Biology Class 12 NCERT Solutions: Chapter 14 Ecosystem Part 4 (For CBSE, ICSE, IAS, NET, NRA 2023)

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


  • It is defined as the amount of organic matter or biomass produced by producers per unit area over a period of time.
  • Primary productivity of an ecosystem depends on the variety of environmental factors such as light, temperature, water, precipitation, etc.
  • It also depends on the availability of nutrients and the availability of plants to carry out photosynthesis.

Q. 10 Define decomposition and describe the processes and products of decomposition.


  • Decomposition is the process that involves the breakdown of complex organic matter or biomass from the body of dead plants and animals with the help of decomposers into inorganic raw materials such as carbon dioxide, water, and other nutrients.
  • The various processes involved in decomposition are as follows:
  1. Fragmentation:
  2. It is the first step in the process of decomposition. It involves the breakdown of detritus into smaller pieces by the action of detritivores such as earthworms.
  3. Leaching:
  4. It is a process where the water soluble nutrients go down into the soil layers and get locked as unavailable salts.
  5. Catabolism:
  6. It is a process in which bacteria and fungi degrade detritus through various enzymes into smaller pieces.
  7. Humification:
  8. The next step is humification which leads to the formation of a darkcoloured colloidal substance called humus, which acts as reservoir of nutrients for plants.
  9. Mineralization:
  10. The humus is further degraded by the action of microbes, which finally leads to the release of inorganic nutrients into the soil. This process of releasing inorganic nutrients from the humus is known as mineralization.
  11. Decomposition produces a dark coloured, nutrient-rich substance called humus. Humus finally degrades and releases inorganic raw materials such as , water, and other nutrient in the soil.

Q. 11 Give an account of energy flow in an ecosystem.


  • Energy enters an ecosystem from the Sun. Solar radiations pass through the atmosphere and are absorbed by the Earth՚s surface. These radiations help plants in carrying out the process of photosynthesis.
  • Also, they help maintain the Earth՚s temperature for the survival of living organisms. Some solar radiations are reflected by the Earth՚s surface.
  • Only 2 - 10 percent of solar energy is captured by green plants (producers) during photosynthesis to be converted into food. The rate at which the biomass is produced by plants during photosynthesis is termed as ‘gross primary productivity’ .
  • When these green plants are consumed by herbivores, only 10 % of the stored energy from producers is transferred to herbivores. The remaining 90 % of this energy is used by plants for various processes such as respiration, growth, and reproduction.
  • Similarly, only 10 % of the energy of herbivores is transferred to carnivores. This is known as ten percent law of energy flow.
Energy Flow at Tropic Levels