(a) Glycosidic linkage

Definition : The oxygen bridge $(-O-)$ that joins two monosaccharide units together, formed by the loss of a water molecule between them, is called a glycosidic linkage.

$$\text{Monosaccharide} + \text{Monosaccharide} \rightarrow \text{Disaccharide} + H_2O$$

Examples :

• In maltose, C-1 of the first glucose is joined to C-4 of the second through an $\alpha$-1,4-glycosidic linkage.
• In sucrose, C-1 of glucose is joined to C-2 of fructose through an $\alpha$-1,2-glycosidic linkage.

This linkage is broken on hydrolysis, which is how disaccharides and polysaccharides are split back into their monosaccharide units.

(b) Primary structure of protein

Definition : The specific sequence in which the amino acids are linked to one another by peptide bonds in a polypeptide chain is called the primary structure of a protein.

Key points :

• It is fixed by covalent peptide bonds, so it is the most stable level of structure.
• Every protein has its own unique sequence.
• Even a single change in this sequence can completely alter the protein's function — sickle-cell anaemia is caused by one amino acid substitution in haemoglobin.
• The primary structure survives denaturation, since peptide bonds are not broken; only the secondary and tertiary structures are destroyed.

(c) Disaccharides

Definition : Carbohydrates that give two monosaccharide units on hydrolysis are called disaccharides.

$$C_{12}H_{22}O_{11} + H_2O \rightarrow 2\ \text{monosaccharide units}$$

Examples :

Sucrose is non-reducing because both its anomeric carbons are locked in the glycosidic linkage, leaving no free $-CHO$ group.