Question

The functions of zinc are:

(A) It is involved in the synthesis of indole acetic acid, metabolism of gibberellic acid and synthesis of RNA.

(B) It is a constituent of enzymes such as carbonic anhydrase (CA), alcoholic dehydrogenase and superoxide dismutase (SOD).

(C) Because of its preferential binding to sulphydryl group, Zn plays an important role in the stabilization and structural orientation of the membrane proteins.

(D) It influences translocation and transportation of P in plants. Under Zn-deficiency, poor translocation of P occurs, resulting in P-deficiency.

Choose the correct answer from the options given below:

• A. (A), (B) and (D) only
• B. (A), (C) and (D) only
• C. (A), (B) and (C) only
• D. (B), (C) and (D) only

Hint:

Recall which way the zinc-phosphorus relationship actually runs: zinc deficiency raises P uptake, it does not cause P deficiency.

Answer 1

The Correct Option is C

This question lists four claims about zinc's roles in plants and asks us to sort out the true ones from the one planted error.

1. (A) auxin, gibberellic acid metabolism and RNA synthesis: Zinc is required to make tryptophan, which is the building block for the auxin indole acetic acid, and zinc also supports RNA synthesis and ribosome integrity, so this statement matches known zinc biochemistry.
2. (B) constituent of carbonic anhydrase, alcohol dehydrogenase and superoxide dismutase: All three of these are genuine zinc metalloenzymes described in plant nutrition textbooks, so this statement is accurate.
3. (C) sulphydryl binding and membrane protein stabilization: Zinc's affinity for sulphydryl, or thiol, groups in membrane proteins is a well established mechanism by which zinc keeps cell membranes structurally sound, so this statement is also accurate.
4. (D) Zn deficiency causes poor P translocation and P deficiency: This is backwards. Zinc deficient plants are actually known to take up and move more phosphorus to their shoots than normal, sometimes to the point of phosphorus toxicity, and this excess P uptake is one reason zinc deficiency symptoms can get worse in high phosphorus soils. So this statement does not match the real relationship.

Since three of the four statements, A, B and C, line up with accepted zinc physiology, while D reverses the true zinc-phosphorus relationship, the combination that leaves out D is the correct one.

Let's summarize:

• Zinc supports hormone precursor synthesis, RNA metabolism, key metalloenzymes and membrane stability.
• Zinc deficiency raises, rather than lowers, phosphorus translocation, so statement D is false.

So the correct answer is (A), (B) and (C) only.