Question

Critical elements are:
(a) N, P, S
(b) N, K, S
(c) N, P, K
(d) P, K, S

Answer 1

Hint: Critical elements are elements necessary for the successful growth and development of a species.

Complete answer:
Nitrogen, phosphorus and sulphur. Nitrogen is one of the major constituents of proteins, nucleic acids, vitamins and hormones. Phosphorus is a constituent of cell membranes, certain proteins, all nucleic acids and nucleotides, and is needed for all phosphorylation reactions. Sulphur is present in two amino acids – cysteine and methionine and is the main constituent of several coenzymes, vitamins (thiamine, biotin, Coenzyme A) and ferredoxin.

Additional Information:
The criteria for essentiality of an element are -
1. The element must be absolutely necessary for supporting normal growth and reproduction. In the absence of the element the plants do not complete their life cycle or set the seeds.
2. The requirement of the element must be specific and not replaceable by another element. In other words, deficiency of any one element cannot be met by supplying some other element.
3. The element must be directly involved in the metabolism of the plant.

Essential elements can be grouped into four broad categories on the basis of their diverse functions -
1. Essential elements as components of biomolecules and hence structural elements of cells (e.g., carbon, hydrogen, oxygen and nitrogen).
2. Essential elements that are components of energy-related chemical compounds in plants (e.g., magnesium in chlorophyll and phosphorus in ATP).
3. Essential elements that activate or inhibit enzymes, for example $Mg^{2+}$ is an activator for both ribulose bisphosphate carboxylase oxygenase and phosphoenol pyruvate carboxylase, both of which are critical enzymes in photosynthetic carbon fixation; $Zn^{2+}$ is an activator of alcohol dehydrogenase and Mo of nitrogenase during nitrogen metabolism.
4. Some essential elements can alter the osmotic potential of a cell. Potassium plays an important role in the opening and closing of stomata.

So the correct answer is N, P, S.

Notes:
1. Macronutrients and micronutrients are both essentially required, however their quantities differ.
2. Growing plants in a nutrient solution is known as hydroponics.
3. The concentration of the essential element below which plant growth is retarded is termed as critical concentration.