Question

7. (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. Use this information to describe the carbon cycle further, indicating how carbon returns to the soil and atmosphere.
(b)(i) Nitrogen-fixing bacteria convert nitrogen into ammonium compounds for plants' use. Give an example. (ii) State the roles of nitrifying and denitrifying bacteria in nature (in a laboratory/field context), and give examples.

Accepted Answer

(a) Living organisms respire aerobically and release carbon(IV) oxide into the atmosphere. Combustion (of fuels) and decay (of dead organisms) are other processes that release carbon(IV) oxide into the atmosphere. Carbon is removed from the atmosphere by photosynthesis, in which green plants use carbon(IV) oxide, together with water and sunlight, to build carbohydrates. These carbon compounds are passed along the food chain from plants to herbivores to carnivores, and are eventually returned to the atmosphere or soil through respiration, excretion, death and decay, and combustion, thereby completing the cycle.

(b)(i) Nitrogen-fixing bacteria convert nitrogen into ammonium compounds plants can use. An example is Rhizobium leguminosarium.
(ii) Nitrifying bacteria (such as Nitrobacter and Nitrosomonas) convert ammonium compounds into nitrite and then into nitrate. Denitrifying bacteria are those bacteria that break down ammonium compounds to release nitrogen back into the atmosphere.

WAEC Biology 2013 Theory — Question 7

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