AP Board Class 9 Biology Chapter 11 Bio-Geo Chemical Cycles

Chapter 11 is the final chapter given in Andhra Pradesh Board class 9 biology textbook. The chapter deals with the topic of Biogeochemical cycles and here students will develop an understanding of how nature is maintaining itself and what are the impacts of human activities on this self-contained ecological unit. Some of the important topics covered in this unit include;

  • Water Cycle or hydrologic Cycle
  • Nitrogen Cycle
  • Nitrogen Fixation
  • Denitrification
  • Greenhouse Effect
  • Global warming and Ozone Depletion

Biogeochemical Cycles

Biogeochemical cycles are basically representations of biological, geological and chemical processes that involve the movement of an element or compound about the surface of the earth. Biogeochemical cycles are more or less complex in nature and contain a mixture of several elements (oxygen, nitrogen, carbon, water, etc) which are all essential for life.

Water Cycle

Water cycle is the process of continuously circulating water in the atmosphere and the earth. Water cycle is usually carried via several processes of evaporation, condensation, precipitation, and through the transpiration of plants and animals. Water cycle is also known as a hydrologic cycle.

Nitrogen fixation

The conversion of atmospheric nitrogen gas into ammonium and nitrates is known as nitrogen fixation. Fixation occurs mostly due to lightning and bacteria (into nitrates and into ammonium ions )


When bacterias convert compounds of nitrogen in the soil and aquatic systems to the gases, nitrogen (N2 ) and nitrous oxide (N2O) where they are eventually released into the atmosphere, this process is known as denitrification.

Greenhouse Effect

The warming of the Earth’s atmosphere and surface by the atmospheric greenhouse gases such as Carbon dioxide, methane and water vapour is called ‘greenhouse effect’.

Question 1: What is Nitrification?

Answer: Nitrification is the process of the conversion of ammonium to nitrite and nitrate by bacteria.

Question 2: Explain Biological oxygen demand.

Answer: Biological oxygen demand is an indicator of the amount of biodegradable waste in an ecosystem.

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Amphibians are the first vertebrates to possess a three chambered heart.