Lesson 1 of 13 · 5 min

What counts as a carbohydrate

NCERT §10.1–10.1.2

On Monday Meera sorts the store shelf: cane sugar, glucose powder, honey, cornflour, cotton muslin cloth and a bottle of vinegar. Her aunt asks which of them are carbohydrates.

The story this chapter follows: Meera's week at the sweet shop

Meera, a Class 12 student, spends a week helping her aunt, who runs a sweet shop with a small food-testing lab at the back. That week the shop turns 342 g of cane sugar into invert syrup, thickens halwa with starch, sets custard with eggs and milk, and checks every jar of honey. Each job turns out to be a lesson in the molecules of life.

The lesson in notes

In short

Living things are built from lifeless atoms and molecules; biochemistry studies the chemistry going on inside them. The large biomolecules are carbohydrates, proteins, nucleic acids and lipids, helped by small ones such as vitamins and mineral salts.

Carbohydrates are made mostly by plants. Many fit the formula Cx(H₂O)y, which is why they were once thought of as 'hydrates of carbon': glucose, C₆H₁₂O₆, can be written C₆(H₂O)₆.

The formula is not a test. Acetic acid, CH₃COOH, fits it as C₂(H₂O)₂ yet is not a carbohydrate, and rhamnose, C₆H₁₂O₅, is a carbohydrate that does not fit it.

The chemical definition: carbohydrates are optically active polyhydroxy aldehydes or ketones, or compounds that give such units when hydrolysed. The sweet ones are called sugars; household sugar is sucrose and milk sugar is lactose. Another name is saccharides, from the Greek sakcharon, sugar.

Grouping by hydrolysis. A monosaccharide cannot be hydrolysed into a simpler polyhydroxy aldehyde or ketone; about 20 occur in nature, among them glucose, fructose and ribose.

An oligosaccharide gives two to ten monosaccharide units on hydrolysis (disaccharides, trisaccharides and so on). Disaccharides are the commonest: sucrose gives one glucose and one fructose, maltose gives two glucose.

A polysaccharide gives a large number of units: starch, cellulose, glycogen and gums. They do not taste sweet and are called non-sugars.

Sugars that reduce Fehling's solution and Tollens' reagent are reducing sugars. Every monosaccharide, aldose or ketose, is a reducing sugar.

Naming monosaccharides (Table 10.1): an aldehyde group makes it an aldose, a keto group a ketose, and the carbon count is built in. Three to seven carbons give triose, tetrose, pentose, hexose and heptose, so glucose is an aldohexose and fructose a ketohexose.