Lipids

AQA spec ref: 3.1.3 - Lipids

Lipids are a diverse group of biological molecules defined by their insolubility in water (hydrophobic) and solubility in organic solvents (e.g. ethanol). They are not polymers - they are assembled from smaller units but are not made of repeating monomers like polysaccharides or proteins. The two most important lipids in AQA biology are triglycerides (fats and oils) and phospholipids (the basis of all membranes).

Triglycerides

A triglyceride consists of one glycerol molecule joined to three fatty acid chains by ester bonds. The formation of each ester bond is a condensation reaction, releasing one water molecule (so three water molecules are released when one triglyceride is formed). Hydrolysis (adding water) breaks the ester bonds, releasing glycerol and three fatty acids.

Glycerol is a 3-carbon alcohol with three hydroxyl (-OH) groups - one on each carbon. It forms the backbone of all glycerolipids.

Fatty acids are long hydrocarbon chains with a carboxyl group (-COOH) at one end. They vary in:

Chain length: typically 14 - 22 carbons in biological lipids
Degree of saturation: the number of C=C double bonds in the hydrocarbon chain

Saturated vs Unsaturated Fatty Acids

Saturated fatty acids - no C=C double bonds; all carbons fully bonded to hydrogens. The hydrocarbon chains are straight and can pack closely together. This allows strong van der Waals forces between chains, making saturated fats solid at room temperature (e.g. butter, lard, beef fat). Animal fats are mostly saturated.

Unsaturated fatty acids - one or more C=C double bonds. Each double bond introduces a kink (bend) in the hydrocarbon chain, preventing close packing. This reduces van der Waals forces → the lipid has a lower melting point → liquid at room temperature (oil). Plant fats and fish oils are mostly unsaturated.

Monounsaturated: one C=C double bond (e.g. oleic acid in olive oil)
Polyunsaturated: two or more C=C double bonds (e.g. linoleic acid in vegetable oils)

Why this matters biologically: the degree of saturation affects membrane fluidity. Membranes with more unsaturated fatty acids in their phospholipids are more fluid at low temperatures - this is why organisms living in cold environments (fish, Arctic plants) have more unsaturated fatty acids in their cell membranes. See Transport Across Cell Membranes.

Functions of Triglycerides

Energy storage: lipids contain more hydrogen atoms per gram than carbohydrates, so complete oxidation releases more energy (approximately 39 kJ/g vs 17 kJ/g for carbohydrate). Adipose tissue stores triglycerides.
Thermal insulation: subcutaneous fat (under the skin) insulates against heat loss - especially important in aquatic mammals (seals, whales) where water conducts heat away rapidly.
Buoyancy: fat is less dense than water - important in buoyancy of marine mammals.
Protection: surrounds and cushions delicate organs (kidneys, eyes).
Source of metabolic water: oxidation of lipids releases water - important for desert animals (camel's hump stores fat, not water, but fat oxidation produces water).

Phospholipids

A phospholipid is similar to a triglyceride, but one of the three fatty acid chains is replaced by a phosphate group. The phosphate group is charged and polar, and is usually attached to a small alcohol or choline molecule.

This gives the phospholipid two distinct regions:

Hydrophilic head - the phosphate group and glycerol. Charged (phosphate is negatively charged) → attracted to water.
Hydrophobic tail - the two fatty acid hydrocarbon chains → repelled by water.

This amphipathic (dual) nature is fundamental to membrane structure. When phospholipids are placed in water, they spontaneously arrange into a bilayer with:

Hydrophilic heads on the outside (facing water on both sides)
Hydrophobic tails on the inside (shielded from water)

This is thermodynamically spontaneous - it minimises the unfavourable exposure of hydrophobic tails to water.

The bilayer forms a selectively permeable barrier: small non-polar molecules (O₂, CO₂, steroid hormones) can dissolve in and cross the hydrophobic core. Large or charged molecules (ions, glucose, amino acids) cannot - they need protein channels or carriers. This selective permeability is essential for cellular regulation.

See Cell Structure and Transport Across Cell Membranes for how phospholipids function in the fluid mosaic model.

Other Lipids

Waxes - long-chain fatty acids esterified to long-chain alcohols. Form the cuticle on leaves (prevents water loss) and give leaves their waxy coating. Also form earwax and the waterproofing of bird feathers.

Steroids - lipids with a characteristic four-ring carbon structure. Not glycerol-based. Examples:

Cholesterol - integral component of animal cell membranes; regulates fluidity; precursor to steroid hormones
Steroid hormones (testosterone, oestrogen, cortisol) - lipid-soluble, so they can diffuse through the plasma membrane and act on intracellular receptors

Glycolipids - lipid with a carbohydrate attached. Found on the outer surface of the plasma membrane; involved in cell recognition. See Cell Recognition and the Immune System.

The Emulsion Test for Lipids

Add a small amount of the sample to a dry test tube. Add absolute ethanol (≈100% ethanol) and shake to dissolve any lipid. Add an equal volume of distilled water and shake.

Positive result: a milky white emulsion forms - lipid is present (tiny lipid droplets refract light)
Negative result: solution remains clear

Why ethanol first? Lipids dissolve in ethanol (organic solvent) but not in water. Adding water then causes the dissolved lipid to come out of solution as an emulsion.

Summary

Triglyceride=glycerol+3 fatty acids, joined by ester bonds via condensation
Saturated fatty acids: no C=C double bonds, straight chains, pack tightly, solid at room temp
Unsaturated fatty acids: one or more C=C double bonds, kinked chains, lower melting point, liquid at room temp
Triglyceride functions: energy storage, insulation, protection, metabolic water
Phospholipid = glycerol + 2 fatty acids + phosphate group. Amphipathic → spontaneously forms bilayer
Hydrophilic head (phosphate) faces water; hydrophobic tails face inward → selectively permeable membrane
Emulsion test: dissolve in ethanol→add water→milky white emulsion=lipid present

AQA Exam Tips

Ester bonds not glycosidic bonds: triglycerides are joined by ester bonds (-COO-). Glycosidic bonds are in carbohydrates. Do not confuse them.
Why lipids are better energy stores than carbohydrates: lipids contain more hydrogen per unit mass (and hydrogens are what are oxidised to release energy). This gives ~39 kJ/g vs ~17 kJ/g for carbohydrate.
Phospholipid in a membrane: the specific reason for bilayer formation is that the hydrophobic tails are repelled from water → they cluster together inward, while hydrophilic heads face outward toward the aqueous environment. Mention "thermodynamically spontaneous" or "minimises free energy" for top marks.
Saturated vs unsaturated - exam question: if asked why a membrane is more fluid at low temperatures, say: more unsaturated fatty acids → more double bonds → more kinks in tails → tails cannot pack closely → remain fluid at lower temperatures.
Emulsion test specificity: the emulsion test detects all lipids. It is not specific to triglycerides (also detects phospholipids, cholesterol, waxes). AQA may ask you to state it as a test for lipids in general.
Condensation vs hydrolysis: always link to bond formation (condensation) and bond breaking (hydrolysis). Triglyceride formation: 3 condensation reactions, 3 water molecules released, 3 ester bonds formed.