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| Nutrition in Animals |
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| All animals are heterotrophic (there are exceptions like Euglena, which has chlorophyll). The different modes of heterotrophic nutrition have already been dealt with in the earlier part of the chapter. Of all the methods of heterotrophism, holozoic is the most commonly found. |
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| Holozoic nutrition involves the following steps: |
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 Ingestion |
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Digestion |
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Absorption |
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Assimilation |
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Egestion |
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| Ingestion |
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| Taking in of food or eating is termed ingestion. The food taken in can be big or small particles. There may be by specialised organs such as mouth in higher animals and through the general body surface with the help of structures such as pseudopodia in lower animals. Ingestion by pseudopodia is called phagocytosis. |
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| Digestion |
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| Breaking down food into simpler forms is called digestion. Digestion is both mechanical and chemical. In mechanical digestion, food is physically broken down into smaller particles by processes such as chewing. Chemical digestion makes use of certain chemicals called enzymes. They are proteins which help in the simplification of food matter. The required enzymes are secreted by the organism itself depending on the type of food to be digested. Digestion converts the food into soluble form. This is done in order to absorb the food into the cells. Foods like glucose and vitamin C, that are already small and water-soluble, need not undergo digestion. They can directly enter the cell. |
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| The digestion may take place outside cells (extracellular) or within the cells (intracellular). In unicellular organisms the digestion is intracellular with the enzymes being present in vesicles. In more advanced multicellular forms, the digestive enzymes are secreted outside into the surrounding medium. The digested products are then absorbed back into the cell. In higher invertebrates and vertebrates, the digestion takes place in a specialised separate canal called the alimentary canal. There are different openings for ingestion and egestion. Each portion of the canal has specific enzymes targeting specific type of food. This increases the efficiency of the digestive system. |
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| Absorption |
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| Taking food in the soluble forms into the tissues. This involves the absorption of food in the soluble form from the region of digestion into the tissues or into where it has to be utilised or into the blood stream which transports it to the different tissues. This takes place through the cell membranes. The absorption may be passive or active. Passive absorption is through diffusion or osmosis without using energy. It takes place in both directions. For example, water is absorbed by osmosis. Active absorption needs energy and can be inhibited by poisons such as cyanide. It takes place only in one direction. For example, absorption of glucose and sodium ions. |
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| Assimilation |
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| Utilisation of the nutrients for the various functions enlisted in the beginning of the chapter. |
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| Egestion |
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| Removal of undigested or waste matter is called egestion. In lower unicellular organisms like the Amoeba, the egestion is also through the general body surface or the cell surface. In multicellular organisms like the Hydra, ingestion and egestion have a common opening. However, in higher animals, the digestive tract is complex and has a separate posterior opening called the anus for egestion. |
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