Nutrient cycle is defined as the circulation of certain nutrient such as carbon, sulphur and water in nature

Carbon Cycle

The carbon cycle is the way carbon (iv) oxide is being used from the atmosphere by photosynthesis and put back into the atmosphere by respiration and decay. The carbon cycle involves the following processes Photosynthesis, Respiration, Putrefaction and Combustion. Only photosynthesis removes carbon (iv) oxide from the atmosphere, the other three processes return carbon (iv) oxide to the atmosphere

  1. Photosynthesis: The air contains carbon (iv) oxide which diffuses into the leaves and it built up into sugar and other complex compound. When the plant is eaten by animal, sugar gets into the cells in the animals body.
  2. Respiration: Within the animal body when the chemical food substance is broken down, it releases carbon (iv) oxide, water and energy. The carbon (iv) oxide is released to the atmosphere.
  3. Putrefaction: When animals and plants die, they decay. In the process of decay, bacteria and other microbes feed on them. The bacteria and other microbes too respire therefore returning C02 back to the atmosphere
  4. Combustion: carbon (iv) oxide is also released into the atmosphere when fuels such as coal, wood, petrol, kerosene are burned
  5. From the sea: Where there is little quantity of carbon (iv) oxide in the atmosphere, more is liberated from the seas. Conversely when there is too much carbon (iv) oxide in the atmosphere, more dissolve in the sea


Importance of Carbon cycle

  1. All living things are made up of carbon component
  2. All food components contain carbon in item

iii.       The process produces carbon (iv) oxide for the use of photosynthesis

  1. The process maintain carbon oxygen balance in the atmosphere



The water cycle is defined as the way water is being used and being returned to the atmosphere in form of water vapour to fall back as rain. The water cycle involves two processes namely condensation and evaporation. When rain falls, some of it is absorbed by plants through their root hairs. After spending sometime in plants, water evaporates from them in the process of transportation. A lot of the water is drunk by animals which they use to release water vapour to the environs during respiration and excretion. Some of the rain falls directly into pond, stream and rivers. Some of the water filters down through the soil and form underground water empties into the oceans. Water returns to the atmosphere by evaporation. When water vapour accumulates into the atmosphere as cloud when it has reached condensation level which may fall once again as rain.

         Importance of water to plant

  1. it cools the plant during the process of transportation
  2. It serves as an agent of weathering for soil formation

iii.       It aids germination of seeds

  1. It helps to dissolve plant nutrient for easy absorption by plant
  2. Water is required for photosynthesis to commence
  3. It gives plant support through rigidity

           Importance of water to animal

  1. It is the main component of animal
  2. It is the solvent for soluble food substance to dissolve during digestion

iii.       It is a medium of transportation for nutrient

  1. It constitutes a large part of the blood
  2. it aids excretion and regulate body temperature
  3. It is the basis for the body secretions such as hormones and enzymes

vii.      It is the medium in which biochemical reactions can take place in living organism.



  1. Name the constituent elements, two food sources and two function of each of the following
  2. Carbohydrate
  3. Protein
  4. Lipid
  5. What is an enzyme?
  6. What are the characteristics of an enzyme?


See also


Organization of Life




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