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Class - 12 : Biology

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Biology - Chapter : 14


Ecosystem


class 12 textbook biology

NCERT Solution for Class 12 Science Biology - Page/Excercise 256


Question : 1 :
Fill in the blanks.
(a) Plants are called as ___________ because they fix carbon dioxide.
(b) In an ecosystem dominated by trees, the pyramid (of numbers) is ___________ type.
(c) In aquatic ecosystems, the limiting factor for the productivity is _________.
(d) Common detritivores in our ecosystem are _________.
(e) The major reservoir of carbon on Earth is ___________.

Answer :
(a) Producers
(b) Upright
(c) Availability of sunlight
(d) Bacteria, fungi and earthworm
(e) Oceans

NCERT Solution for Class 12 Science Biology - Page/Excercise 257


Question : 2 :
Which one of the following has the largest population in a food chain?
(a) Producers
(b) Primary consumers
(c) Secondary consumers
(d) Decomposers

Answer :
(a) Producers have the largest population because they occupy the first trophic level in a food chain, and the number of organisms decreases from the producer level to the top carnivore level.


Question : 3 :
The second trophic level in a lake is
(a) Phytoplankton
(b) Zooplankton
(c) Benthos
(d) Fishes

Answer :
(b) Zooplankton is the second trophic level in a lake.


Question : 4 :
Secondary producers are
(a) Herbivores
(b) Producers
(c) Carnivores
(d) None of the above

Answer :
(d) 'None of the above' is the correct option. Herbivores and carnivores are consumers.


Question : 5 :
What is the percentage of photosynthetically active radiation (PAR) in incident solar radiation?
(a) 100%
(b) 50%
(c) 1-5%
(d) 2-10%

Answer :
(b) 50% is the correct option. Only 50% of the incident solar radiation is photosynthetically active radiation.


Question : 6 :
Distinguish between
(a) Grazing food chain and detritus food chain
(b) Production and decomposition
(c) Upright and inverted pyramids
(d) Food chain and food web
(e) Litter and detritus
(f) Primary and secondary productivity

Answer :
(a) Grazing food chain and detritus food chain

Grazing Food Chain Detritus Food Chain
i. Energy for the food chain comes from the Sun. i. Energy comes from detritus (organic matter).
ii. First trophic level organisms are producers. ii. First trophic level organisms are detritivores and decomposers.

(b) Production and decomposition

Production Decomposition
i. It refers to the process of synthesis of organic compounds from inorganic substances utilising sunlight. i. It is the phenomenon of degradation of waste biomass.
ii. Example: Plants perform the function of production of food. ii. Example: Bacteria and fungi decompose dead organic matter.

(c) Upright pyramid and inverted pyramid

Upright Pyramid Inverted Pyramid
When the number of producers or their biomass is maximum in an ecosystem and it decreases progressively at each trophic level in a food chain, an upright pyramid is formed. When the number of individuals or their biomass at the producer level is minimum and it increases progressively at each trophic level in a food chain, an inverted pyramid is formed.

(d) Food chain and food web

Food Chain Food Web
i. A food chain is a single pathway where energy is transferred from producers to successive orders of consumers. i. A food web is a network of various food chains which are interconnected with each other like an interlocking pattern.
ii. All food chains start with green plants which are the original source of all food. ii. It has many linkages and intercrosses among producers and consumers.

(e) Litter and detritus

Litter Detritus
The dead remains of plants (leaves, flowers etc.) and animals and animal excreta which falls on the surface of the Earth in terrestrial ecosystems is called litter. The dead remains of plants and animals constitute detritus. It is differentiated into litter fall (above ground detritus) and below ground detritus.

(f) Primary and secondary productivity

Primary Productivity Secondary Productivity
i. It is the rate at which organic matter is built up by producers. i. It is the rate of synthesis of organic matter by consumers.
ii. It is due to photosynthesis. ii. It is due to herbivory and predation.

Question : 7 :
Describe the components of an ecosystem.

Answer :
There are two components of an ecosystem:
(a) Biotic components: These include all living organisms present in an ecosystem such as producers, consumers and decomposers. Examples: Plants, animals and microorganisms.
(b) Abiotic components: These include all the non-living factors. Examples: Light, temperature, wind, humidity, rain, pressure, inorganic materials such as carbon, nitrogen, oxygen etc. and dead organic matter containing proteins, carbohydrates and lipids.


Question : 8 :
Define ecological pyramids and describe with examples pyramids of number and biomass.

