Chapter 14 - Natural Resources revision notes

Biogeochemical cycles:

Biogeochemical cycles are repeated circulation of biogeochemical (water, nitrogen, oxygen etc) between abiotic and biotic components of the environment which result in their repeated withdrawal and replacement of their pool.

Water cycle:

  • Water is an essential and a principle component of living beings and also vita for life processes. It forms 60% to 90% of self contained.
  • The whole process in which water evaporates and falls on the land as rain and later flows back into the sea by a river is known as water cycle.
  • All of the water that falls on the land does not immediately flows back into the sea. Some of it seeps into the soil and become part of the underground reservoir of fresh water.
  • Some of these underground water finds its way to the surface through springs or we bring it to the surface for our use through wells and tube wells.
  • Rain may fall directly into the ocean, some water flows into the river and return to the sea.
  • Water is also used by terrestrial animals and plants for various life processes.
  • Plants add water to the air as water vapors by transpiration. Animals return water to the air as vapors by respiration or to the soil as fluid by excretion.
  • So, there is a constant exchange of water between air, land, sea and between living organisms and their environment.
water cycle

Oxygen cycle:

  • Oxygen is an essential component of all biomolecules. It is also required as such by all organisms that respire aerobically.
  • It is found in the elemental form is the atmosphere to the extent of also occurs in combines forms as carbon dioxide, water and it the earth’s crust as oxides of metals, carbonates, sulphate etc.
  • Oxygen from the atmosphere is used up in 3 processes namely combustion, respiration and in the formation of oxides of nitrogen.
  • It return to the atmosphere by only one major process called photosynthesis.
oxygen cycle

Carbon cycle:

  • The carbon is an important constituent of organic compounds found in all living beings in the form of carbohydrates, fats, proteins and nucleic acid.
  • It occurs in the elemental form as diamond and graphite.
  • The endoskeletons of various animals are also formed from carbonates salts. It is found as carbon dioxide in the atmosphere.
  • Carbon is incorporated into lie forms through the basic process of photosynthesis. This process converts carbon dioxide from the atmosphere or dissolved in water into glucose molecule.
  • The process of respiration converts glucose back into carbon dioxide. This carbon dioxide then goes back into atmosphere.
  • Another process that adds to the carbon dioxide in the atmosphere is the process of combustion where fuels are burnt to provide energy for various needs. So, there is a constant cycling of carbon in the environment by various living and non-living beings.

Nitrogen cycle:

Importance of nitrogen:

  • Nitrogen gas make up 78% of our atmosphere and nitrogen is also a part of many molecules essential to life like proteins, nucleic acids and some vitamins
  • .
  • Nitrogen is found in other biologically important compounds such as alkaloids and urea. So, nitrogen is an essential nutrient for all life forms.


  • Other than few forms of bacteria life forms are not able to convert the comparatively inert nitrogen molecule into forms like nitrates and nitrites which can be taken up and used to make the require molecules.
  • The conversion of inert nitrogen to nitrates and nitrites can be done either by industrial nitrogen fixation or by some nitrogen fixing bacteria such as Azotobacter (occurs freely in soil) and Rhizobium (occurs in root nodules of leguminous plants as pea etc). The process of biofixation of nitrogen is called nitrogen fixation.
  • In a physical process such as during lightning, the high temperature and pressure created in the air convert nitrogen into oxide of nitrogen. These oxides dissolve in water to give nitric and nitrous acids and fall on land along with rain (acid rain). These are then utilized by various life forms.
  • Plant generally take up nitrates and nitrites and convert them into amino acids which are used to make proteins. These proteins and other complex compounds are subsequently consumed by animals. Once the animals or plants dies, other bacteria in the soil convert the various compounds of nitrogen back into nitrates and nitrites. A different type of nitrogen releasing bacteria converts the nitrates and nitrites into elemental nitrogen.
  • So, there is a nitrogen cycle in nature which maintains the overall amount of nitrogen constant in atmosphere, soil and water.

Green House Effect:

  • A building mostly made of glass for growing or sheltering the delicate plants is called green house.
  • In green house, the heat is trapped by glass; hence the temperature in a glass enclosure is much higher than the surrounding. Due to higher temperature in green house, the tropical plants can be kept warm protected from cold temperature during winter.
  • Some gases like carbon dioxide, methane and nitrous oxide (green house gases) also prevent the escape of heat from the earth. An increase in the percentage of such gases in the atmosphere would cause the average temperature to increase worldwide and this is called green house effect. It leads to global warming

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Class 9 Maths Class 9 Science
Reference Books for class 9 science

Given below are the links of some of the reference books for class 9 science.

These books by S.Chand Publications are detailed in their content and are must have books for class 9 students.
  1. Science for Ninth Class Part1 Physics
  2. Science for Ninth Class Part2 Chemistry
  3. Science for Ninth Class Part3 Biology
Some other books that might be worth considering are
  1. Oswaal CBSE Question Bank Class 9 Science Chapterwise & Topicwise We all know importance of practicing what we have studied. This book is perfect for practicing what you have learned and studied in the subject.
  2. Pearson Foundation Series (IIT-JEE/NEET) Physics, Chemistry, Maths & Biology for Class 9 (Main Books) | PCMB Combo These Foundation books would be helpful for students who want to prepare for JEE/NEET exams. Only buy them if you are up for some challenge and have time to study extra topics. These might be tough for you and you might need extra help in studying these books.
  3. Foundation Science Physics for Class - 9 by H.C. Verma This is one of my favorite Physics book for class 9. Most of the book is within the limits of CBSE syllabus. It might overwhelm you with its language but I feel if you can understand the content is authentic with plenty of problems to solve.

You can use above books for extra knowledge and practicing different questions.

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