(a) Peptide linkage

A peptide linkage is the amide bond $-CO-NH-$ formed between the $-COOH$ group of one amino acid and the $-NH_2$ group of the next amino acid, with the loss of one water molecule.

$$H_2N-CHR-COOH + H_2N-CHR'-COOH \rightarrow H_2N-CHR-\underset{\text{peptide bond}}{CO-NH}-CHR'-COOH + H_2O$$

(b) Bonds holding the DNA double helix together

Hydrogen bonds between complementary nitrogenous bases on the two strands :

• Adenine pairs with Thymine through two hydrogen bonds ($A = T$)
• Guanine pairs with Cytosine through three hydrogen bonds ($G \equiv C$)

(c) The polysaccharide

Starch.

Glucose and fructose are monosaccharides (single sugar units); sucrose is a disaccharide (glucose + fructose). Starch is a polysaccharide made of many glucose units joined by glycosidic linkages.

(d) Examples of vitamins

Water-soluble vitamin : Vitamin C (ascorbic acid) — or any of the B-complex vitamins such as $B_1$, $B_2$, $B_6$, $B_{12}$

Fat-soluble vitamin : Vitamin A — or vitamin D, E or K

Note : Water-soluble vitamins are not stored in the body and must be supplied regularly in the diet; fat-soluble vitamins are stored in the liver and adipose tissue.