NCERT Solutions for Class 11th: Ch 15 Life on the Earth


Page No. 133

1. Multiple choice questions.

(i) Which one of the following is included in biosphere?
(a) only plants
(b) all living and non-living organisms
(c) only animals
(d) all living organisms
► (b) all living and non-living organisms 

(ii) Tropical grasslands are also known as :
(a) the prairies
(b) the savannas 
(c) the steppes
(d) none of the above
► (b) the savannas 

(iii) Oxygen combines with iron found in the rocks to form :
(a) iron carbonate
(b) iron nitrites 
(c) iron oxides
(d) iron sulphate
► (c) iron oxides

(iv) During photosynthesis, carbon dioxide combines with water in the presence of sunlight to form :
(a) proteins
(b) amino acids 
(c) carbohydrates
(d) vitamins
► (c) carbohydrates

2. Answer the following questions in about 30 words.

(i) What do you understand by the term ‘ecology’?


Ecology is a scientific study of the interactions of organisms with their physical environment and with each other.

(ii) What is an ecological system? Identify the major types of ecosystems in the world.


The interactions of a particular group of organisms with abiotic factors within a particular habitat resulting in clearly defined energy flows and material cycles on land, water and air, are called ecological systems.
Ecosystems are of two major types:
• Terrestrial
• Aquatic

(iii) What is a food-chain? Give one example of a grazing food-chain identifying the various levels.


Thes sequence of eating and being eaten and the resultant transfer of energy from one level to another is known as the food-chain.
Grass (first level) is eaten by deer (intermediate level) is eaten by lion (last level).

(iv) What do you understand by the term ‘food web’? Give examples.


A network of food chains mixed together to form a food web. For example, a mouse feeding on grain may be eaten by different secondary consumers (carnivores) and these carnivores may be eaten
by other different tertiary consumers (top carnivores). 

(v) What is a biome?


A biome is a plant and animal community that covers a large geographical area.

Page No. 134

3. Answer the following questions in about 150 words.

(i) What are bio-geochemical cycles? Explain how nitrogen is fixed in the atmosphere.


The balance of the chemical elements is maintained by a cyclic passage through the tissues of plants and animals. The cycle starts by absorbing the chemical elements by the organism and is returned to the air, water and soil through decomposition. These cycles are largely energised by solar insolation. The cyclic movements of chemical elements of the biosphere between the organism and the
environment are referred to as biogeochemical cycles. Bio refers to living organisms and geo to rocks, soil, air and water of the earth.
Ninety per cent of fixed nitrogen is biological. The principal source of free nitrogen is the action of soil micro-organisms and associated plant roots on atmospheric nitrogen found in pore spaces of the soil. Nitrogen can also be fixed in the atmosphere by lightning and cosmic radiation. In the oceans, some marine animals can fix it. 

(ii) What is an ecological balance? Discuss the important measures needed to prevent ecological imbalances.


Ecological balance is a state of dynamic equilibrium within a community of organisms in a habitat or ecosystem. It can happen when the diversity of the living organisms remains relatively stable.

Ecological imbalances: Ecological balance may be disturbed due to the introduction of new species, natural hazards or human causes.
The important measures to prevent ecological imbalances are:
• By not interfering in nature.
• By controlling population.
• By reusing and recycling things.
• By using natural resources judiciously.
• Afforestation should be encouraged.
Proper knowledge and management are necessary for protecting and conserving the ecosystems.

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