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1.1.5Unit 1: Lifestyle, Transport, Genes and Health

Triglycerides and Ester Bonds — IAL Edexcel Linear Revision Notes & Topical Questions

5 min read 4 parts Lesson 12 of 29 Unit quiz

📚 IAL Edexcel Linear Hub / Topical Revision
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Curriculum Equation Key معادلة تفاعل تكوين الإستر (الأسترة) - Esterification Reaction EquationFast Exam Solution
R-COOH + R2-OH ⇌ (conc. H₂SO₄) R-COO-R2 + H₂O
Word Equation & Conditions: حمض كربوكسيلي + كحول (في وجود حمض الكبريتيك المركز كعامل حفاز ونازع للماء) ⇌ إستر + ماء
Location & Specifications: الكيمياء العضوية للثانوية العامة والتحصيلي و Edexcel Chemistry.
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Part 1 of 4Building a Triglyceride

1
A molecule of glycerol, an alcohol with three –OH groups, is the backbone of the triglyceride.
2
A fatty acid has a carboxyl group (–COOH) at one end of a hydrocarbon chain, written as R–COOH.
3
The –OH of glycerol reacts with the –COOH of a fatty acid in a condensation reaction, forming an ester bond and releasing water.
4
This happens three times, so three fatty acids are attached to one glycerol and three water molecules are released.
5
The overall equation is glycerol + 3 fatty acids → triglyceride + 3 H₂O, and the reverse hydrolysis reaction breaks the ester bonds.

Part 2 of 4Saturated and Unsaturated Fatty Acids

Saturated
  • Saturated The hydrocarbon chain has no carbon-to-carbon double bonds,
    • so it is straight and the molecules pack tightly,
    • making fats solid at room temperature.
Monounsaturated
  • Monounsaturated There is one C=C double bond in the chain, which causes a bend or kink in the shape of the molecule.
Polyunsaturated
  • Polyunsaturated Two or more C=C double bonds produce more kinks, so molecules cannot pack closely and are liquid oils at room temperature.
Dietary link
  • Dietary link A diet rich in saturated fats is linked with raised blood cholesterol and an increased risk of cardiovascular disease.
FeatureSaturatedUnsaturated
C=C double bondsNoneOne or more
Shape of chainStraightKinked
State at room temperatureUsually solidUsually liquid

Part 3 of 4Functions of Triglycerides

Functions of Triglycerides
Energy store
  • Energy store Triglycerides contain many C–H bonds, so they release about twice as much energy per gram as carbohydrates when respired.
Insulation
  • Insulation A layer of fat under the skin reduces heat loss, and fat around organs protects them from physical shock.
Waterproofing
  • Waterproofing Lipids are hydrophobic and form waterproof coatings on skin, fur or feathers and plant cuticles.
Metabolic water
  • Metabolic water Oxidising lipids releases water, which is useful for desert animals that have little access to drinking water.

Part 4 of 4Exam Checklist & Tips

1.1.5 – Key Points for the Exam

  • Triglyceride A lipid made of one glycerol joined by ester bonds to three fatty acids.
  • Ester bond The bond formed between the –OH of glycerol and the –COOH of a fatty acid.
  • Saturated fatty acid A fatty acid with no double bonds between carbon atoms in its chain.
  • Hydrophobic
    • Repelling water;
      • lipids do not dissolve in water.

Common Mistakes

  • Saying that three fatty acids and one glycerol release one water molecule;
    • three water molecules are released.
  • Describing triglycerides as polymers; they are not made of repeating identical monomers.
  • Stating that unsaturated fatty acids have no double bonds; they have at least one C=C.
  • Confusing ester bonds with glycosidic or peptide bonds.

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