medium difficulty8 min

Classification of Carbohydrates

Carbohydrates are optically active polyhydroxy aldehydes or ketones, or compounds that give such units on hydrolysis.

In one line: Mono means one, di means two, poly means many sugar units after hydrolysis.

Practice This Concept

Understand it

Carbohydrates are classified mainly by the number of sugar units produced on hydrolysis.
Monosaccharides cannot be hydrolysed further into simpler carbohydrates; glucose and fructose are common examples.
Disaccharides give two monosaccharide units on hydrolysis; sucrose, lactose, and maltose belong here.
Polysaccharides give many monosaccharide units and include starch, glycogen, and cellulose.
This classification is chemical because it depends on hydrolysis behaviour, not just sweetness or solubility.

Example

Glucose is a monosaccharide, sucrose is a disaccharide, and starch is a polysaccharide made from many glucose units.

Rule to remember

Rule: no hydrolysis means monosaccharide; hydrolysis to two monosaccharides means disaccharide; hydrolysis to many monosaccharides means polysaccharide.

General carbohydrate formula is often written as Cx(H2O)y, but this is not a reliable definition for every carbohydrate.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

Classify glucose, maltose, and cellulose.

1Glucose cannot be hydrolysed further, so it is a monosaccharide.
2Maltose gives two glucose units, so it is a disaccharide.
3Cellulose gives many glucose units, so it is a polysaccharide.

Method you can reuse

1First identify whether hydrolysis occurs.
2Next count the number of monosaccharide units produced.
3Then write the class with one correct example.

Visual tip

A flowchart is useful: carbohydrates branch into monosaccharides, disaccharides, and polysaccharides, each with examples.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

What class of carbohydrate is sucrose, and why?

Think first, then show answer

Sucrose is a disaccharide because one molecule gives two monosaccharide units on hydrolysis.

Common wrong answer

Calling starch a disaccharide because it is made of glucose units is wrong; it contains many glucose units and is a polysaccharide.

Don't mix this up

Students often classify all sweet substances as carbohydrates or assume all carbohydrates are sweet. Cellulose and starch are carbohydrates but are not sweet like glucose or sucrose.

Exam tip

When asked to classify, check the number of monosaccharide units obtained on hydrolysis before writing the class.

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

Usually asked as definition, classification with examples, or a comparison between mono-, di-, and polysaccharides.

1
Enough for 1 mark
Carbohydrates are optically active polyhydroxy aldehydes or ketones, or compounds that give such units on hydrolysis.
2
Add for 2 marks
Example: Glucose is a monosaccharide, sucrose is a disaccharide, and starch is a polysaccharide made from many glucose units.
3
Add for 3 marks
Carbohydrates are classified mainly by the number of sugar units produced on hydrolysis.

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