WAEC Biology 2013 Theory — Question 5

Model answer

(a) The aerobic respiration of living organisms leads to the release of carbon(IV) oxide into the atmosphere. Combustion and decay are other processes that release carbon(IV) oxide into the atmosphere. This explains how carbon is released into the environment. Carbon is also removed from the atmosphere by photosynthesis, which uses up carbon(IV) oxide to build carbohydrates in green plants; these carbon compounds pass along the food chain to animals, and are also released back through respiration, decay and combustion, completing the cycle. (b)(i) Nitrogen-fixing bacteria convert nitrogen gas into ammonium compounds for plants' use. An example is Rhizobium leguminosarium (found in root nodules of leguminous plants). (ii) Nitrifying bacteria (such as Nitrobacter and Nitrosomonas) convert ammonium compounds into nitrite and then into nitrate, making nitrogen available to plants in a usable form. (iii) Denitrifying bacteria are those bacteria that break down ammonium compounds to release nitrogen back into the atmosphere as nitrogen gas.