Biomolecules — NCERT Solutions

A monosaccharide is the simplest carbohydrate unit. It cannot be hydrolysed further to give a simpler carbohydrate. It is a polyhydroxy aldehyde or polyhydroxy ketone.

Reducing sugars are those carbohydrates which can reduce Fehling’s solution and Tollens’ reagent. All monosaccharides are reducing sugars, whether they are aldoses or ketoses.

Carbohydrates perform two main functions in plants:

1. They act as food storage molecules, mainly as starch.
2. They provide structural support, as in cellulose of the cell wall.

• Monosaccharides: ribose, 2-deoxyribose, galactose, fructose
• Disaccharides: maltose, lactose

A glycosidic linkage is the oxygen bridge formed between two monosaccharide units when a molecule of water is lost. It joins the two sugar units together.

Glycogen is the storage polysaccharide in animals, also called animal starch. It is stored mainly in the liver, muscles and brain.

Difference from starch:

• Starch is the main storage polysaccharide of plants.
• Glycogen is the storage polysaccharide of animals.
• Glycogen is more highly branched than starch (its structure is similar to amylopectin but more branched).

On hydrolysis:

• Sucrose gives D-(+)-glucose and D-(-)-fructose.
• Lactose gives D-glucose and D-galactose.

The basic structural difference is that:

• Starch is made of $alpha$-D-glucose units.
• Cellulose is made of $beta$-D-glucose units.

So, starch and cellulose differ in the kind of glucose unit and the linkage pattern between them.

When D-glucose is treated with:

1. HI: It gets reduced to n-hexane.
2. Bromine water: It is oxidised to gluconic acid.
3. HNO$_3$: It is oxidised to the dicarboxylic acid saccharic acid.

The following reactions of D-glucose cannot be explained by its open-chain structure:

1. It does not give Schiff’s test and does not form the hydrogensulphite addition product with NaHSO$_3$, although it has an aldehyde group in the open-chain formula.
2. Its pentaacetate does not react with hydroxylamine, showing the absence of a free –CHO group.
3. It exists in two crystalline forms, $alpha$-glucose and $beta$-glucose.

These facts led to the cyclic hemiacetal structure of glucose.

Essential amino acids are those which cannot be synthesised in the body and must be supplied through diet. Non-essential amino acids can be synthesised by the body.

Examples:

• Essential: valine, lysine
• Non-essential: glycine, alanine

(i) Peptide linkage: The amide bond formed between the $-\mathrm{COOH}$ group of one amino acid and the $-\mathrm{NH}_2$ group of another amino acid. It is written as $-\mathrm{CO}-\mathrm{NH}-$.

(ii) Primary structure: The specific sequence of amino acids in a polypeptide chain. Any change in this sequence gives a different protein.

(iii) Denaturation: The process in which a protein in its native state loses its secondary and tertiary structures and hence its biological activity due to change in temperature or pH. The primary structure remains intact.

The two common secondary structures of proteins are:

• $alpha$-helix
• $beta$-pleated sheet