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MHT-CET Chemistry · Biomolecules

Lipids, Enzymes and Other Biomolecules

Fats and oils are triglycerides — triesters of glycerol with three fatty acids, hydrolysed by three water molecules and saponified to soap, the sodium or potassium salt of a higher fatty acid; the common fatty acids are counted by carbons and double bonds (linolenic 18:3), enzymes are globular protein catalysts named for their substrate and site, and haemoglobin is the biological coordination compound.

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Why this matters

5 PYQs, none HARD — a short page. Two ask the number of C=C bonds in linolenic acid (three), one the water needed to hydrolyse n mol of triglyceride (3n), one what a soap is (K or Na salt of a higher fatty acid), one the enzymes in saliva; haemoglobin as a coordination compound sits here too. One card.

Concept 1 of 1

Triglycerides, Fatty Acids, Soap and Enzymes

Intuition

A triglyceride has three ester bonds, so complete hydrolysis needs three waters per molecule and returns glycerol plus three fatty acids; done with NaOH or KOH it gives the salts instead — soap. A fatty acid is described by two numbers, carbons and C=C: palmitic 16:0 and stearic 18:0 are saturated (solid fats), oleic 18:1, linoleic 18:2 and linolenic 18:3 are unsaturated (oils). Enzymes are proteins that speed one reaction each; the name usually says what they act on (amylase on starch, lipase on fats, pepsin and trypsin on proteins).

Definition

  • Triglyceride = glycerol + 3 fatty acids (3 ester links). Hydrolysis: nn mol triglyceride needs 3n3n mol H2O\text{H}_2\text{O} → glycerol + 3 RCOOH.
  • Soap = sodium or potassium salt of a higher fatty acid (C12–C18); Na gives hard soap, K soft soap. Formic-acid or ammonium salts are not soaps.
  • Fatty acids: palmitic C15H31COOH\text{C}_{15}\text{H}_{31}\text{COOH} (16:0), stearic C17H35COOH\text{C}_{17}\text{H}_{35}\text{COOH} (18:0), oleic 18:1, linoleic 18:2, linolenic 18:3 (9,12,15-octadecatrienoic acid — three C=C).
  • Enzymes: globular proteins, highly specific, work at body temperature and pH. Saliva: amylase (ptyalin), lysozyme, lingual lipase; stomach: pepsin (with HCl); intestine: trypsin, lactase; invertase hydrolyses sucrose.
  • Haemoglobin: Fe(II) held by the porphyrin ring — a coordination compound in blood; chlorophyll (Mg) is the plant counterpart, and vitamin B12 holds cobalt.

Triglyceride hydrolysis

triglyceride+3 H2O→glycerol+3 RCOOH;linolenic acid: C18, 3 C=C\text{triglyceride} + 3\,\text{H}_2\text{O} \to \text{glycerol} + 3\,\text{RCOOH};\qquad \text{linolenic acid: } \text{C}_{18},\ 3\ \text{C=C}

Worked example

How many moles of NaOH saponify 2 mol of a triglyceride completely, and what are the products?
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From the bank · past-year question

Example 1BiomoleculesMODERATE
What is the number of >C=C<> C = C < bonds present in a linolenic acid molecule?

[Q72 · 19 April Shift II · 2025]

Linoleic for linolenic

One letter apart: linoleic acid has TWO C=C, linolenic THREE. The '-trien-' in 9,12,15-octadecatrienoic acid gives the count.
Drill 4 more on triglycerides, fatty acids, soap and enzymes

Summary — formulas & gotchas at a glance

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Formulas (1)

  • Triglycerides, Fatty Acids, Soap and Enzymes

    Triglyceride hydrolysis

    triglyceride+3 H2O→glycerol+3 RCOOH;linolenic acid: C18, 3 C=C\text{triglyceride} + 3\,\text{H}_2\text{O} \to \text{glycerol} + 3\,\text{RCOOH};\qquad \text{linolenic acid: } \text{C}_{18},\ 3\ \text{C=C}

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