Class 12 Biology


NCERT Exercise

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) producer, (b) upright, (c) sunlight, (d) earthworm, (e) ocean

Question 2: Which one of the following has the largest population in a food chain?

  1. Producers
  2. Primary consumers
  3. Secondary consumers
  4. Decomposers

Answer: (d) Decomposers

Question 3: The second trophic level in a lake is

  1. Phytoplankton
  2. Zooplankton
  3. Benthos
  4. Fishes

Answer: (b) Zooplankton

Question 4: Secondary producers are

  1. Herbivores
  2. Producers
  3. Carnivores
  4. None of the above

Answer: (d) None of the above

Question 5: What is the percentage of photosynthetically active radiation (PAR), in the incident solar radiation.

  1. 100%
  2. 50%
  3. 1-5%
  4. 2-10%

Answer: (b) 50%

Question 6: Distinguish between

(a) Grazing food chain and detritus food chain

Answer: Grazing food chain begins with a producer, while detritus food chain begins with dead organic matter.

(b) Production and decomposition

Answer: Production involves conversion of inorganic materials into organic materials, while decomposition involves conversion of organic materials into inorganic materials. Production is mainly done by green plants, while decomposition is mainly done by bacteria and fungi. Sunlight is necessary for production in most of the cases, while sunlight is not required for decomposition.

(c) Upright and inverted pyramid

Answer: Upright pyramid is the norm in a terrestrial ecosystem, while inverted pyramid is found in ocean ecosystem. The pyramid of energy is always upright. In some exceptional case, pyramid of number and biomass can be inverted.

(d) Food chain and Food web

Answer: Food chain is the simple linear representation of energy flow from producer to consumers. Food chain does not exist in nature. A network of interconnected food chains is called food web. It is the food web that is found in nature.

(e) Litter and detritus

Answer: Litter is always present above the surface of earth, while detritus is present above and below the surface of earth. Litter contains both biodegradable and non-biodegradable substances, while detritus always contains biodegradable substances.

(f) Primary and secondary productivity

Answer: Primary productivity is associated with producers, while secondary productivity is associated with consumers. Primary productivity is the result of photosynthesis or chemosynthesis. Secondary productivity is the result of predation.

Question 7: Describe the components of an ecosystem.

Answer: Following are the two main components of ecosystem:

Biotic Components: All living organisms in an ecosystem compose the biotic component. Biotic component can be further divided into three main types, viz. producers, consumers and decomposers.

Abiotic Components: All non-living things in an ecosystem compose the abiotic component. Abiotic components provide the required raw materials for creation of biomass. Air, soil, minerals, water, etc. are examples of abiotic components.

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

Answer: The three ecological pyramids that are usually studied are

Pyramid of number: It shows a sharp decrease in number as we move up the trophic levels. This pyramid is generally upright. But there are exceptions to this rule. For example; number of insects feeding on a large banyan tree is numerous.

Pyramid of biomass: It shows a sharp decrease in biomass as we move up the trophic levels. This pyramid is upright in terrestrial ecosystems. But this pyramid is inverted in oceans because the biomass of fishes far exceeds the biomass of phytoplankton.

Pyramid of energy. This pyramid is always upright. The primary producers convert only 1% of the energy in the sunlight available to them into NPP. Out of this, only 10% is available for the organisms at the next trophic level.

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

Answer: The amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis is called primary production. Primary production is expressed in terms of weight (g-2 or energy (kcal m-2. When we need to compare the productivity of different ecosystems, then primary productivity is expressed in terms of g–2 yr–1 or (kcal m–2) yr–1.

Primary productivity depends on plant species in a given area. It also depends on many environmental factors, availability of nutrients and photosynthetic capacity of plants.

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

Answer: Conversion of complex organic matter into inorganic substances by decomposers is called decomposition.

Steps of decomposition are; fragmentation, leaching, catabolism, humification and mineralization.

Fragmentation: Detritivores break down detritus into smaller particles. This process is called fragmentation. Earthworm is an example of detritivore.

Leaching: The loss of water soluble plant nutrients from soil is called leaching. During this process, water soluble inorganic nutrients go down into the soil horizon and get precipitated as unavailable salts.

Catabolism: Degradation of detritus into simpler organic substances by bacterial and fungal enzymes is called catabolism.

Humification: Formation of humus is called humification. Humus is a dark coloured amorphous substance. It is highly resistant to microbial action and undergoes decomposition at an extremely slow rate. Humus serves as a reservoir of nutrients.

Mineralization: Degradation of humus to release inorganic nutrients is called mineralization.

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

Answer: Less than 50% of the incident solar radiation is photosynthetically active radiation (PAR). Plants capture only 2 – 10% of the PAR. All organisms are directly or indirectly dependent for food on producers. So, there is unidirectional flow of energy from the sun to producers and then to consumers. This obeys the First Law of Thermodynamics.

Ecosystems need a constant supply of energy to synthesize the required molecules to counteract the universal tendency towards increasing disorderliness. This obeys the Second Law of Thermodynamics.

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

Answer: Following are the features of sedimentary cycle in an ecosystem:

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

Answer: As per an estimate, 4 × 1013 kg of carbon is fixed in the biosphere through photosynthesis annually.

A considerable amount of carbon returns to the atmosphere as CO2 through respiration by organisms. Substantial amount of CO2 is contributed by decomposers during processing of waste materials. Burning of fossil fuels, wood, forest fire, combustion of organic matter and volcanic activities are other sources of CO2 release into the atmosphere.