Question

Which of the following hormones is a weak organic acid having unsaturated ring structures and derived from amino acid?
A. Cytokinin
B. Auxin
C. Gibberellins
D. Ethylene

Answer 1

Plant hormones are also known as Plant Growth Regulators or PGRs. These are chemicals that alter the physiological functions of the plant body. There are different PGRs that carry out different functions in the plant body.

Complete answer:
All the auxins are molecules that consist of a carboxylic acid or –COOH group, hence making it weak acids due to their lesser ability to undergo protonation. The molecule also consists of aromatic rings, which contain double bonds making it unsaturated. The naturally occurring auxin is Indole-3-acetic acid, and synthetic auxins are indole-3- butyric acid and 2, 4- dichloro phenoxy acetic acid. The indole acetic acid is derived from the amino acid tryptophan.

Figure: The ball and stick model structure of Indole-3-acetic acid

Considering the other given options:
A. Cytokinins are hormones that contain Adenine molecules which is a purine. It contains isoprene or an aromatic side chain and an attached amino group. The chemical name of zeatin, which is a naturally occurring cytokinin, is 6-(4- hydroxy-3-methylbut-2-enyl amine) purine. Thus option A is not the right option.
C. Gibberellins have the chemical formula $C_{19}H_{22}O_6$. It is a carboxylic acid. And contains ring structures but is not derived from any amino acids. Thus option ‘C’ 3 is not the right option.
D. Ethylene is a gaseous PGR. It is a hydrocarbon molecule that contains only carbon and hydrogen atoms. The chemical formula of the molecule is $C_2H_2$. Thus D is not the right option.

Hence, from the above-given options, the most appropriate answer is option B. That is, auxin is a weak organic acid that contains unsaturated rings and is derived from amino acid tryptophan.

Note: Auxins have a variety of effects during plant growth. It is a growth-promoting hormone. It helps in the elongation of the shoot, increases cell growth and also helps in the formation of roots.