The correct answer is option 2: Calcium.

Plants require a variety of nutrients to survive and thrive. These nutrients can be classified into macronutrients and micronutrients. Macronutrients are required in large quantities, while micronutrients are required in smaller amounts.

Nitrogen, phosphorus, and potassium are all macronutrients that play crucial roles in plant growth. Nitrogen is an essential component of proteins and nucleic acids, which are involved in various cellular processes. It is responsible for leaf and stem development. Phosphorus is involved in energy transfer and storage, as well as the formation of DNA and RNA. Potassium helps regulate water uptake in plants and is involved in various enzymatic activities.

On the other hand, calcium is not considered a structural component of plants. It is primarily involved in the formation and stability of cell walls, as well as signaling pathways within the plant. While calcium does contribute to the overall structure of the plant, it is not considered a primary structural component like cellulose or lignin.

Therefore, option 2, Calcium, is the correct answer as it is not a structural component of the plant.