BiomoleculesClass 12 Chemistry NCERT Solutions

Monosaccharides are carbohydrates that cannot be hydrolysed further to give simpler units of polyhydroxy aldehydes or ketones. They are the simplest form of carbohydrates. About 20 monosaccharides are known to occur in nature. Some common examples include glucose, fructose, and ribose.

Reducing sugars are carbohydrates that can reduce Fehling's solution and Tollens' reagent. This property is due to the presence of a free aldehyde or ketonic group. All monosaccharides, whether they are aldoses or ketoses, are reducing sugars. Disaccharides like maltose and lactose are also reducing sugars as they have a free hemiacetal group which can open up to give a free aldehyde group.

Two main functions of carbohydrates in plants are:

The classification is as follows:

A glycosidic linkage is an oxide linkage formed between two monosaccharide units with the loss of a water molecule. This covalent bond joins a carbohydrate (sugar) molecule to another group, which can be another carbohydrate. For example, in disaccharides and polysaccharides, monosaccharide units are joined together by glycosidic linkages.

Glycogen is a polysaccharide of $\alpha$-D-glucose units that serves as the main form of energy storage in animals and fungi. It is often referred to as animal starch.

The basic structural difference between starch and cellulose lies in the type of glycosidic linkage between their glucose monomer units.

This difference in linkage ($\alpha$ vs $\beta$) results in starch having a helical structure and cellulose having a straight, rigid structure.

The following reactions and facts about D-glucose cannot be explained by its open-chain structure:

These observations are explained by the cyclic hemiacetal structure of glucose.

Essential Amino Acids: These are amino acids that cannot be synthesized by the human body and must be obtained through diet. Their deficiency can lead to diseases.

Non-essential Amino Acids: These are amino acids that can be synthesized by the human body from other compounds and are therefore not required to be supplied through diet.

The secondary structure of a protein refers to the shape in which a long polypeptide chain can exist due to regular folding of its backbone. The two most common types of secondary structure are:

The $\alpha$-helix structure of proteins is stabilized by intramolecular hydrogen bonds. These bonds form between the carbonyl oxygen (>C=O) of one amino acid residue and the amide hydrogen (-NH) of the amino acid residue that is four positions ahead in the polypeptide chain. This regular pattern of hydrogen bonding causes the chain to coil into a helical shape.

The differences between globular and fibrous proteins are as follows:

Amino acids exhibit amphoteric behaviour because they contain both an acidic group (the carboxyl group, -COOH) and a basic group (the amino group, -$\mathrm{NH}_2$) in the same molecule.

In an aqueous solution, the carboxyl group can donate a proton, and the amino group can accept a proton, leading to the formation of a dipolar ion called a zwitterion (e.g., $^{+} \mathrm{NH}_3-\mathrm{CHR}-\mathrm{COO}^{-}$).

This zwitterion can act as both an acid and a base:

Since amino acids can react with both acids and bases, they are considered amphoteric.

Enzymes are biological catalysts that increase the rate of chemical reactions within living organisms. Almost all enzymes are globular proteins. They are highly specific in their action, meaning a particular enzyme typically catalyzes only one specific reaction or acts on a specific substrate. They function under mild conditions of temperature and pH in the body and are required only in small quantities.

Denaturation has a significant effect on the higher levels of protein structure. When a protein is denatured, its secondary and tertiary structures are disrupted and destroyed. The hydrogen bonds, disulphide linkages, and other forces that maintain the folded, three-dimensional shape of the protein are broken. This causes the coiled polypeptide chains to unfold and uncoil into a random configuration. However, the primary structure, which is the sequence of amino acids held by peptide bonds, remains intact. The loss of the 3D structure also results in the loss of the protein's biological activity.

Vitamins are classified into two groups based on their solubility:

The vitamin responsible for the coagulation of blood is Vitamin K. A deficiency of this vitamin leads to an increased blood clotting time.

Nucleic acids are long-chain polymers of nucleotides, also known as polynucleotides. They are biomolecules found in the nucleus of living cells and are responsible for the transmission of inherent characters (heredity). The two main types of nucleic acids are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).

Two important functions of nucleic acids are:

The main difference between a nucleoside and a nucleotide is the presence of a phosphate group.

Nucleotides are the monomer units that build up the polymer chains of nucleic acids (DNA and RNA).

The statement means that the sequence of bases in one strand of the DNA double helix is not the same as the sequence in the other strand. Instead, the sequence of one strand determines the sequence of the other due to specific base pairing rules.

The two strands are held together by hydrogen bonds formed between specific pairs of nitrogenous bases:

Because of this specific pairing, if the sequence of bases on one strand is, for example, 5'-ATGC-3', the sequence on the opposite strand must be 3'-TACG-5'. Thus, the two strands are not identical copies of each other but are complementary, like a lock and key.

There are three main types of RNA found in the cell, each with a specific function in protein synthesis: