Short on time? This page is the recap. For explanations, figures and worked examples, use the full study guide.
Recap
- A polymer is a large molecule made of many monomers joined by covalent bonds. Joining many monomers is polymerization.
- Dehydration synthesis (condensation) joins two monomers. One monomer loses H, the other loses –OH, and these leave as one water molecule.
- Hydrolysis breaks the bond between two monomers by adding water. The H from water joins one monomer; the –OH joins the other.
- Digestion is hydrolysis. Building starch, glycogen, proteins or DNA in cells is dehydration synthesis.
- Carbohydrates, proteins and nucleic acids are polymers. Lipids are not true polymers, but their parts are joined by the same reaction.
- Background: building needs energy, hydrolysis releases it, and enzymes speed up both (Unit 3).
Key relationships
| Feature | Dehydration synthesis | Hydrolysis |
|---|---|---|
| Molecule size | increases | decreases |
| Covalent bond | formed | broken |
| Water | product (released) | reactant (used) |
| H and –OH | removed from monomers | added to monomers |
| Rule | Meaning |
|---|---|
| bonds = n − 1 | for a straight or branched chain of n monomers (no rings) |
| water released to build = water used to break = n − 1 | one water per bond |
| mass of polymer = n × (monomer mass) − (n − 1) × (water mass) | the polymer is lighter than its separate monomers |
Assumptions behind the numbers
- The mass rule assumes all the monomers have the same mass. For mixed monomers, add their masses, then subtract (n − 1) × 18.0.
- The n − 1 rule assumes the chain does not close into a ring.
- Simple diagrams leave out enzymes and energy; they show only atoms and bonds.
Mistakes to avoid
- Saying hydrolysis is “dissolving”. It breaks covalent bonds.
- Giving both the H and the –OH from water to the same monomer.
- Counting n bonds instead of n − 1.
- Forgetting to subtract the water when finding a polymer’s mass.
- Calling the links between monomers hydrogen bonds. They are covalent.
- Reading a diagram without checking which side the water is on.
Quick self-check
- A straight polymer of 30 monomers is fully hydrolysed. How many water molecules are used? (30 − 1 = 29)
- Two glucose units (180.2 g mol⁻¹ each) join. What is the mass of the product? (2 × 180.2 − 18.0 = 342.4 g mol⁻¹)
- A diagram shows water on the left of the arrow and two small molecules on the right. Which reaction is it? (Hydrolysis: water is a reactant and the molecule is split.)
Next: practice questions.