High concentration of DDT in water of a lake disturb:

The correct answer is Option (1) → Calcium metabolism in birds

Option (1) Calcium metabolism in birds — Correct. DDT builds up to high levels in fish-eating birds through biomagnification. In birds it disturbs calcium metabolism, so the eggshell becomes thin, the eggs break too early, and the bird population falls.

Option (2) Potassium metabolism in birds — Incorrect. DDT is not linked to potassium metabolism. In birds, high DDT disturbs calcium metabolism, which thins the eggshell.

Option (3) Carbohydrate metabolism in birds — Incorrect. DDT does not upset carbohydrate metabolism in birds. It affects calcium metabolism, so eggshells become thin and break early, causing a decline in bird populations.

Option (4) Protein metabolism in birds — Incorrect. Protein metabolism is not the target of DDT. Its harm to birds comes through disturbed calcium metabolism and thin, fragile eggshells.

Note: This question is from Chapter 16, Environmental Issues, of the pre-rationalised NCERT book. This chapter is not in the present NCERT book (2026-27). The extract below is from the pre-rationalised edition.

NCERT Extract (Pre-rationalised edition, Chapter 16, Environmental Issues):
Domestic Sewage and Industrial Effluents – “… Biomagnification refers to increase in concentration of the toxicant at successive trophic levels. This happens because a toxic substance accumulated by an organism cannot be metabolised or excreted, and is thus passed on to the next higher trophic level. This phenomenon is well-known for mercury and DDT. Figure 16.5 shows biomagnification of DDT in an aquatic food chain. In this manner, the concentration of DDT is increased at successive trophic levels; say if it starts at 0.003 ppb (ppb = parts per billion) in water, it can ultimately reach 25 ppm (ppm = parts per million) in fish-eating birds, through biomagnification. High concentrations of DDT disturb calcium metabolism in birds, which causes thinning of eggshell and their premature breaking, eventually causing decline in bird populations.”