Lipids — Triglycerides and Ester Bonds

Spec mapping — OCR H420 Module 2.1.2 — Biological molecules. This lesson covers the structure of triglycerides (one glycerol + three fatty acids joined by three ester bonds) and the contrast between saturated and unsaturated fatty acids in determining physical properties (melting point, packing). Triglycerides are framed as the principal long-term energy storage molecule of animals and plants, contrasted with glycogen as short-term store (refer to the official OCR H420 specification document for exact wording).

Lipids are a diverse group of organic molecules that share one key property: they are insoluble in water but soluble in organic solvents (e.g., ethanol, ether, chloroform). They include triglycerides (fats and oils), phospholipids, steroids such as cholesterol, and waxes. This lesson focuses on triglycerides — the main energy storage lipids in animals and plants.

1. Overview of Lipids

Unlike carbohydrates and proteins, lipids are not polymers in the strict sense. They are built from smaller subunits but without the regular repeating structure characteristic of polymerisation. The defining feature is their hydrophobic character, due to a predominance of C–H and C–C bonds with few polar groups.

Lipid functions include:

Energy storage (triglycerides in adipose tissue, oil in seeds)
Thermal insulation (subcutaneous fat in mammals)
Physical protection (fat around kidneys and other organs)
Membrane structure (phospholipids and cholesterol)
Hormones (steroid hormones such as testosterone and oestrogen)
Waterproofing (cuticle waxes of plants and insects)
Metabolic water (lipid oxidation produces more water per gram than carbohydrate)

2. Structure of a Triglyceride

A triglyceride consists of one molecule of glycerol covalently bonded to three fatty acids by three ester bonds. It is formed by three condensation reactions, each releasing a water molecule.

Key Definition — Triglyceride: A lipid formed by the condensation of one glycerol and three fatty acids, linked by three ester bonds. The main energy storage molecule of many organisms.

2.1 Glycerol

Glycerol (propane-1,2,3-triol) is a three-carbon alcohol with three –OH groups, one on each carbon:

Glycerol is soluble in water because of its three hydroxyl groups, which form hydrogen bonds with water.

2.2 Fatty Acids

A fatty acid consists of a long hydrocarbon tail (typically 4–24 carbons) ending in a carboxyl group (–COOH):

The hydrocarbon tail is non-polar and hydrophobic.
The carboxyl group is polar and hydrophilic, but in a triglyceride it forms the ester bond and is no longer exposed.
Fatty acids can be saturated (no C=C double bonds) or unsaturated (one or more C=C double bonds).

2.3 Formation of a Triglyceride

Each –OH of glycerol reacts with the –COOH of a fatty acid. Water is released (condensation) and an ester bond (–COO–) is formed. Three such reactions produce a triglyceride and three water molecules.

Reaction	Glycerol site	Fatty acid	Product link	Water released
1	C1 –OH	HOOC—(tail₁)	C1 — O — CO — (tail₁)	+ H₂O
2	C2 –OH	HOOC—(tail₂)	C2 — O — CO — (tail₂)	+ H₂O
3	C3 –OH	HOOC—(tail₃)	C3 — O — CO — (tail₃)	+ H₂O

Overall: one glycerol + three fatty acids → one triglyceride with three ester bonds + 3 H₂O.

The three fatty acid chains may be identical (simple triglyceride) or different (mixed triglyceride). Mixed triglycerides predominate in nature.

2.4 The Ester Bond

An ester bond is the covalent linkage formed between a hydroxyl group (–OH) and a carboxyl group (–COOH) with loss of water. Structurally, it is a –C(=O)–O– linkage.

Exam Tip: The carbonyl oxygen is part of the ester bond; students often forget to draw it.

3. Saturated vs Unsaturated Fatty Acids

Saturated Fatty Acids

All C–C bonds in the tail are single bonds.
The tail is straight and can pack tightly with neighbouring chains.
Higher melting point — solid at room temperature.
Common in animal fats (e.g., butter, lard, tallow).
Linked to increased blood cholesterol and cardiovascular disease when consumed in excess.

Unsaturated Fatty Acids

Contain one or more C=C double bonds in the tail.
Each double bond introduces a kink in the chain, preventing tight packing.
Lower melting point — liquid at room temperature (oils).
Monounsaturated: one double bond (e.g., oleic acid in olive oil).
Polyunsaturated: more than one double bond (e.g., linoleic acid in sunflower oil).
Common in plant oils and oily fish; considered healthier for the cardiovascular system.
Saturated: CH₃-CH₂-CH₂-CH₂-CH₂-COOH (straight chain)
Unsaturated: CH₃-CH₂-CH=CH-CH₂-COOH (kinked at double bond)

4. Structure-Function of Triglycerides

Triglycerides are ideal energy storage molecules for several reasons:

4.1 High Energy Density

A gram of triglyceride yields approximately 38 kJ on complete oxidation, compared with about 17 kJ g⁻¹ for carbohydrate. This is because fatty acid tails contain many C–H bonds in a highly reduced state; oxidation to CO₂ and H₂O releases a large amount of energy.

