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Carbohydrates: Overview, Monosaccharides, Practice Problems and FAQs

What are the world’s most abundant biomolecules? Can you answer this question? I will give you a clue. These compounds are prepared by plants during photosynthesis. In fact, the sugar that you add to your tea or coffee and the sugar that makes a fruit sweet in taste also belong to this group of biomolecules. Yes! It is carbohydrates. What does the term carbohydrate mean? ‘Carbohydrates’ means hydrates of carbon. These compounds have carbon, hydrogen and oxygen in the ratio 1:2:1. 

ALSO READ: Plant Definition, Characteristics and Types

Come let us learn some more about these compounds.

Table of Contents

General Structure of Carbohydrates

Carbohydrates have at least 3 carbon atoms and multiple hydroxyl (-OH) groups. They have mainly two types of functional groups, either an aldehyde (-CHO) group or a ketone (C=O) group.

aldehyde and ketone group in carbohydrates

General Formula 

The general formula of a carbohydrate is Cn (H2O)n . Their names generally end in ‘-ose’.

general formula of carbohydrates

  • There are some exceptions to this general formula: 
  • Rhamnose and Deoxyribose are carbohydrates, yet do not follow the general formula.

1

Classification of Carbohydrates

On the basis of the number of monomeric units present, carbohydrates are classified as follows:

  • Monosaccharides
  • Disaccharides
  • Oligosaccharides
  • Polysaccharides

Monosaccharides

The word monosaccharide means single sugar (Mono = Single; Saccharide = Sugar). Thus, monosaccharides have only one monomeric unit present. They are the simplest carbohydrates and are the building blocks of larger carbohydrates.

On the basis of the number of carbon atoms present in a monosaccharide unit, they are classified as follows:

  • Trioses
  • Tetroses
  • Pentoses
  • Hexosez

Type of Monosaccharide

Example

Structure

Trioses

Have 3 carbon atoms


 

Glyceraldehyde

1

Tetroses

Have 4 carbon atoms

Erythrose

1

Pentoses

Have 5 carbon atoms

Ribose and Deoxyribose

1

Hexoses

Have 6 Carbon atoms

Glucose, Fructose (Fruit sugar) and Galactose

1

Optically Active Forms of Monosaccharides

Monosaccharides exist in two optically active forms which are non-superimposable mirror images of each other. 

monosaccharides

These forms are known as enantiomers and are of the following types -

Dextrorotatory 

Laevorotatory

Turn the plane polarised light to the right or clockwise.

Turn the plane polarised light to the left or anticlockwise.

It is represented as D

It is represented as L

tow forms of glyceraldehyde

aldehyde and ketone group in carbohydrates

Reducing and Non-reducing Sugars

Reducing sugars have a free aldehyde/ketone group, that is, their anomeric carbon is free to react with functional groups. Non-reducing sugars do not have a free aldehyde/ketone group.

All monosaccharides such as glucose, fructose and some disaccharides such as (lactose, maltose) have a free aldehyde or ketone group, that is, they are reducing sugars. Most polysaccharides and sucrose (disaccharide) lack free aldehyde or ketone groups, thus, they are non-reducing sugars.

reducing sugar

Benedict’s Test

Benedict's test is a chemical test used to determine whether or not an analyte contains reducing sugars. As a result, this test can identify simple carbohydrates having a free ketone or aldehyde functional group. Benedict's reagent (also known as Benedict's solution) is a complicated mixture of sodium citrate, sodium carbonate, and copper(II) sulphate pentahydrate that is used in the test. When Benedict's reagent is mixed with reducing sugars, the cuprous ions are reduced to cupric ions which results in the production of a brick-red precipitate, indicating a positive Benedict's test. The colour of Benedict’s reagent varies from clear blue to brick-red upon reaction with reducing sugars.

benedict test for reducing sugar

Practice Problems of Carbohydrates

1. Match the following.

Column I

Column II

Tetrose

Deoxyribose

Hexose

Fructose

Pentose

Erythrose

a. i - 1, ii- 2, iii - 3
b. i - 2, ii- 1, iii - 3
c. i - 3, ii- 2, iii - 1
d. i - 1, ii- 3, iii - 2

Solution: The correct match is as follows

Column I

Column II

Tetrose

Erythrose

Hexose

Fructose

Pentose

Deoxyribose

Hence, the correct answer is option c.

2. Which of the following sugars is abundantly found in fruits?

a. Glucose
b. Erythrose
c. Fructose
d. Ribose

Solution: Fruits are generally rich in fructose which is a monosaccharide.

Thus, the correct option is c.

3. A teacher gave two carbohydrates lactose and sucrose to test whether the carbohydrate present in them is reducing or non-reducing. Students performed a Benedict’s test and concluded that lactose is a reducing sugar while sucrose is non-reducing. Which of the following observations might have helped the students reach one of these conclusions?

a. Lactose gives a blue colouration upon mixing with Benedict’s solution
b. Lactose gives a brick red precipitate upon mixing with Benedict’s solution
c. Sucrose gives a brick red precipitate upon mixing with Benedict’s solution
d. Sucrose gives a dirty green precipitate upon mixing with Benedict’s solution

Solution: Lactose is a reducing sugar because it has a free aldehyde/ketone group, that is, their anomeric carbon is free to react with functional groups while sucrose is a non-reducing sugar as it does not have a free aldehyde/ketone group. 
When Benedict's reagent is mixed with reducing sugars, the cuprous ions are reduced to cupric ions which results in the production of a brick-red precipitate, indicating a positive Benedict's test. Thus, lactose gives a brick red precipitate with Benedict’s solution.

Thus, the correct option is b.

4. The general formula of a carbohydrate is Cn (H2O)n. Which of the following carbohydrates do not follow this general formula?

a. Glucose
b. Rhamnose
c. Mannose
d. Fructose

Solution: Rhamnose and Deoxyribose are carbohydrates, yet do not follow the general formula. The correct option is b.

FAQs of Carbohydrates

Question 1. Mention the four classes of carbohydrates.

Answer: The four classes of carbohydrates are as follows:

  • Monosaccharides
  • Disaccharides
  • Oligosaccharides
  • Polysaccharides

Question 2. What is the general formula of carbohydrates?

Answer: The general formula of a carbohydrate is Cn (H2O)n .

Question 3. Name the monosaccharides found in nucleic acids.

Answer: Nucleic acids have 5-carbon sugar, that is, pentose sugar. In DNA, monosaccharide is Deoxyribose sugar and in RNA, monosaccharide is ribose sugar.

Question 4. Define reducing sugar.

Answer: Reducing sugars have a free aldehyde/ketone group, that is, their anomeric carbon is free to react with functional groups.

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