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| Modes of Nutrition |
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| The mechanism by which organisms obtain food are referred to as modes of nutrition. The organisms either synthesize their own food or obtain food prepared by other organisms in various ways. There are basically two modes of nutrition - autotrophic and heterotrophic. |
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| 'Auto' means self and 'trophic' refers to food. So, the organisms which synthesise their own food are called the autotrophs and the process is called autotrophic nutrition. |
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| Autotrophs include all green plants and some bacteria such as the nitrifying bacteria. |
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| The source of energy for the autotrophs may be either light energy or chemical energy. Accordingly they are classified as |
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 Photoautotrophic |
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Chemoautotrophic |
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| Photoautotrophic organisms synthesize food with the help of light energy of the sun, carbon dioxide and water. The process of synthesis of food in this method is called photosynthesis, which will be dealt later in this chapter. |
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| For example: |
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cyanobacteria (blue-green bacteria, prokaryotes) |
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algae and all green plants. |
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| All these use water as the source of hydrogen. |
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| However, there are some other photoautotrophic bacteria, such as the green sulphur bacteria which derives the hydrogen from hydrogen sulphide as follows: |
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| Chemoautotrophic organisms synthesise food with the help of chemical energy. They do not require sunlight. Thus they include organisms which are found deep in the soil where sunlight does not penetrate such as nitrifying bacteria. These organisms use carbon dioxide and water as sources of carbon and hydrogen but obtain energy from chemical compounds. |
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| For example: |
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| Compounds such as ammonia and nitrite are oxidized in order to release energy. |
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| Examples of chemosynthetic bacteria are nitrifying bacteria such as Nitrobacter and Nitrosomonas, hydrogen bacteria and iron bacteria. |
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| 'Hetero' refers to other or different and 'trophic' refers to food. Thus, those organisms which obtain their food from other organisms are called heterotrophic and the process of obtaining the food from other organisms is called heterotrophic nutrition. All the heterotrophs depend directly or indirectly on the autotrophic organisms for their food and energy requirements. Heterotrophs include most of the bacteria, fungi and all animals. |
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| Heterotrophic nutrition is broadly classified as holozoic, saprophytic and parasitic. |
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