Biomolecules

After studying this Unit, you will be able to

“It is the harmonious and synchronous progress of chemical reactions in body which leads to life”.

A living system grows, sustains and reproduces itself. The most amazing thing about a living system is that it is composed of non-living atoms and molecules. The pursuit of knowledge of what goes on chemically within a living system falls in the domain of biochemistry. Living systems are made up of various complex biomolecules like carbohydrates, proteins, nucleic acids, lipids, etc. Proteins and carbohydrates are essential constituents of our food. These biomolecules interact with each other and constitute the molecular logic of life processes. In addition, some simple molecules like vitamins and mineral salts also play an important role in the functions of organisms. Structures and functions of some of these biomolecules are discussed in this Unit.

Carbohydrates are primarily produced by plants and form a very large group of naturally occurring organic compounds. Some common examples of carbohydrates are cane sugar, glucose, starch, etc. Most of them have a general formula, $\text{ce}{C}_x(H2O{)}_y$, and were considered as hydrates of carbon from where the name carbohydrate was derived. For example, the molecular formula of glucose ($\text{ce}C6H12O6$) fits into this general formula, $\text{ce}C6(H2O)6$. But all the compounds which fit into this formula may not be classified as carbohydrates. For example acetic acid ($\text{ce}CH3COOH$) fits into this general formula, $\text{ce}C2(H2O)2$ but is not a carbohydrate. Similarly, rhamnose, $\text{ce}C6H12O5$ is a carbohydrate but does not fit in this definition. A large number of their reactions have shown that they contain specific functional groups. Chemically, the carbohydrates may be defined as optically active polyhydroxy aldehydes or ketones or the compounds which produce such units on hydrolysis. Some of the carbohydrates, which are sweet in taste, are also called sugars. The most common sugar, used in our homes is named as sucrose whereas the sugar present

in milk is known as lactose. Carbohydrates are also called saccharides (Greek: sakcharon means sugar).

Carbohydrates are classified on the basis of their behaviour on hydrolysis. They have been broadly divided into following three groups.

Monosaccharides are further classified on the basis of number of carbon atoms and the functional group present in them. If a monosaccharide contains an aldehyde group, it is known as an aldose and if it contains a keto group, it is known as a ketose. Number of carbon atoms constituting the monosaccharide is also introduced in the name as is evident from the examples given in Table 10.1

Carbon atoms | General term | Aldehyde | Ketone

3 | Triose | Aldotriose | Ketotriose

4 | Tetrose | Aldotetrose | Ketotetrose

5 | Pentose | Aldopentose | Ketopentose

6 | Hexose | Aldohexose | Ketohexose

7 | Heptose | Aldoheptose | Ketoheptose

Glucose occurs freely in nature as well as in the combined form. It is present in sweet fruits and honey. Ripe grapes also contain glucose in large amounts. It is prepared as follows:

1. From sucrose (Cane sugar): If sucrose is boiled with dilute $\text{ce}HCl$ or $\text{ce}H2SO4$ in alcoholic solution, glucose and fructose are obtained in equal amounts.

Sucrose, Glucose, Fructose. 2. From starch: Commercially glucose is obtained by hydrolysis of starch by boiling it with dilute $\text{ce}H2SO4$ at 393 K under pressure.

Starch or cellulose, Glucose

Glucose is an aldohexose and is also known as dextrose. It is the monomer of many of the larger carbohydrates, namely starch, cellulose. It is probably the most abundant organic compound on earth. It was assigned the structure given below on the basis of the following evidences:

3. Glucose reacts with hydroxylamine to form an oxime and adds a molecule of hydrogen cyanide to give cyanohydrin. These reactions confirm the presence of a carbonyl group ($\text{ce}>C=O$) in glucose.

4. Glucose gets oxidised to six carbon carboxylic acid (gluconic acid) on reaction with a mild oxidising agent like bromine water. This indicates that the carbonyl group is present as an aldehydic group.

5. Acetylation of glucose with acetic anhydride gives glucose pentaacetate which confirms the presence of five –OH groups. Since it exists as a stable compound, five –OH groups should be attached to different carbon atoms.

6. On oxidation with nitric acid, glucose as well as gluconic acid both yield a dicarboxylic acid, saccharic acid. This indicates the presence of a primary alcoholic (–OH) group in glucose.

The exact spatial arrangement of different —OH groups was given by Fischer after studying many other properties. Its configuration is correctly represented as I. So gluconic acid is represented as II and saccharic acid as III.