Question

Biosynthesis of cytokinin takes place via
A. Mevalonic acid cycle
B. Citric acid cycle
C. Calvin cycle
D. $C_4$ cycle

Answer 1

Hint: Biosynthesis is a process that turns simple substances to complex substances. Here, the cycle which is responsible for biosynthesis of cytokinin is also responsible for synthesis of gibberellic acid and abscisic acid.

Complete answer:
First of all, you should know what cytokinin is and what its biosynthesis means. Cytokinin is a plant growth hormone which is responsible for the cell growth, chloroplast differentiation and it promotes cell division in the roots and shoots of plants. Biosynthesis is an enzyme-catalysed process which includes multiple steps where enzyme substrates are converted into more complex products in living organisms or you can say simple compounds are converted into complex compounds.

Biosynthesis of cytokinin takes place via Mevalonic acid cycle. In this cycle, the mevalonic acid first gets converted into isopentenyl pyrophosphate which then further gets converted into isopentenyl AMP which is then converted into isopentenyl adenosine. This finally gets converted into cytokinin.

Now if we look at the options,
> Option D is C4 cycle. It is a photosynthetic process in some plants to convert carbon dioxide into a form which is usable by plants during the process of photosynthesis. So this cannot be the right option.

> Option C which is Calvin cycle. It has four main steps which are carbon fixation , reduction phase, carbohydrate formation and regeneration phase. In this cycle, biosynthesis of cytokinin does not take place. So this is not the right option.

> Option B which is the Citric acid cycle, biosynthesis of cytokinin does not take place in this cycle too. So, this option is also incorrect one.

> Option A is mevalonic acid cycle, this option is correct as per the explanation given above. In this cycle, biosynthesis of cytokinin takes place.

Hence, the correct answer is option (A).

Note: In mevalonic acid cycle, there is synthesis of gibberellic acid and abscisic acid and these are also responsible for growth of plants. Cytokinin biosynthesis is done in various steps which start from mevalonic acid and then from isopentenyl adenosine cytokinin are formed.