Science: Life Processes-1: Nutrition, Transportation, Respiration and Excretion (For CBSE, ICSE, IAS, NET, NRA 2022)

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We Need Food Because

  • provide energy to carry out life processes, such as respiration, digestion, excretion etc.
  • help in growth of the body and repair of worn-out and damaged cells and tissues.
  • help in the production of enzymes and hormones in the body.


Nutrition is defined as a process by which living beings obtain food, change food into simple absorbable forms and use it to make substances needed by the body.

Types of Nutrition

Autotrophic Nutrition

The green plants, algae and certain bacteria manufacture their own food through photosynthesis. They are termed autotrophs and their mode of nutrition autotrophic nutrition. They are the producers of the food chain as all organisms depend for food on them.

Heterotrophic Nutrition

The organisms, which depend on other organisms for their food, are called heterotrophs and their mode of nutrition is heterotrophic nutrition.

Holozoic Nutrition

Holozoic nutrition includes ingestion, digestion and absorption of food as in Amoeba, frogs and human beings.

Parasitic Nutrition

The organisms that live on or inside other living organisms and derive their food from them are called parasites and the nutrition is called parasitic nutrition. Cuscuta or Dodder plant (Amar bel) is a parasite on green plants.

Saprotrophic Nutrition

  • Organisms that derive their food from dead and decaying organisms are called saprotrophs. Saprotrophs help in cleaning the environment by decomposing the dead and decaying organic matter.
  • E. g. You must have seen a white cottony growth developing on your wet leather shoes or belts especially when they get wet during rainy days. This is a fungus. The fungus grows and feeds on substances, which were once part of the living organisms, such as stored food, wood, leather and rotten plant products. Some common examples are mushrooms, bread mold, yeast, etc.


  • Photosynthesis is ‘a biochemical process by which green plants manufacture their own food using carbon dioxide and water as raw materials in the presence of sunlight and chlorophyll’ . Oxygen is released as a by-product in this process.
  • Photosynthesis is the only process by which solar (sun՚s) energy is converted into chemical energy. The overall equation of photosynthesis is given here.

6CO2 + 12H2O ⇾ C6H12O6 + 6H2O + 6O2

Carbon dioxide Water Glucose Water Oxygen

Photosynthesis Biochemical Process

Raw Materials for Photosynthesis


  • The green color of plants is due to chlorophyll. Chlorophyll is in the chloroplasts. It can trap light.
  • To carry out photosynthesis, plants require as raw materials, carbon dioxide (CO2) , water (H2O) , light and chlorophyll. Light gives energy for photosynthesis. Photosynthesis takes place in chloroplasts in the cells of leaves.


Sunlight is absorbed by chlorophyll as solar energy.

Carbon Dioxide and Water

Carbon dioxide and water are combined in the chloroplast with the help of a number of enzymes to yield sugar which is converted into starch. Oxygen formed during photosynthesis diffuses out into the atmosphere through the stomata.

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