A condensation reaction joins two molecules by forming a covalent bond and releasing a molecule of water. In IB Biology, condensation reactions build carbohydrates and lipids from smaller components.
This is required at both SL and HL in subtopic B1.1, Carbohydrates and lipids.
How does a condensation reaction work?
An group from one molecule and an from another combine to form . The molecules then become linked by a new covalent bond.
For carbohydrates, two monosaccharides join by condensation to form a glycosidic bond. For example:
Repeated condensation reactions join many monosaccharides into polysaccharides. Alpha-glucose forms starch or glycogen, while beta-glucose forms cellulose. Each new glycosidic bond formed releases one water molecule.
In lipid formation, glycerol reacts with fatty acids. Each fatty acid forms an ester bond with glycerol through condensation:
A phospholipid forms when glycerol joins to two fatty acids and a phosphate-containing group. Lipids are assembled by condensation, but they are not polymers made from repeating monomers.
Bond formation and water release have a fixed relationship. If ten glucose molecules are joined into one unbranched chain, nine glycosidic bonds form because each bond connects two previously separate units. Therefore, nine water molecules are released. More generally, joining monomers into one linear chain forms bonds and releases water molecules, provided no branches or rings are formed.
| Condensation | Hydrolysis |
|---|---|
| Joins smaller molecules | Splits larger molecules |
| Forms a covalent bond | Breaks a covalent bond |
| Releases water | Uses water |
| Builds polysaccharides and lipids | Supports digestion of biological molecules |
A common misconception is that condensation means water simply evaporates. Instead, water is chemically produced from atoms removed from the reacting molecules.
Exam technique
For an explain question, state that condensation forms a covalent bond, identify the bond type, and state that water is released. Do not confuse condensation with hydrolysis, which uses water to break bonds.
