Question

DDT causes eggshell thinning in birds because it inhibits:
(A) calmodulin
(B) calcium ATPase
(C) magnesium ATPase
(D) carbonic anhydrase

Answer 1

Hint: DDT is a pesticide and thus it is a toxin, so it undergoes the process of biomagnification. DDT present in water bodies reaches up to birds through the food chain and successively increases toxicity from one trophic level to another by the process of biomagnification. So, it is maximum in birds. The eggshell contains calcium, it interferes with calcium metabolism and causes thinning of eggshells in birds.
Step-by-step solution:
DDT is used as a pesticide, it reaches up to birds by the process of biomagnifications. Biomagnifications in the aquatic food chain lead to the accumulation of DDT in birds. Toxic substances present on industrial wastewater can undergo magnification in the aquatic food chain. Biomagnification means an increase in the concentration of a particular toxin from one trophic level to a higher trophic level. This happens because a toxin substance cannot be metabolised by the organism and so it passes onto the next trophic level. This phenomenon works in the case of mercury as well as DDT.
Let us take an example, if the concentration of DDT in water is 0.003 oob (ppb stands for parts per billion), then it reaches up to 0.04 ppm (ppm stands for parts per million) in zooplanktons, then up to 0.5 ppm in small fishes, up to 2 ppm in large fishes and then finally up to 25 ppm in fish-eating birds. A high concentration of toxins such as DDT in birds disturbs calcium metabolism which leads to thinning of their egg shells and thus their premature breaking which eventually leads to a decline in the bird population.
Hence, the option (B) is correct.
Note: Sewage from homes does not contain toxic substances but the sewage discharge from industries like petroleum, paper manufacturing, metal extraction and processing, chemical manufacturing etc contains toxins so effluents from such industries are the major cause of biomagnifications.