Class 12th Biology 2012 Set3 Delhi Board Paper Solution

Key process involved in carbon cycle is photosynthesis and respiration. 

 

In photosynthesis, carbon-dioxide and water produce carbohydrate and oxygen while respiration oxidizes food to give carbon-dioxide and water.  In oceans, carbon-dioxide dissolves readily in water. In rock it gets stored as calcium carbonate. Carbon can either be liberated to atmosphere through respiration or can be passed to animal when being eaten or remain in plant even after death. After death of plant or animal, decomposition takes place and carbon dioxide gets released into the atmosphere. They can also be buried inside the earth and converted to coal or natural gas. Natural gas and coal are fuels and on combustion they release carbon dioxide  back into the atmosphere. Excess of carbon-dioxide can cause global warming. 

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(a)  The species of organisms that first invade a bare area are called pioneer species. The pioneer species on a bare rock are usually lichens. Lichens secrete acids which dissolve rocks, thereby leading to weathering and soil formation. This paves the way for small plants which hold the soil. They are succeeded by bigger plants, and ultimately, an entire forest gets established. Forests represent the climax community in this succession.

(b)    The algae and plants are the primary producers in a lake in which the algae contribute to the maximun productivity. In an industrial area the runoff of nutrients such as inorganic chemicals and sewage, leads to an increase in the fertility of the lake. As a result, it causes a tremendous increase in the primary productivity of the lake ecosystem. This leads to an increased growth of algae, resulting in algal blooms. Later, the decomposition of these algae depletes the supply of oxygen, leading to the death of other aquatic animal life. This ageing process of a lake caused due to nutrient enrichment is called Eutrophication.