one key substitution: instead of a third fatty acid, the glycerol backbone carries a phosphate group linked to a polar molecule such as choline, serine, or ethanolamine.

This gives phospholipids their defining property: they are amphipathic. The phosphate-containing head is hydrophilic (attracted to water), while the two fatty acid tails are hydrophobic (repelled by water). In an aqueous environment, phospholipids spontaneously arrange into a bilayer, with hydrophobic tails facing inward and hydrophilic heads facing outward toward the water on both sides. This bilayer is the structural foundation of all cell membranes.

A common phospholipid you'll encounter is lecithin (phosphatidylcholine), which also functions as an emulsifier in food products, helping oil and water mix.

Structure and Composition of Triglycerides, Lipids | Boundless Anatomy and Physiology

Glycolipids and Their Roles

Glycolipids share the amphipathic character of phospholipids but replace the phosphate group with a carbohydrate group, which can range from a single sugar (like glucose or galactose) to a complex oligosaccharide chain.

Found on the outer leaflet of cell membranes, with the carbohydrate portion extending into the extracellular space
Play roles in cell recognition, cell signaling, and cell adhesion
The carbohydrate portions of glycolipids contribute to the glycocalyx, the sugar-rich coating on cell surfaces that helps cells identify each other (this is relevant to blood type determination and immune function)

Sterols and Waxes

Cholesterol and Its Functions

Sterols are structurally distinct from the other lipids covered so far. Instead of long fatty acid chains, sterols are built around a four-fused-ring structure (three six-carbon rings and one five-carbon ring) with a hydroxyl group ( $-OH$ ) attached.

Cholesterol is the most important sterol in animal tissues. It serves several critical functions:

Membrane component: Cholesterol inserts itself between phospholipids in cell membranes, where it modulates fluidity. At high temperatures it restricts movement; at low temperatures it prevents tight packing. This keeps membranes functional across a range of conditions.
Precursor molecule: Your body uses cholesterol as the starting material to synthesize steroid hormones (estrogen, testosterone, cortisol), bile acids (needed for fat digestion), and vitamin D.

Plants don't contain cholesterol but do produce related sterols called phytosterols (like beta-sitosterol), which can actually compete with cholesterol for absorption in the gut.

Properties and Uses of Waxes

Waxes are esters formed from long-chain fatty acids bonded to long-chain alcohols. Their long, saturated hydrocarbon chains make them extremely hydrophobic and resistant to hydrolysis, which is why they excel as protective barriers.

In nature: plant leaf cuticles (preventing water loss), insect exoskeletons, and coatings on animal fur and feathers
In food science and industry: fruit coatings to extend shelf life, candles, polishes, and cosmetic formulations

Waxes are not a significant energy source in the human diet because we lack the enzymes to efficiently digest them.