Nutrition is the process by which food is taken in by living organisms in order to supply the nutrients required for the continuous metabolic reactions going on in the body. Food is the source of nutrients.


Usefulness of Food

Living cells or organisms require food for various reasons. These reasons are:

  1. To provide energy needed for various physical and metabolic activities.
  2. To make essential substances such as hormones and enzymes.
  3. To make new cells for growth and replacement of worn-out tissues.
  4. To supply various substances required for healthy growth and development.


Types of Nutrition

There are two major types of nutrition. These are autotrophic and heterotrophic nutrition.


Autotrophic Nutrition

Autotrophic nutrition is the type of nutrition in which organisms are able to manufacture their own food. Organisms which can manufacture or synthesize their own food are called autotrophs. Two types of autotrophic nutrition are photosynthetic (holophytic) and chemosynthetic nutrition.


Heterotrophic Nutrition

Heterotrophic nutrition is the type of nutrition in which organisms cannot manufacture their own food but depends directly or indirectly on plants (autotrophs) for their own food. Organisms which cannot manufacture their own food are called heterotrophs. Most animals, fungi, protozoa and some bacteria belong to this group.

Forms or types of heterotrophic nutrition include holozoic, parasitic, saprophytic and symbiotic nutrition.


Roles of Enzymes

Enzymes are organic catalysts, usually proteinous in nature, which promote or speed up chemical changes in living cells but are not themselves used up in the process. Enzymes accelerate metabolic reaction without changing their composition in the process.


Mineral Salts

Plant cells require mineral salts or elements obtained from the soil in form of solution for healthy growth and development. The soil is the main source of mineral salts while gaseous elements such as oxygen, hydrogen and carbon are mainly derived from the atmosphere. These elements or plant nutrients are grouped into two classes depending on the quantity that is required by plants.

  • Macro-nutrient or major elements: Macro-nutrients are mineral clements or nutrients required by plants in large quantities for healthy growth and development. Examples are nitrogen, phosphorus, potassium, magnesium, calcium, oxygen, hydrogen, carbon, sulphur and hydrogen.
  • Micro-nutrients or trace elements: Micro-nutrients are mineral clements or nutrients required by plants in small quantities for healthy growth and development. Examples are zinc, copper, boron, molybdenum, cobalt, iron, chlorine and manganese.
See also  PRIMATES


Experiment to Show the Effect of Fertilizers on Bean Seedlings

Plant bean seeds in different boxes. Apply the right quantity of nitrogenous fertilizer to the bean seedlings in box A. Box B is given an excess treatment of the fertilizer and box C has no fertilizer at all, but supplied with water. After about 2-3 weeks, the seedlings are compared. The bean seedlings in box A will grow normally with sturdy stem and rich green leaves. The seedlings in box B will have soft elongated stem and few leaves. The seedlings in box C will have stunted stem and yellow leaves.



Metabolism in living organisms is grouped into two parts. These are anabolism and catabolism.



Anabolism is defined as the building up of complex organic molecules from simple ones in a biological system. In anabolic process, energy is usually consumed because some forms of energy are required first to break the old molecules before building up the complex ones. Common examples of anabolic processes are:

  • The formation of glycogen from glucose.
  • The formation of starch from glucose.
  • The formation of proteins from amino acids.
  • The formation of fats and oil from fatty acids and glycerol.
  • Photosynthesis in green plants.



Catabolism is defined as the breaking down of complex organic molecules into simple substances coupled with the release of energy. Examples of catabolic processes in the body are respiration, fermentation and digestion.


Feeding in Amoeba

Amoeba exhibits holozoic mode of feeding. Although it has no mouth, it feeds by a process called engulfing. Amoeba feeds on other micro-organisms like diatoms, bacteria and flagellates. When Amoeba comes across its food, it produces its pseudopodium and flows around the food particle and gradually encloses (i.e. engulfs) the food along with a drop of water into the body of the animal.


In the process, a food vacuole is formed inside where digestion takes place with the aid of digestive enzymes produced by the protozoan. The digested food diffuses into the protoplasm while the undigested parts of the food are left behind by the cell which simply flows away from it.


Feeding in Paramecium

Paramecium also exhibits holozoic mode of nutrition. It feeds mainly on yeast and other organic food substances. Paramecium feeds bybeating the locomotive organelles (i.e cillia) against the surrounding water. A current of water with food particles are swept into the gullet of the animal. These food particles, e.g. yeast particles are taken to the food vacuoles situated at the end of the gullet. The endoplasm then secretes digestive enzymes on the food. While the digestion of food progresses, the food vacuoles move around the endoplasm. The undigested food is passed out as waste products through the anal pore.


Feeding in Spirogyra

Spirogyra exhibits holophytic or photosynthetic mode of nutrition. In other words, it is capable of manufacturing its own food through the process of photosynthesis. It has chloroplast which enables it, with the aid of sunlight in the presence of carbon dioxide and water to carry out photosynthesis. Through the process of photosynthesis, organic food required by the Spirogyra is synthesized and eventually used by the organism.

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