Introduction to Lipids: Triglycerides Revision Notes | A Level AQA Biology

1. Why Lipids Matter: The Energy Store You Cannot Ignore

Gram for gram, triglycerides store more than twice the energy of carbohydrates. That is why your body turns to fat reserves during prolonged exercise or fasting — not because fat is a backup plan, but because it is the most efficient long-term energy currency available.

Lipids are a chemically diverse group, but at A Level the two groups you need to know are triglycerides and phospholipids. This lesson focuses on triglycerides: how they are built, what holds them together, and how their fatty acid tails determine their properties. Compared to phospholipids—which have a hydrophilic phosphate group in place of one fatty acid—triglycerides are fully hydrophobic and are suited for long-term energy storage, whereas phospholipids form the bilayer structure of cell membranes. The difference in structure leads to distinct biological roles.

2. The Building Blocks: Glycerol and Fatty Acids

A triglyceride contains two types of component: one molecule of glycerol and three molecules of fatty acid. Glycerol is a small, three-carbon alcohol — each carbon carries a hydroxyl group (-OH). Those three -OH groups are the attachment points for the fatty acids.

The R in RCOOH represents the hydrocarbon tail — a chain of carbon and hydrogen atoms. This tail is the part that differs between fatty acids and, crucially, it is the R-group that determines whether a fatty acid is saturated or unsaturated. The -COOH end is the reactive end: it is where the bond to glycerol forms.

The diagram above shows glycerol. Notice the three -OH groups, one on each carbon. Each of these will react with one fatty acid molecule to build the complete triglyceride.

3. Forming the Triglyceride: Condensation and the Ester Bond

When glycerol and a fatty acid react, the -OH from glycerol and the -COOH from the fatty acid combine. A molecule of water is released — this is the hallmark of a condensation reaction. The bond that remains after the water leaves is called an ester bond.

This happens three times in total: once for each of the three fatty acids joining to the three -OH groups on glycerol. Each reaction releases one water molecule, so forming a complete triglyceride releases three water molecules overall.

Worked example: Building a Triglyceride Step by Step

Describe, with reference to bonds and molecules, how a triglyceride is formed from glycerol and three fatty acid molecules.

1. Glycerol has three -OH groups. Each fatty acid (RCOOH) has one -COOH group. — These are the functional groups that participate in the condensation reaction. Knowing which groups react is the foundation of any mark-scheme answer.
2. One -OH (from glycerol) + one -COOH (from fatty acid 1) → ester bond + H₂O — Water is eliminated: the H comes from the -OH on glycerol and the OH comes from the -COOH of the fatty acid. The remaining bond between the oxygen and the carbonyl carbon is the ester bond.
3. Two further condensation reactions occur at the remaining -OH groups on glycerol, each releasing one H₂O and forming one ester bond. — Three condensation reactions in total, three ester bonds formed, three water molecules released.
4. 1 glycerol + 3 fatty acids → 1 triglyceride + 3 H₂O — This is the complete equation for triglyceride synthesis. In the reverse direction (hydrolysis), water is added back and the ester bonds are broken.

Answer: A triglyceride is formed by three condensation reactions between one glycerol and three fatty acids. Each reaction forms one ester bond and releases one water molecule, giving three ester bonds and three water molecules in total.

A useful way to remember the direction of this reaction: condensation removes water and builds molecules. Hydrolysis adds water and breaks molecules apart. Digestion of dietary fats is hydrolysis — enzymes (lipases) add water across each ester bond to release glycerol and fatty acids.

4. Saturated and Unsaturated Fatty Acids: Reading the R-Group

Now that you know how the triglyceride is assembled, the next question is: what makes one fatty acid different from another? The answer lies entirely in the R-group — the hydrocarbon tail attached to the -COOH end.

In a saturated fatty acid, every carbon in the R-group chain is joined to the next by a single bond (C-C). Each carbon carries the maximum number of hydrogen atoms — the chain is, in chemical terms, saturated with hydrogen. On a diagram, the tail appears as a straight, unbranched zigzag chain.

In an unsaturated fatty acid, at least one pair of adjacent carbons in the R-group is joined by a double bond (C=C). At each double bond, two hydrogen atoms are absent compared with the saturated equivalent. A fatty acid with one double bond is monounsaturated; one with two or more double bonds is polyunsaturated. On a diagram, the double bond introduces a distinct kink or bend in the chain.

The kink introduced by a C=C double bond has a direct physical consequence. Saturated fatty acid chains pack tightly together, allowing strong van der Waals forces between adjacent molecules — hence animal fats are solid at room temperature. Unsaturated chains cannot pack as closely because the kinks prevent tight alignment, so the intermolecular forces are weaker and the lipid remains liquid at room temperature.

5. Recognising Fatty Acids from Diagrams

AQA exam questions frequently present structural diagrams of fatty acids and ask you to identify whether they are saturated or unsaturated. There is a reliable two-step method for this.

• Locate the -COOH group at one end of the molecule — this confirms you are looking at a fatty acid.
• Scan along the R-group (hydrocarbon tail) for any C=C double bonds.
• If you find at least one C=C: the fatty acid is unsaturated.
• If all bonds in the chain are C-C single bonds: the fatty acid is saturated.
• Count the number of C=C bonds to distinguish monounsaturated (one) from polyunsaturated (two or more).

On a skeletal formula (the zigzag line diagram common in AQA papers), a double bond appears as two parallel lines between two carbon vertices. A straight, uninterrupted zigzag with no parallel lines indicates a saturated chain. Practise identifying this quickly — exam questions often give you 30 seconds of reading time per mark, so pattern recognition matters.

6. Putting It Together: Triglyceride Structure as a Whole

A complete triglyceride consists of the glycerol backbone with three fatty acid tails extending from it, each attached via an ester bond. The three fatty acid tails do not have to be identical — a single triglyceride molecule can carry a mixture of saturated and unsaturated fatty acids on its three positions.

The diagram above shows a triglyceride with three saturated fatty acid chains. Notice the three ester linkages (-COO-) connecting each fatty acid to the glycerol backbone. In an unsaturated triglyceride, one or more of those chains would show a C=C double bond and the associated kink in the tail.

The non-polar nature of the long hydrocarbon tails means triglycerides are hydrophobic — they do not dissolve in water. This is why fat droplets form in aqueous environments and why lipids must be emulsified by bile salts before digestion can proceed efficiently in the small intestine.

7. Exam Technique: Scoring Full Marks on Triglyceride Questions

Triglyceride questions appear regularly in AQA papers and tend to target three areas: naming the bond formed, describing the condensation reaction, and identifying saturation from a diagram. Each area has its own mark-winning language.