Important Questions for CBSE Class 12 Biology Nutrient Cycling and Ecosystem Services

Ecosystem Important Questions for CBSE Class 12 Biology Nutrient Cycling and Ecosystem Services

1.The movement of nutrient elements through the various components of an ecosystem is called nutrient cycling or biogeochemical cycles.
(i)The amount of nutrients, i.e. carbon, nitrogen, phosphorus, calcium, etc., present in the soil at any given time, is called standing rate. It varies with the kind of ecosystem and season.
(ii)Nutrient cycles are of two types:
(a)Gaseous Cycle

Reservoir is the atmosphere.
Nutrients occur as gas or vapour.
Comparatively quick and fast.

e.g. carbon cycle and nitrogen cyole.
(b)Sedimentary Cycle

Reservoir is located in the earth’s crust.
Nutrient is non gaseous.
Very slow.

e.g. phosphorus cycle and sulphur cycle.
(iii) Environmental factors, e.g. soil, moisture, pH, temperature, etc., regulate the rate of nutrients into the atmosphere.
(iv) Function of reservoir is to meet with the deficit which occurs due to imbalance in the rate of influx and efflux.

2.Phosphorus Cycle
(i)Phosphorus is a major constituent of biological membranes, nucleic acids and cellular energy transfer systems.
(ii)Many animals also need phosphorus to make shells, bones and teeth.
(iii) The natural reservoir is rock, which contains phosphorus in the form of phosphates.
(iv)When rocks are weathered, minute amounts of these phosphates dissolve in soil solution and are absorbed by the roots of the plants.
(v)Herbivores and other animals obtain this element from plants.
(vi)Organic waste products and the dead organisms are decomposed by the phosphate solubilising bacteria releasing phosphorus.

3. Carbon cycle is the most common nutrient or biogeochemical cycle of an ecosystem.
(i) Carbon constitutes 49% of dry weight of an organism, next to water.
(ii) About 71% of carbon is found dissolved in oceans, which is responsible for its regulation In atmosphere.
(iii) Carbon cycle occurs through atmosphere, ocean and through living and dead organisms. Fossil fuel also represent a reservoir of carbon

(iv)According to an estimate, 4x 10¹³ kg of carbon is fixed in the biosphere through photosynthesis annually.
(v) A considerable amount of carbon is returned to atmosphere as C0₂, released by the animals and plants during respiration and by the activities of decomposers.
(vi) Some amount of fixed carbon is lost to sediments and removed from circulation.
(vii) The additional sources of carbon for the release in atmosphere are burning of wood, forest fire and combustion of organic matter, fossil fuel, volcanic activity.
(viii) Human activities like deforestation, massive burning of fossil fuel for energy and transport have increased the rate of release of carbon dioxide into the atmosphere.

4.Difference between carbon and phosphorus cycle are:
(i) Unlike carbon, there is no respiratory release of phosphorus.
(ii)Atmospheric inputs of phosphorus through rainfall are much smaller than carbon inputs
(iii)Gaseous exchange of phosphorus between organism and environment are negligible

5.Ecosystem services are the products of ecosystem processes.
(i)Some examples of ecosystem services are:

Purification of air and water by healthy forest ecosystems.
Mitigation of droughts and floods.
Cycling of nutrients.
Generate fertile soils.
Provide wildlife habitat.
Maintain biodiversity.
Pollinate crops by components of natural ecosystem, i.e. bees and insects.
Carbon fixation huge amounts of C0₂ in the atmosphere is removed naturally and fixed by plants into organic molecules and energy through photosynthesis. All the other trophic levels, i.e. consumers depend upon this energy produced by them.
Release of oxygen by the producers, as a byproduct in the process of photosynthesis, improves the air quality and supports life on earth.
Dispersal of seeds by various abiotic components (e.g. water, wind) and biotic components (birds, insects, animals, humans) of ecosystem.
Provide aesthetic cultural and spritual values.

(ii) Robert Constanza and his colleagues have tried to put price tags on nature’s life support services.
(iii) Researchers have estimated the price to be 33 trillion US dollars a year, while Global Gross Production (GGP) is only 18 trillion US dollers.
(iv) Out of the total cost of various ecosystem services, the soil formation accounts for about 50% and contributions of other services like recreation and nutrient cycling are less than 10% each. The cost of climate regulation and habitat for wildlife are about 6% each.

Previous YearExamination Questions

2 Marks Questions

1.Explain the function of reservoir in nutrient cycle. List the two types of nutrient cycles in nature. [Foreign 2011]
Ans.Reservoir in an ecosystem meets the deficit that arises due to the imbalance in the influx and efflux of nutrients. The two types of nutrient cycles are:
(i) Gaseous cycle (ii) Sedimentary cycle

2.Name the two types of nutrient cycles existing in nature. Where are their reservoirs present? State the function of reservoirs. [All India 2010 c]
Ans.Two types of nutrient cycles in nature:
(i) (a) Gaseous cycles (carbon and nitrogen cycle).
(b) Sedimentary cycles (phosphorus and sulphur cycle).
(ii) (a) Reservoir for gaseous cycle is atmosphere.
(b) Reservoir for sedimentary cycle is earth’s crust.
Function of Reservoir It meets the deficit which occurs due to the imbalance in the state of influx and efflux of nutrients

3 Marks Questions

3. State the function of a reservoir in a nutrient cycle. Explain the simplified model of carbon cycle in nature.
Ans.For function of a reservoir in a nutrient cycle.
Two types of nutrient cycles in nature:
(i) (a) Gaseous cycles (carbon and nitrogen cycle).
(b) Sedimentary cycles (phosphorus and sulphur cycle).
(ii) (a) Reservoir for gaseous cycle is atmosphere.
(b) Reservoir for sedimentary cycle is earth’s crust.
Function of Reservoir It meets the deficit which occurs due to the imbalance in the state of influx and efflux of nutrients

4.Draw and complete the following model of carbon cycle filling A, B,C,D,E and F.

