Biomolecules: Overview, Questions, Preparation

Biomolecules 2021

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Updated on Jun 4, 2024 15:18 IST

Introduction

Carbohydrates are naturally occurring organic substances that are primarily produced by plants. Starch, glucose, and cane sugar are some common examples. Carbohydrates have a general formula of Cx (H2O) y. They are polyhydroxy- aldehydes or ketones or compounds which produce such units on hydrolysis. These are of three types, namely -

1) Monosaccharide

2) Oligosaccharide

3) Polysaccharide

Glucose

Glucose is a monosaccharide with the chemical formula of C6H12O6. It is made of 6 carbon atoms and one aldehyde group, which is the reason why it is also referred to as an aldohexose. Glucose occurs freely and in the combined form. Glucose is naturally present in fruits and honey. It’s a primary source of energy for living beings. Plants produce it via photosynthesis, and animals obtain it via the process of glycogenolysis.

The structure of glucose is as follows:

Above are the acyclic and cyclic structures of glucose, respectively. 

Glucose is correctly named D(+) Glucose, wherein D represents the configuration, and + indicates the molecule’s dextrorotatory nature.  

Baeyer introduced the acyclic or the open chain structure of glucose. However, this structure could not explain certain properties, like no reaction to Schiff’s test or with hydroxylamine or sodium hydrogen sulphite, or even the process of mutarotation.

The cyclic structure of glucose was introduced by Haworth, which confirmed mutarotation and the existence of alpha and beta forms of glucose. The ring structure has no free aldehyde group. The cyclic structure has one chiral centre more than the acyclic form. Therefore, two isomeric forms are possible, which differ in the configuration of the hydroxyl group at C1.

The six-membered ring structures of glucose are called pyranose structures, and hence the isomers are alpha D + and beta D + glucopyranose.

Fructose

The structure of fructose is as follows:

Fructose is an important ketohexose, obtained along with glucose during the hydrolysis of disaccharides like sucrose. The pure form of fructose is used as a sweetener and is commonly found in fruits, vegetables, and honey. Discovered in 1847 by French chemist Augustin-Pierre Dubrunfaut, it consists of a 6-carbon polyhydroxy ketone.

Fructose has the molecular formula C6H12O6 an, and it contains a ketonic group at C2. It belongs to the D series and is a levorotatory compound written as D-(-) fructose. It’s open, and cyclic is as shown in the above image, respectively.

It exists in 2 cyclic forms by the addition of -OH group at C5, resulting in a five-membered ring that has the analogy of Furan. Its two isomers are named alpha D-(-) fructofuranose and beta D-(-) fructofuranose.

Weightage

This topic is part of the Class 12 Chemistry syllabus of NCERT. Biomolecules and Alcohols, Phenols and Ethers, Aldehydes, Ketones, and Carboxylic acids and Amines carry a weightage of 28 marks.

Illustrated Examples

1. Preparation of Glucose and Fructose from Cane Sugar (Sucrose)

Answer: If sucrose is boiled with dilute HCl or H2SO4, glucose and fructose are obtained in equal amounts.

2. Preparation of Glucose from Starch
Answer: Glucose is obtained by boiling starch with dilute H2SO4 at 393K under pressure.
3. Preparation of n-hexane
Answer: When heated with HI for a long time, glucose forms n-hexane which suggests that all 6 carbon atoms are linked in a straight chain.
[Image courtesy: NCERT]

FAQs on Structure of Glucose and Fructose

Q: What are glucose and fructose?

A:  These are sugars belonging to the monosaccharide’s category of carbohydrates. These are primary sources of energy for living organisms.

Q: Who introduced the cyclic structures for glucose?

A:  Norman Haworth was the scientist who introduced the cyclic structures.

Q: What are the crystalline forms of glucose?

A:  Crystalline forms of glucose are alpha -D-(+) glucose and beta-D-(+) glucose.

Q: Which analogies are applied to the cyclic structures of glucose and fructose?

A:  Pyran and Furan are the compounds whose analogies are applied to glucose and fructose’s cyclic structures, respectively.

Q: What is the function of glucose and fructose?

A:  The primary and most important function is to provide the cells with energy.

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