What are carbohydrates?
Carbohydrates are organic molecules made of carbon, hydrogen and oxygen, with the hydrogen and oxygen usually in the ratio 2:1 as in water. Their main role is to provide and store energy, and some also give structural support. They are grouped by size into monosaccharides, disaccharides and polysaccharides. Carbohydrates are among the most common organic molecules in living things and form the sugars, starches and fibres in the food we eat.
Three groups
- Monosaccharides are single sugar units, for example glucose, fructose and galactose. Glucose is the main respiratory substrate.
- Disaccharides form when two monosaccharides join by a condensation reaction, releasing water. Examples: maltose (glucose + glucose), sucrose (glucose + fructose) and lactose (glucose + galactose).
- Polysaccharides are long chains of many monosaccharides. Starch stores energy in plants, glycogen stores energy in animals, and cellulose forms plant cell walls.
Storage carbohydrates such as starch and glycogen are useful because they are compact and insoluble, so they do not affect the water balance of the cell and can be broken down to glucose when energy is needed. Cellulose, in contrast, is never digested by humans; it passes through the gut as dietary fibre and helps food move along the intestine.
Key idea
Small sugars join by condensation (removing water) to build larger carbohydrates; they are broken by hydrolysis (adding water) back into sugars.
Food tests
The Benedict's test detects reducing sugars such as glucose: on heating, the blue solution turns green, yellow, orange then brick-red. The iodine test detects starch: iodine solution turns from brown to blue-black.
Example
A drop of iodine on a slice of potato turns blue-black, showing the potato stores energy as starch.
Remember
Starch = plant store, glycogen = animal store, cellulose = plant wall. Benedict's for reducing sugar; iodine for starch.