Question
Which of the following is NOT a criterion for the essentiality of an element in plants?
(A) The element must be required for completing the life cycle of the plant (B) The element must be directly involved in plant metabolism (C) The element must be present in large quantities in the plant body (D) The requirement for the element must be specific and cannot be substituted by another element
(NEET 2023)
Solution — Step by Step
In 1939, Arnon and Stout gave three criteria that an element must satisfy to be called essential. These criteria are about functional necessity, not about concentration. We need to check each option against these criteria.
Criterion 1 says: the element must be absolutely necessary for the plant to complete its life cycle (germination → reproduction → seed formation). If the plant cannot reproduce in the absence of that element, it’s essential. Option A is a valid criterion.
Criterion 2 says: the element must play a direct biochemical role — either as a structural component of molecules (like N in amino acids, Mg in chlorophyll) or as an activator of enzymes. Option B is a valid criterion.
Criterion 3 says: the deficiency must be specific to that element and cannot be corrected by supplying any other element. This rules out accidental accumulation. Option D is a valid criterion.
Option C says the element must be present in large quantities. This is the criterion for a macronutrient, not for essentiality itself. Micronutrients like Zinc, Molybdenum, and Boron are essential even though they’re present in trace amounts (parts per million). Concentration has nothing to do with whether an element is essential.
Answer: (C)
Why This Works
The key insight here is separating essentiality from quantity. An element is essential based on what it does, not how much of it is present. Molybdenum, for instance, is needed in nanogram quantities yet is absolutely essential for nitrate reduction — no other element can substitute for it.
This is why the classification into macronutrients (C, H, O, N, P, K, Ca, Mg, S) and micronutrients (Fe, Mn, Zn, B, Cu, Mo, Cl, Ni) is based purely on required quantity, while all 17 essential elements satisfy the same three Arnon-Stout criteria.
Think of it like a lock-and-key: Molybdenum fits a specific enzyme active site (nitrogenase, nitrate reductase). Whether you need one key or a hundred, you still need that key.
Alternative Method
If you blank on the exact criteria, use the elimination approach based on examples:
Molybdenum is present in plants at roughly 0.1 ppm — yet remove it and the plant cannot reduce nitrates and dies. Clearly essential, clearly not “large quantity.” This immediately rules out Option C as a criterion for essentiality.
A handy mnemonic for the three criteria: LSD — Life cycle completion, Specificity (non-substitutable), Direct metabolic role. If you remember these three, you can rule out any impostor criterion in MCQs.
Common Mistake
Many students confuse “essential” with “macronutrient” and think that if an element is present in small amounts, it must not be essential. This mix-up is deliberately exploited in NEET options. The correct relationship: all essential elements satisfy the same three criteria, but they’re classified as macro or micro based on concentration (>10 mmol/kg dry weight = macro). Never use quantity as a test for essentiality.
A related trap: some students list Carbon, Hydrogen, and Oxygen as micronutrients because “they come from air and water, not soil.” Wrong — C, H, O are macronutrients by quantity and essential by all three criteria. Source of supply has nothing to do with the classification.