Relate triglyceride structure to storage functions
A triglyceride contains glycerol joined to three fatty acids by ester bonds and is a non-polar, hydrophobic molecule.
AS Level; Cambridge International Biology 9700, 2028–2030. Original explanatory diagrams and examples support syllabus learning; written answers are self-checked and teacher review remains pending.
Before you begin
- Recognise that biological molecules are built from smaller molecules and that structure is related to function.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
How many fatty acids are joined to glycerol in a triglyceride?
What you will learn
- Describe how glycerol and three fatty acids form a triglyceride
- Distinguish saturated and unsaturated fatty acids
- Relate triglyceride structure to biological storage functions
Three ester bonds connect glycerol and fatty acids
A triglyceride contains one glycerol molecule joined to three fatty acids. Each fatty acid is joined to glycerol by an ester bond. These bonds form through condensation reactions that remove water.
A saturated fatty acid has no carbon–carbon double bonds in its hydrocarbon chain. An unsaturated fatty acid has one or more carbon–carbon double bonds. These structural differences affect chain shape and packing, but a triglyceride may contain different fatty acids.
Non-polar molecules are suited to storage
Triglycerides are non-polar and hydrophobic. They do not dissolve in water and can be stored in droplets without making the cell cytoplasm strongly more concentrated. They are a concentrated energy store; oxidation in respiration releases energy.
Triglycerides also provide thermal insulation in some organisms. Relate a stated structure to a relevant role and avoid claiming that hydrophobicity alone explains every function.
Pause and explain
What bond joins a fatty acid to glycerol in a triglyceride?
Worked example
A molecule has glycerol joined to three fatty acids, with several ester bonds and no polar region. Classify it and state one function.
Show the worked solution
- One glycerol plus three fatty acids identifies the molecule.
- Three ester links connect the components.
- Hydrophobicity supports storage.
Answer The molecule is a triglyceride: one glycerol is joined to three fatty acids by ester bonds. It is non-polar and hydrophobic and can act as a concentrated energy store; it can also provide thermal insulation in some organisms.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Describe the structure of a triglyceride.
Give me a hint
State the component numbers and bond type.
Compare my reasoning
- One glycerol and three fatty acids are present.
- Each fatty acid is joined to glycerol by an ester bond.
A triglyceride contains one glycerol molecule joined to three fatty-acid molecules by three ester bonds. The ester bonds form through condensation reactions.
Look for these in your work
- Correct components and counts
- Ester bond stated
Explain how triglyceride properties suit energy storage.
Give me a hint
Connect non-polarity with water and storage.
Compare my reasoning
- Triglycerides are non-polar and hydrophobic.
- They store energy in a concentrated form without dissolving in cytoplasm.
Triglycerides are non-polar and hydrophobic, so they do not dissolve in the watery cytoplasm. They can be stored in droplets as a concentrated energy store; oxidation in respiration releases energy.
Look for these in your work
- Property stated
- Storage role explained
A fatty acid has one carbon–carbon double bond. Classify it and explain what the word “saturated” would mean instead.
Give me a hint
Use carbon–carbon double bonds.
Compare my reasoning
- One or more C=C bonds means unsaturated.
- Saturated chains have no C=C bonds.
The fatty acid is unsaturated because it has a carbon–carbon double bond. A saturated fatty-acid chain has no carbon–carbon double bonds.
Look for these in your work
- Correct classification
- Definition contrasted
Common mistakes
- Using two fatty acids to describe a triglyceride
- Calling an unsaturated fatty acid one with no double bonds
- Calling triglycerides polar or water-soluble
What can you explain now?
How many fatty acids are joined to glycerol in a triglyceride?
Compare with the explanation
Three
“Tri-” indicates three fatty-acid chains.
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Make it stick
How do triglyceride components and non-polar properties relate to energy storage?
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