Ans.Carbon cycle in

5 Marks Questions

5.(i) Draw a simplified model of phosphorus cycling in a terrestrial ecosystem.
(ii) Write the importance of such cycles in ecosystems. [All India 2014 C]
Ans. (i) Simplified model of phosphorus cycling in a terrestrial ecosystem.

(ii)The importance of such nutrient cycles in ecosystem can be briefed as:

It allows exchange, storage and transfer of biogenetic nutrients through various biotic components of ecosystem.
It also allows exchange of nutrients between abiotic and biotic components of ecosystem.
Recycling of nutrients allows them to be used repeatedly and indefinitely.

6.(i) (a) Name the given biogeochemical (nutrient)cycle.
(b) Name an activity of the living organisms not depicted in the cycle by which this nutrient is returned to the atmosphere.
(ii)How would the flow of nutnent m the cycle be affected due to the large scale deforestation? Explain giving reasons.[Delhi 2011]

Ans.(i)(a) It is a carbon cycle.
(b) Decomposition of organic wastes by decomposers cycles the carbon back into atmosphere. (ii)Deforestation leads to increase in carbon dioxide levels in the air. Because the C0₂ present is not being utilised, i.e. fixed for photosynthesis in the absence of plants.

7.Carbon cycle in nature is a biogeochemical event. Explain.[Delhi 2009 C]
Ans.Carbon cycle is the most common nutrient or biogeochemical cycle of an ecosystem.
(i) Carbon constitutes 49% of dry weight of an organism.
(ii) About 71 % of carbon is found dissolved in oceans, which is responsible for its regulation in atmosphere.
(iii) Carbon cycle occurs through atmosphere, ocean and through living and dead organisms.
(iv)According to an estimate, 4 x 10¹³ kg of carbon is fixed in the biosphere through photosynthesis \ annually.
(v)Carbon is returned to atmosphere as C0₂ released by the animals and plants during respiration and by the activities of decomposers.
(vi) Some amount of fixed carbon is lost to sediments and removed from circulation.
(vii) The additional sources of carbon for the release in atmosphere are burning of wood, forest fire and combustion of organic matter, fossil fuel, volcanic activity.
(viii)Human activities like deforestation, massive burning of fossil fuel for energy and transport have increased the rate of release of carbon dioxide into the atmosphere.

8.What does the term standing state of soil signify? How are the nutrients recycled in the ecosystem? Write a cyclic account of carbon movement imnature. [All India 2009 c]
Ans.Standing state of soil The amount of nutrients, such as carbon, nitrogen, phosphorus, calcium, etc.,the present in the soil at any given time, is referred to as standing state of soil. It varies in different kind of ecosystem and also on a seasonal basis.
Nutrient cycling The movement of nutrients through various components of an ecosystem is called nutrient cycling or biogeochemical cycles. These are of two types, i.e. gaseous and sedimentary.
The cyclic account of carbon movement in nature.
Carbon cycle is the most common nutrient or biogeochemical cycle of an ecosystem.
(i) Carbon constitutes 49% of dry weight of an organism.
(ii) About 71 % of carbon is found dissolved in oceans, which is responsible for its regulation in atmosphere.
(iii) Carbon cycle occurs through atmosphere, ocean and through living and dead organisms.
(iv)According to an estimate, 4 x 10¹³ kg of carbon is fixed in the biosphere through photosynthesis annually.
(v)Carbon is returned to atmosphere as C0₂ released by the animals and plants during respiration and by the activities of decomposers.
(vi) Some amount of fixed carbon is lost to sediments and removed from circulation.
(vii) The additional sources of carbon for the release in atmosphere are burning of wood, forest fire and combustion of organic matter, fossil fuel, volcanic activity.
(viii)Human activities like deforestation, massive burning of fossil fuel for energy and transport have increased the rate of release of carbon dioxide into the atmosphere.

Miscellaneous Questions

1 Mark Question

1.Differentiate between standing state and standing crop in an ecosystem.[Foreign 2010]
Ans.Differences between standing state and standing crop are

3 Marks Questions

2.(i) Healthy ecosystems are the base of wide range of (ecosystem) services. Justify.
(ii) Explain the differences and the similarities between hydrarch and xerarch successions of plants. [Delhi 2011]
Ans.(i) Healthy ecosystem provides following ecological services:

Purification of air and maintenance of gaseous composition.
Mitigation of droughts and floods. Cycling of nutrients.
Store house of carbon.
Maintenance of biodiversity.
Habitat for a number of wildlife.
Influence on hydrological cycle.

(ii)Differences between hydrarch and xerarch succession are:

Similarities Both hydrarch and xerarch succession lead to mesic conditions.
3.(i) Trace the succession of plants on a dry bare rock,
(ii) How does phosphorus cycle differ from carbon cycle?
Ans.(i)Primary succession rocks 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 or bryophytes 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
(ii)Differences between phosphorus cycle and carbon cycle are

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