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2.1.2Additional lessons

Macromolecules — IB Biology SL Revision Notes & Topical Questions

5 min read 5 parts Lesson 12 of 142

📚 IB Biology SL Hub / Topical Revision
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Part 1 of 5Monomers and Polymers

2.1.2 Macromolecules
Macromolecule
  • A very large molecule built from many smaller repeating units joined together by covalent bonds.
Carbohydrates
  • Polysaccharides are built from monosaccharide monomers such as glucose.
Proteins
  • Polypeptides are built from amino acid monomers joined by peptide bonds.
Nucleic acids
  • DNA and RNA are built from nucleotide monomers joined in long strands.
Lipids
  • Fats and phospholipids are built from fatty acids, glycerol and phosphate groups, though they are not true polymers.
MacromoleculeMonomerBond formed
PolysaccharideMonosaccharideGlycosidic bond
PolypeptideAmino acidPeptide bond
Nucleic acidNucleotidePhosphodiester bond
TriglycerideFatty acid and glycerolEster bond

Part 2 of 5Condensation and Hydrolysis

1
Two monomers are brought together so that a hydroxyl group on one and a hydrogen on the other are positioned next to each other.
2
A covalent bond forms between the monomers and a molecule of water is released: this is a condensation reaction.
3
Repeating condensation reactions adds more monomers, so the chain grows and one water molecule is lost for each bond made.
4
In hydrolysis a water molecule is added across the bond, which breaks it and splits the polymer into smaller units.
5
Enzymes catalyse both reactions, for example amylase hydrolyses starch into maltose during digestion.

Part 3 of 5Reaction Examples

Reaction Examples
Glucose to maltose
  • Two glucose molecules join by condensation to form maltose and one molecule of water.
Amino acids to dipeptide
  • Two amino acids join to form a dipeptide with a peptide bond, releasing water.
Digestion
  • Hydrolysis in the gut breaks starch, proteins and fats into small molecules that can be absorbed.
Reversibility
  • The same bond can be made by condensation and broken by hydrolysis, so cells can build and recycle macromolecules.

Part 4 of 5Worked Example

Question

  • A polypeptide chain contains 120 amino acids.
  • How many water molecules are released when it forms, and how many are needed to hydrolyse it completely?

  1. Joining 120 amino acids in a single chain needs 120 - 1 = 119 peptide bonds.
  2. Each condensation reaction releases one water molecule, so 119 are released.
  3. Complete hydrolysis breaks all 119 bonds and each needs one water molecule.

Answer 119 water molecules are released on formation, and 119 are required for complete hydrolysis.

Part 5 of 5Exam Checklist & Tips

2.1.2 – Key Points for the Exam

  • Monomer A small molecule that can be joined with others to form a polymer.
  • Polymer A large molecule made of many repeating monomers.
  • Condensation A reaction that joins molecules together and releases water.
  • Hydrolysis A reaction that splits a molecule by adding water.
  • Macromolecule A very large biological molecule such as a protein or polysaccharide.

Common Mistakes

  • Saying condensation uses water; it releases water, while hydrolysis uses it.
  • Forgetting that for a chain of n monomers, n - 1 bonds and n - 1 waters are involved.
  • Describing lipids as true polymers made of repeating identical units.
  • Naming the wrong bond, such as peptide bonds in carbohydrates.

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