Fat stores about twice as much energy per gram as carbohydrate.

4.2 Insoluble / Osmotically Inert

Triglycerides are insoluble in water because the hydrocarbon tails are non-polar. They can therefore be stored in large quantities without affecting the water potential of cells or tissues.

4.3 Compact / Low Mass

Because of their high energy density and insolubility, large amounts of energy can be stored in a small volume and mass — important for mobile animals (imagine having to carry twice the mass of glycogen instead).

4.4 Thermal Insulation

Adipose (fat) tissue beneath the skin provides thermal insulation. Blubber in whales, seals and polar bears is a particularly thick layer of subcutaneous fat — essential for maintaining body temperature in cold environments.

4.5 Physical Protection

Fat pads protect organs (e.g., kidneys, heart) from mechanical damage.

4.6 Source of Metabolic Water

When triglycerides are respired aerobically, water is a product. Kangaroo rats, camels and other desert animals rely on metabolic water from fat oxidation as a significant water source. One gram of fat yields approximately 1.1 g of water on oxidation.

4.7 Buoyancy

In some fish (e.g., sharks), oil-rich livers provide buoyancy in the absence of a swim bladder.

5. Why is Fat a Better Long-Term Store than Glycogen?

Feature	Triglyceride	Glycogen
Energy per gram	~38 kJ	~17 kJ
Solubility	Insoluble (osmotically inert)	Slightly soluble
Water storage needed	Minimal (no hydration shell)	Stored with 2–3 × its mass of water
Speed of mobilisation	Slower	Faster
Use	Long-term storage	Short-term storage

Animals use glycogen for rapid energy demands and triglycerides for long-term energy storage, hibernation and thermal insulation.

6. Digestion and Transport of Triglycerides

Triglycerides ingested in the diet are:

Emulsified by bile salts in the small intestine, breaking large fat droplets into smaller ones (increasing surface area).
Hydrolysed by pancreatic lipase into monoglycerides and fatty acids.
Absorbed into intestinal epithelial cells, where they are re-esterified.
Packaged with cholesterol and phospholipids into chylomicrons (a type of lipoprotein) and released into the lacteals of the lymphatic system.
Transported to tissues for storage (adipose) or oxidation (muscles, liver).

Exam Tips

Always draw the ester bond correctly: –C(=O)–O– with the carbonyl oxygen included.
Count carefully: three ester bonds and three water molecules per triglyceride.
When explaining why triglycerides are good energy stores, mention both high energy density and insolubility (osmotic inertness).
Distinguish saturated (single bonds) from unsaturated (C=C double bonds) with the correct terminology.

Common Exam Mistakes

Writing "ether bond" instead of "ester bond".
Forgetting that glycerol has three –OH groups, not two.
Saying lipids are polymers — they are macromolecules but not true polymers (no repeating identical monomer).
Writing "unsaturated fatty acids have a single bond" — they have at least one double bond.
Confusing "insoluble" with "non-polar" — lipids are insoluble because they are predominantly non-polar.
Saying fat yields more ATP than glucose "per mole" — the correct comparison is per gram.

7. Triglyceride Synthesis — SVG of the Condensation Step

8. Synoptic Links

This lesson connects across the OCR H420 specification:

ocr-alevel-biology-biological-molecules Lesson 7 — phospholipids and cholesterol. Diacylglycerol-3-phosphate is the immediate biosynthetic precursor of all membrane phospholipids; the chemistry of ester-bond formation taught here is identical.
ocr-alevel-biology-exchange-transport — lipid transport in blood. Dietary triglycerides are packaged into chylomicrons and released into lacteals; VLDL, LDL and HDL particles transport endogenous triglycerides and cholesterol systemically. The lipoprotein classification underpins clinical interpretation of lipid panels.
ocr-alevel-biology-neuronal-hormonal — insulin/glucagon control of lipolysis. Glucagon and adrenaline stimulate hormone-sensitive lipase to release fatty acids from adipocytes; insulin suppresses this pathway, illustrating reciprocal control of glucose and lipid metabolism.
ocr-alevel-biology-photosynthesis-respiration — β-oxidation and the citric acid cycle. Fatty acids are catabolised by β-oxidation in mitochondria to acetyl-CoA, which enters the citric acid cycle. Each 2-carbon unit yields ~10 ATP equivalents, dramatically more than glucose oxidation per carbon.

9. Specimen Question (Modelled on the OCR H420 Paper Format)

Q (6 marks): Explain why triglycerides are more suitable than carbohydrates for long-term energy storage in animals.

AO breakdown

AO	Marks	Earned by
AO1	2	Triglyceride structure (glycerol + 3 fatty acids, ester bonds)
AO2	3	Energy density, insolubility, osmotic comparison
AO3	1	Long-term vs short-term storage rationale

Lesson 6: Lipids — Triglycerides and Ester Bonds