Answer :
An ecological pyramid (or trophic pyramid) is a graphical representation designed to show the biomass or productivity at each trophic level in a given ecosystem.
A pyramid of numbers is a graphic representation depicting the arrangement of the number of individuals of different trophic levels in a food chain in an ecosystem.
Example: A pyramid of numbers in a grassland ecosystem. In this, the pyramid of numbers is a straight pyramid. The size of organisms increases with each trophic level, i.e. from the producer level to the top carnivore level, while their number decreases in a food chain.
Pyramid of biomass: An ecological pyramid of biomass shows the relationship between biomass and trophic level by quantifying the amount of biomass present at each trophic level.
The pyramid of biomass can be straight or inverted. For example, in grassland and forest ecosystems, there is a gradual decrease in biomass of organisms at successive trophic levels from producers onwards to top carnivores (upright or straight pyramid). In a pond ecosystem, on the other hand, producers are the smallest organisms, while carnivores are large in size. Consequently, there is a gradual increase in biomass of organisms at successive trophic levels from producers onwards to top carnivores, resulting in an inverted pyramid.


Question : 9 :
What is primary productivity? Give a brief description of factors which affect primary productivity.

Answer :
Primary production is the amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis.
Factors: Primary productivity depends on sunlight, temperature, moisture, nutrients and photosynthetic efficiency of producers.
i. Sunlight: Maximum sunlight is available in tropical regions. There is a progressive reduction of its availability towards the polar regions. Therefore, more photosynthesis and high productivity occur in tropical regions. It decreases progressively towards the polar regions.
ii. Temperature: Temperate forests have lesser productivity as compared to tropical forests due to cold climate during winter.
iii. Nutrients: A regular availability of nutrients is required for sustaining plant growth and productivity of an ecosystem.


Question : 10 :
Define decomposition and describe the processes and products of decomposition.

Answer :
Decomposition is the physical and chemical breakdown of complex organic remains into inorganic raw materials for recycling. It is carried out by decomposers.
Process: Three types of processes occur during decomposition. They are fragmentation of detritus, catabolism and leaching.
i. Fragmentation of detritus: It is the breakdown of dead plants and animals into smaller particles.
ii. Catabolism: It involves the secretion of digestive enzymes by decomposers on the detritus. It changes insoluble complex organic substances into simple and soluble organic compounds and inorganic substances.
iii. Leaching: Soluble simpler substances formed during decomposition along with percolation of water pass down to deeper layers of soil.
The decomposition process gives rise to two products:
i.Humus
ii.Inorganic nutrients (minerals)
Humus is a dark-coloured amorphous substance which acts as a reservoir of nutrients and maintains soil moisture and aeration.


Question : 11 :
Give an account of energy flow in an ecosystem.

Answer :
Only 50% of the incident solar radiation is photosynthetically active radiation. Of this, plants capture about 2-10% of the energy. This small fraction of energy is what sustains life on the Earth. The energy flows from producers (green plants) to consumers or to detritus. The flow of energy follows the 10% rule. According to this rule, 90% of the energy is spent in metabolic activities by an organism and only 10% is available for the next component of the food chain.

Producers ⇒ Primary Consumers ⇒ SecondaryConsumers ⇒ Tertiary Consumers

Because of the 10% rule, a food chain can have a limited number of levels. Numerically also, the number of organisms in a particular level will be less than that in the previous level.


Question : 12 :
Write important features of a sedimentary cycle in an ecosystem.

Answer :
The sedimentary cycle involves cycling and recycling of various minerals such as phosphorous or sulphur of the lithosphere or soil between living and non-living things. Its main features are
i.It is a cycle where the nutrients are not lost from the system but are recycled again and again.
ii.It operates through soil, water bodies, air and living organisms.
iii.It converts the nutrients into usable form with the help of decomposers.
iv. It helps in maintaining the nutrient pool of the Earth.


Question : 13 :
Outline salient features of carbon cycling in an ecosystem.

Answer :
Salient features of carbon cycling:
i.71% of carbon is found dissolved in ocean water, so the oceans are the main reservoir of carbon on the Earth. Fossil fuel also represents a reservoir of carbon.
ii.Carbon cycling occurs through the atmosphere, ocean and living and dead organisms.
iii.About 4 x 1013 kg of carbon is fixed in the biosphere through photosynthesis annually.
iv.A considerable amount of carbon returns to the atmosphere as CO2 through respiratory activities of producers and consumers.
v.Decomposers also contribute substantially to the CO2 pool by the processing of waste materials and dead organic matter of land or oceans.
vi.Some amount of the fixed carbon is lost to sediments and removed from circulation. Burning of wood, forest fire and combustion of organic matter, fossil fuel and volcanic activity are additional sources for releasing CO2 into the atmosphere.
vii.Human activities have significantly influenced the carbon cycle. Rapid deforestation and massive burning of fossil fuel for energy and transport have significantly increased the rate of release of carbon dioxide into the atmosphere.


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