Short on time? This page is the recap. For explanations, figures and worked examples, use the full study guide.
Recap
- Monosaccharides (simple sugars) are the monomers. Glucose, fructose and galactose are all C₆H₁₂O₆ but with different arrangements of atoms. Ribose and deoxyribose are five-carbon sugars in RNA and DNA.
- Two monosaccharides join by dehydration synthesis (one H₂O removed) to make a disaccharide. The link is a covalent bond.
- Many monosaccharides joined in this way make a polysaccharide (complex carbohydrate). It can be linear or branched.
- Hydrolysis breaks each link using one H₂O. Digestion of starch is hydrolysis.
- Starch, glycogen and cellulose are all polymers of glucose. They differ in branching and in how the units are joined, and so in function.
Key relationships
| Polymer | Where | Shape | Function |
|---|---|---|---|
| Starch | plants (seeds, tubers) | mix of linear and branched chains | energy storage |
| Glycogen | animals (liver, muscle), fungi | highly branched | energy storage, fast release |
| Cellulose | plant cell walls | linear, chains hydrogen-bonded side by side | structural support |
| Disaccharide | Monomers |
|---|---|
| Maltose | glucose + glucose |
| Sucrose | glucose + fructose |
| Lactose | glucose + galactose |
| Counting rule | Example |
|---|---|
| n monomers → n − 1 links → n − 1 waters | 10 glucose → 9 waters; C₆₀H₁₀₂O₅₁ |
| molar mass = n × 180.16 − (n − 1) × 18.02 (g mol⁻¹) | maltose: 2 × 180.16 − 18.02 = 342.3 g mol⁻¹ |
Assumptions behind the numbers
- The counting rule assumes every link joins exactly two monomers and releases one water. It holds for linear and branched chains alike.
- Branching models (rate = number of free ends × rate per end) assume every end is equally reachable. Real molecules release glucose more slowly than such a model predicts.
Mistakes to avoid
- Saying dehydration synthesis adds water.
- Counting n waters instead of n − 1.
- Calling the links in a chain hydrogen bonds; they are covalent. Hydrogen bonds join neighbouring cellulose chains.
- Saying cellulose is not made of glucose.
- Saying branching adds monomers; it adds chain ends.
- Trying to draw detailed structures of starch or cellulose: not required.
Quick self-check
- Name the monomer of glycogen and its formula. (Glucose, C₆H₁₂O₆)
- How many water molecules are released in making a chain of 30 glucose units? (29)
- Why does glycogen release glucose faster than an unbranched chain of the same size? (More branches give more chain ends, and enzymes remove glucose from many ends at once.)
- Why does cellulose form strong fibres? (Straight, unbranched chains lie side by side and form many hydrogen bonds between chains.)
Next: practice questions.