Which of the following nutrients is not a structural component of the plant ?

In plant physiology, structural nutrients are those incorporated into organic molecules or cell structures. Nitrogen is a structural component of proteins, nucleic acids, and chlorophyll [c1, t3, t6]. Phosphorus is a vital structural part of nucleic acids (DNA/RNA), phospholipids in cell membranes, and energy molecules like ATP [t2, t3, t8]. Calcium serves a critical structural role in the cell wall (middle lamella) and membranes, providing rigidity [t1, t7]. In contrast, Potassium (K) is unique because it is not incorporated into the structure of any organic compounds [t2, t10]. Instead, it remains in ionic form (K+) within the cell solution, where it functions primarily as an osmolyte for regulating osmotic pressure, stomatal movement, and enzyme activation [t2, t10]. Therefore, potassium is considered a regulatory and catalytic nutrient rather than a structural one.

Sources

1. [1] Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 25: Agriculture > Nitrogen (N) > p. 363
2. [2] https://pmc.ncbi.nlm.nih.gov/articles/PMC4243668/