All organism need energy to perform various processes of their lives. Energy is necessary for them whether they are working or at rest. We require energy even during sleep because a number of biological activities occur continuously in our body. For example, respiration and beating of heart keeps on going always without any break. All these processes of living beings need a continuous flow of energy. Food is the source of this energy. It is released by the breakdown of food taken by the organism. In the forthcoming text, we study various modes of intake of food and nutrition.


The very word nutrition means the process of taking in and utilizing the various nutrients by an organism. Nutrients are substance that provide energy to the organism. They also help in the biosynthesis of various consituents of the body. These are obtained by organisms from their surroundings. Food an important source of various nutrients required for growth, maintenance and repair of the body. It is also required for building the resistance in the body again various diseases and foregin pathogens. Thus, nutrition is a process by which organism obtains  substances which are utilized for their growth, maintenance, reproduction and to counteract various diseases. Substances used for nutrition are called nutrients.

Modes of nutrition

Organism have different modes of nutrition. They are classified into two major groups on the basis of mode of their taking food.

1.      Autotrophic nutrition: ‘Auto’ means self and ‘trophein’ means to nourish. In this mode of nutrition, organism synthesize their own food in their own bodies from simpler inorganic substances. Such organism are known as autotrophs. Autotrophic nutrition is further classified into two types depending on the source of energy used.

(i)                 Photoautotrophic nutrition: All green plants, certain protists and photosynthesis bacteria have this mode of nutrition. They can directly use the solar energy in the presence of chlorophyll to make their organic food from simpler inorganic substances,i.e., water and carbon dioxide. The green pigment present in them is oftrapping the solar energy . This energy  is used for synthesis of food  from raw materials- carbon dioxide, water and some minerals. These organisms are known as photoautotrophs.

(ii)               Chemoautotrophic nutrition or chemosynthesis: certain bacteria can make organic food from simpler organic substances without using an solar energy. They use energy produced by the breakdown of these inorganic substances. This process is known as chemosynthesis and their mode of nutrition as chemoautotrophic nutrition. These organism are known as chemoautotrophs. For examples, sulphur bacteria and iron bacteria.

2.      Heterotrophic nutrition: ‘Hetro’ a greek word means different. In this mode of nutrition, the nutritions are taken from others. All, animals, fungi, many bacteria and some non-organic plants cannot make their own food. Such organisms are known as heterotrophs. Thus, heterotrophs nutrition may be defined as a type nutrition in which energy is obtained from the digestion of organic matter, of plant and animal source. In this mode of nutrition, first organic is digestion of organic matter, of plants and animals sources.

Different types of hetrotrophic nutrition or Saprotrophic nutrition.

(i)                 Saprophytic nutrition or saprotrophic nutrition

(ii)               Parasitic nutrition.

(iii)             Holozoic nutrition.

(i)                 Saprophytic nutrition or saprotrophic nutrition : The word ‘sapro’ means rotten. In this mode of nutrition, organism obtain nutrition fronm dead decaying organic matter. They secrete digestive juices to the surrounding medium. The enzymes present in these juices convert the non-diffusable food in diffusible form. The it is absorbed by them through their body surface. For example fungi and bacteria. These organism are called saprophaytes.

(ii)               Parasitic nutrition : The word ‘para’ means other. This mode of nutrition is to take food from others. The organism that takes food is known as parasite and the organism from whom food is taken is known as host. Thus the organism show a host-parasite relationship in this type of nutrition. The parasite lives in an intimate association with the host and derives essential nutritions from it. But the host does not get any benefit in return from the parasite. The host may be an animal or a plant. The parasite may live outside the host body. Such a parasite is known as Ectoparasite. The parasite which live inside the host body is known as Endoparasite. Parasite mode of nutrition is found in bacteria, fungi, some animals like round warm and plasmodium. A few higher plants like Cuscuta and Viscum also show parasite mode of nutrition.

(iii)             Holozoic nutrition: most of the animals takes solid or liquid food by process of ingestion. Holozoic mode of nutrition can be defined as the taking in complex oganic matter by ingestion. The food is then digested and absorbed in the body. For example : Amoeba, frog, rabbit, loin and human beings. Depending upon the source of food taken, holozoic nutrition is further of following types.

(a)    Herbivorous.

(b)   Carnivorous.

(c)    Omnivorous.

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