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Lesson Note
Subject: Biology
Topic: Nutrients Cycling in Nature
Subtopic: Carbon, Oxygen and Water Cycle

Lesson Objective: by the end of the lesson, the learners should be able to:

  1. Describe how carbon circulates in nature;
  2. Draw the carbon cycle in detail;
  3. State why the carbon cycle is necessary for life;
  4. Recognise the delicate balance between carbon and oxygen;
  5. Describe the part played by plants and animals in the water cycle;
  6. Draw the water cycle in detail;
  7. State the importance of water to living organisms.

Lesson Summary/Discussions

Nutrient cycling refers to the circulation of certain nutrients like carbon, Oxygen, water, sulphur,
nitrogen etc in nature.

Definition: Carbon cycle involves the series of processes which contribute to the circulation of carbon in nature.


  1. Carbon dioxide is removed from the air by photosynthesis during which plants use it to manufacture their food.
  2. Carbon is lost in form of carbonates of calcium and magnesium through leaching and drainage.
  3. The atmosphere gains carbon dioxide through: (a) Burning of fuel like coal and wood; (b) The action of volcanoes which releases carbon dioxide; (c) The respiration by plants and animals;
  4. Diffusion of carbon dioxide from seas and other bodies of water acting as reservoir of carbon dioxide.

Carbon occurs in the air or atmosphere as carbon dioxide. It also dissolved in water, seas or oceans or other water bodies and is circulated
continuously through the activities of micro organisms or fungi which decompose the dead organisms. Plants and animals release
carbon dioxide and water during respiration. The carbon dioxide released increases the concentration of carbon in the atmosphere or other water bodies, This is further increased by respiration in plants and animals, combustion of wood, coal, peat, oil and fuel. Some of the excess carbon dioxide is absorbed by the sea
or water bodies and also used by plants in photosynthesis. Organisms in seas, oceans or other water bodies release carbon dioxide into the atmosphere or water.


  1. Plant uses carbon dioxide obtained from the air to manufacture their food during photosynthesis.
  2. It provides carbon which is the major building block of all organic matter.
  3. It helps to purify the atmosphere and maintain atmospheric level of carbon dioxide.
  4. Organic matter which is made from carbon helps to replenish soil nutrient.

Oxygen circle involves the circulation of oxygen between the living organism (plant and animals) and the non-living things like fuels, water etc.
the main bodies involve in oxygen circle include plants, animals, water, fuel during combustion and the atmosphere. Oxygen and water are
released to the atmosphere principally during photosynthesis. On the other hand, oxygen is removed from the atmosphere and water during respiration and decay by animals and plants as well as combustion of fuels.

Importance of Oxygen Cycle

  1. It helps to make carbon dioxide available to plants for photosynthesis.
  2. It ensures the purification of the air to avoid pollution during decay.
  3. It ensures the continuation of life by the provision of oxygen derived from plants.

Oxygen constitutes for 21% of the gases in the atmosphere. Respiration, decay and combustion are the processes which remove oxygen from
the atmosphere while photosynthesis is the process that releases oxygen into the atmosphere.
(a) Human activities such as deforestation (less photosynthetic organisms) leads to the release of less oxygen into the atmosphere while less carbon dioxide is removed from the atmosphere.
(b) Increase in the combustion of fuel,
respiration and decay lead to the
removal of more oxygen from the
atmosphere while more carbon dioxide is added.

As a result of the above activities, oxygen level in atmosphere decreases while carbon dioxide level increases. While a decrease in the atmospheric oxygen level by 2-8 % does not cause any significant effect but a slight increase in the atmospheric carbon dioxide may cause a green house effect, that is, increase in the retention of the sun’s radiant (heat) energy. This could result in a warming of the earth’s atmosphere. In order to prevent this, there is the need to always balance the carbon-oxygen level in the atmosphere.


Definition: Water cycle is defined as the continuous movement of water from the atmosphere to the earth and from the earth to the atmosphere.

The Atmosphere receives water through:

  • Evaporation from oceans and land,
  • Transpiration from plants;
  • Breathing or respiration by plants and animals.

The land receives water through:

  • Rainfall or precipitation.
  • Infiltration and percolation.

Water is very important to living organisms, both plants and animals, in a number of ways:

  1. Water provides the medium for
  2. absorption of dissolved mineral salts by plants in the process of photosynthesis.
  3. It acts as a solvent for soluble food
  4. substances in digestion of food.
  5. It acts as a medium of transport for plant.
  6. Water is an essential raw material in the nutrients.
  7. It constitutes a large portion of the blood.
  8. Water aids excretion of waste products.
  9. Water regulates body temperature.
    It maintains osmotic content of body tissues.
  10. Water is the main component of plant and animal protoplasm.
    Water is required for seed germination.
  11. Water provides a natural habitat for aquatic organisms.

See previous lessons in Biology

Lesson Evaluation/Test

  1. Describe how carbon circulates in nature.
  2. With the aid of diagram, illustrate the carbon cycle in detail.
  3. Why is the carbon cycle necessary for life?
  4. Describe how carbon and oxygen can be balanced.
  5. Draw the water cycle in detail.
  6. What are the importance of water to living organisms?

Questions answered correctly? Bravo!

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