OCR · A-Level · Biology
Course version: H420 · Version from Sept 2025
(h) the structure of a triglyceride and a phospholipid as examples of…
Full specification wording
Biological molecules: (h) the structure of a triglyceride and a phospholipid as examples of macromolecules
A triglyceride forms when one glycerol molecule condenses with three fatty acid molecules, releasing three water molecules and forming three ester bonds.
Checked against the OCR H420 specification (opens in a new tab) on .
- Point 8 of 20
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A triglyceride forms when one glycerol molecule condenses with three fatty acid molecules, releasing three water molecules and forming three ester bonds. The fatty acids may be saturated or unsaturated, and the triglyceride is a macromolecule used for energy storage because it is compact and insoluble. A phospholipid forms when one glycerol condenses with two fatty acids and one phosphate group, so it has a hydrophilic phosphate head and two hydrophobic fatty acid tails. This amphipathic structure lets phospholipids form bilayers in membranes. Recognise both as macromolecules built by condensation, and distinguish them by the third glycerol position: a fatty acid in a triglyceride, a phosphate group in a phospholipid.
What earns the marks
- A triglyceride consists of one glycerol molecule bonded to three fatty acid molecules by ester bonds formed in condensation reactions.
- Three water molecules are released when one triglyceride is formed by condensation.
- A phospholipid consists of one glycerol bonded to two fatty acids and one phosphate group.
- The phosphate group gives a hydrophilic head, while the two fatty acid tails are hydrophobic, allowing phospholipids to form bilayers.
Common mistakes and how to put them right
Stating that a phospholipid has three fatty acids.
Put it right by noting it has two fatty acids and one phosphate group.
Confusing ester bonds with peptide bonds.
Put it right by identifying ester bonds between glycerol and fatty acids.
Saying condensation releases one water molecule per triglyceride.
Put it right by noting three water molecules are released, one per bond formed.
Exam tips
- Count the subunits and bonds explicitly: glycerol plus three fatty acids gives three ester bonds in a triglyceride.
- Use the terms hydrophilic and hydrophobic accurately when describing the phospholipid head and tails.
- Link the amphipathic nature of phospholipids to bilayer formation in membranes.
Try the lesson
The start of the Biological molecules: (h) the structure of a triglyceride and a phospholipid as examples of macromolecules lesson, made from checked teaching notes for this specification point.
Have a go first
A triglyceride molecule is formed from which two types of component?
Listen
A triglyceride forms when one glycerol molecule condenses with three fatty acid molecules, releasing three water molecules and forming three ester bonds. The fatty acids may be saturated or unsaturated, and the triglyceride is a macromolecule used for energy storage because it is compact and insoluble. A phospholipid forms when one glycerol condenses with two fatty acids and one phosphate group, so it has a hydrophilic phosphate head and two hydrophobic fatty acid tails. This amphipathic structure lets phospholipids form bilayers in membranes. Recognise both as macromolecules built by condensation, and distinguish them by the third glycerol position: a fatty acid in a triglyceride, a phosphate group in a phospholipid. A strong answer earns marks for these points. A triglyceride consists of one glycerol molecule bonded to three fatty acid molecules by ester bonds formed in condensation reactions. Three water molecules are released when one triglyceride is formed by condensation. A phospholipid consists of one glycerol bonded to two fatty acids and one phosphate group. The phosphate group gives a hydrophilic head, while the two fatty acid tails are hydrophobic, allowing phospholipids to form bilayers. A common slip: Stating that a phospholipid has three fatty acids. Instead, noting it has two fatty acids and one phosphate group. Exam tip: Count the subunits and bonds explicitly: glycerol plus three fatty acids gives three ester bonds in a triglyceride.
See it
Key words
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Condensation and ester bonds
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Triglyceride structure
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Phospholipid structure
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Amphipathic properties and membranes
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See every definition
- Condensation and ester bonds
- A triglyceride forms when one glycerol molecule condenses with three fatty acid molecules, releasing three water molecules and forming three ester bonds.
- Triglyceride structure
- One glycerol molecule bonded to three fatty acid molecules by ester bonds, with the fatty acids saturated or unsaturated.
- Phospholipid structure
- One glycerol bonded to two fatty acids and one phosphate group, giving a hydrophilic phosphate head and two hydrophobic fatty acid tails.
- Amphipathic properties and membranes
- Having a hydrophilic phosphate head and hydrophobic fatty acid tails lets phospholipids form bilayers in membranes.
Label the diagram
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