Bread, pasta, fruit, paper and cotton are mostly carbohydrates. They are built from the sugars you saw joined in Monomers and Polymers, and they contain only carbon, hydrogen and oxygen.
Key idea
Monosaccharides (simple sugars such as glucose) are quick energy. Disaccharides are two sugars joined. Polysaccharides are long chains that store energy (starch in plants, glycogen in animals) or build structure (cellulose in plant cell walls, chitin in insect shells and fungal cell walls).
| Kind | Examples | Job |
|---|---|---|
| Monosaccharide | Glucose, fructose (fruit sugar), galactose | Fuel for cellular respiration; building blocks |
| Disaccharide | Sucrose (table sugar: glucose + fructose), lactose (milk sugar: glucose + galactose), maltose (glucose + glucose) | Carrying or storing sugar |
| Polysaccharide | Starch, glycogen | Storing energy (plants, animals) |
| Polysaccharide | Cellulose, chitin | Building strong structures |
Why it works
Starch and cellulose are both chains of glucose. The difference is how the glucose units are linked. In starch, the links make the chain coil, and our digestive enzymes (amylase) fit those links. In cellulose, every other glucose is flipped, giving straight chains that line up side by side and hydrogen-bond into tough fibers. Our enzymes do not fit those links, so cellulose passes through us as fiber. Cows and termites digest it with the help of microbes that make the right enzyme.
Worked example: Sort the sugars
Classify: glycogen, fructose, sucrose, cellulose.
- Fructose is a single sugar: monosaccharide.
- Sucrose is glucose + fructose: disaccharide.
- Glycogen and cellulose are long chains: polysaccharides (glycogen stores energy in animals; cellulose builds plant cell walls).
Answer: Mono: fructose. Di: sucrose. Poly: glycogen, cellulose.
Common mistake
Thinking starch and cellulose are different kinds of molecule because people can digest only one. Both are polymers of the same monomer, glucose. Only the way the units are linked differs, and enzymes are fussy about the shape of the link. Check: break either one completely by hydrolysis and you get nothing but glucose.