Lipids

Lipids are a diverse group of biological molecules that share the common property of being hydrophobic (insoluble in water) and soluble in organic solvents such as ethanol and acetone. Unlike carbohydrates, proteins, and nucleic acids, lipids are not true polymers — they are not built from repeating monomer units joined by condensation reactions in the same way. However, triglycerides and phospholipids are assembled from smaller components (glycerol and fatty acids) via condensation reactions.

The main types of lipid at A-Level are triglycerides, phospholipids, and cholesterol.

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Fatty Acids

A fatty acid consists of a carboxyl group (–COOH) attached to a long hydrocarbon chain (typically 14–22 carbon atoms). The hydrocarbon tail is non-polar and therefore hydrophobic.

Saturated Fatty Acids

• Every carbon–carbon bond in the hydrocarbon chain is a single bond (C–C).
• The chain is straight, allowing fatty acid molecules to pack closely together.
• Lipids rich in saturated fatty acids tend to be solid at room temperature (e.g., butter, lard, animal fats).
• Common examples: palmitic acid (C16), stearic acid (C18).

Unsaturated Fatty Acids

• One or more carbon–carbon double bonds (C=C) are present in the hydrocarbon chain.
• Monounsaturated: one C=C double bond (e.g., oleic acid).
• Polyunsaturated: two or more C=C double bonds (e.g., linoleic acid, linolenic acid).
• Each double bond introduces a kink (bend) in the chain, preventing close packing.
• Lipids rich in unsaturated fatty acids tend to be liquid at room temperature (e.g., olive oil, sunflower oil).

Exam Tip: If asked why unsaturated fats have lower melting points than saturated fats, explain that the kinks caused by C=C double bonds reduce the strength of van der Waals forces between adjacent fatty acid tails because the molecules cannot pack as closely together.

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Triglycerides

Key Definition: A triglyceride is a lipid formed from one molecule of glycerol and three fatty acid molecules, joined by three ester bonds through condensation reactions.

Formation

Glycerol is a three-carbon alcohol with three hydroxyl (–OH) groups. Each hydroxyl group reacts with the carboxyl group of a fatty acid in a condensation reaction, releasing water and forming an ester bond.

Glycerol + 3 fatty acids → triglyceride + 3H₂O

The reverse reaction (hydrolysis) breaks the ester bonds by adding water, regenerating glycerol and three fatty acids. This occurs during digestion (catalysed by lipase enzymes) and during lipolysis in adipose tissue.

Properties and Functions of Triglycerides

Property	Explanation	Biological Importance
High energy content	More C–H bonds per molecule than carbohydrates; higher ratio of hydrogen to oxygen	Yield approximately 2× more energy per gram than carbohydrates when oxidised during respiration
Insoluble	Non-polar hydrocarbon chains; no effect on water potential	Can be stored in cells without drawing in water by osmosis; adipose tissue is compact
Low density	Less dense than water	Helps aquatic mammals (whales, seals) maintain buoyancy
Thermal insulation	Subcutaneous adipose tissue	Reduces heat loss in endotherms; blubber layer in marine mammals
Protection	Fat deposits around organs	Kidney and heart are cushioned by adipose tissue
Waterproofing	Hydrophobic nature	Sebum on skin and fur; waxy cuticle on leaves (contains lipids)
Metabolic water	Oxidation of fats produces water	Desert animals (e.g., camel, kangaroo rat) obtain metabolic water from fat reserves

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Phospholipids

Key Definition: A phospholipid has a similar structure to a triglyceride, but one of the three fatty acid chains is replaced by a phosphate group (often with an additional polar molecule attached).

Structure