At aqueous solution, the proton from carboxyl group can be transferred to the amino group of an amino acid leaving these groups with opposite charges. Despite having both positive and negative charges, this molecule is neutral and has amphoteric behaviour. These ions are called zwitter ions.

Zwitter ions behave as neutral molecules at pH isoelectric point.

(i) Carbohydrates are polyhydroxy aldehydes or polyhydroxy ketones.
(ii) They are naturally occurring organic substances.
(iii) Carbohydrates are formed in the plants by photosynthesis from water in sunlight.
(iv) They are classified into two types
(a) Sugars Eg: Glucose, Fructose.
(b) Non-sugars Eg: Starch, Cellulose.

Proteins are classified based on their structure (overall shape) into two major types. They are fibrous protein and globular proteins.
(i) Fibrous proteins: These proteins are linear molecules. These are generally insoluble in water and are held together by disulphide bridges and weak intermolecular hydrogen bonds. The Example: Keratin, Collagen etc
(ii) Globular proteins: These proteins have an overall spherical shape. The polypeptide chain is folded into a spherical shape. These proteins are usually soluble in water and have many functions including catalysis.

On hydrolysis, lactose yields galactose and glucose. Here, the β-D-galactose and β-Dglucose are linked by β-1,4 glycosidic bond. The aldehyde carbon is not involved in the glycosidic bond hence; it retains its reducing property and is called a reducing sugar.

On hydrolysis, sucrose yields equal amount of glucose and fructose units.
\(Sucrose\overset { Invertase }{ \longrightarrow } Glucose+Frutose\)
Sucrose (+66.6°) and glucose (+52.5^o) are dextrorotatory compounds while fructose is levo rotatory (-92.4°). During hydrolysis of sucrose, the optical rotation of the reaction mixture changes from dextro to levo. Hence, sucrose is called as invert sugar.

On oxidation with nitric acid, it gives glycolic acid and tartaric acids which contain smaller number of carbon atoms than in fructose.

This shows that a keto group is present in C-2.

(i)

(ii) Glucose forms penta acetate with acetic anhydride suggesting the presence of five alcohol groups.

Glucose gets oxidized to gluconic acid with mild oxidizing agents like bromine water suggesting that the carbonyl group is an aldehyde group.

Glucose reacts with HCN to form cyanohydrin.

(i) An amino acid is bifunctional organic molecule that contains both a carboxyl group, -COOH as well as an amine group, -NH₂.
(ii) Those amino acids that cannot be synthesised by the body and must be supplied in the diet are called essential amino acids
(iii) The amino acids that can be synthesised from other compounds by the tissues of the body are called non-essential amino acids.

(i) Oxidation of fructose yields a mixture of glycollic acid and tartaric acid.
(ii) When fructose is treated with HCN, it forms an addition product which upon hydrolysis and subsequent reduction with hydroiodic acid and red phosphorous gives 2-methyl-hexanoic acid.
This indicates that the ketone group is