CBSE Class 12-science Answered

(a) Chlorofluorocarbons (CFCs) are mainly responsible for the depletion of ozone in the stratosphere. Once released in the troposphere, CFCs reach the stratosphere and produce ‘active chlorine’ in the presence of ultraviolet (UV) radiation. These radicals, through a chain reaction, destroy the ozone by converting it into oxygen. Excess dose of UV-B radiations damages DNA causing mutation, skin cancer, snow blindness, cataract and ageing of skin.

(b) If DDT leaches from agricultural fields, it enters the nearby water body. The concentration of DDT in the water body is low (0.003 ppm). The pesticide enters the food chain and gets accumulated in zooplankton (0.04 ppm) and thereby its concentration goes on increasing at different trophic levels of the food chain from the small fish (0.5 ppm), large fish (2 ppm) to finally the fish-eating birds (5 ppm). Concentration of DDT increases along the food chain, reaching the highest level in the top carnivore bird. The increased concentration of DDT in birds disturbs calcium metabolism. The egg shells become thin, resulting in premature breaking. Consequently, there is a decline in the bird population.