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| Holozoic Nutrition |
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| Holozoic animals are boradly classified into three categories based on their food habits as |
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| Herbivores |
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| Those which consume algae or plant materials. |
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| Examples: Cow, horse and rabbit. |
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| Carnivores |
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| Flesh eating animals. For e.g., cockroach, rat, bear, crow and man. |
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| Holozoic animals can also be sub-classified into various categories such as: |
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| Insectivores |
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| Insect eaters such as toads, lizards and spiny ant eaters. |
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| Frugivores |
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| They are the fruit eaters |
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| Examples: Monkeys, birds and bats. |
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| Sanguinivores |
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| They feed on the blood of the vertebrates. |
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| Examples: Female(anopheles), mosquito, leeches and vampire bats. |
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| Detritivores |
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| They feed on decaying organic matter |
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| Example: Earthworm |
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| (a) |
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| (b) |
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| Complete digestive tracts in (a) Earthworm and (b) Salmander |
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| Fluid feeders |
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| The fluid feeders feed on fluids from plants. |
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| Examples: Butterflies and male mosquitoes. |
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| All animals are heterotrophic (there are exceptions like Euglena, which has chlorophyll and can thus manufacture its own food). 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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| Taking in of food or eating. The food is taken in as big or small particles. This may be by specialised organs such as mouth in higher animals or through the general body surface with the help of structures such as pseudopodia in lower organisms (like amoeba). Ingestion by pseudopodia is called phagocytosis. |
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| Digestion |
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| Digestion is defined as the process by which complex food molecules are broken down into simpler molecules, so that they can be absorbed by the body. Digestion is 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. |
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| The enzymes break the covalent bonds in the food molecules and release energy. This reaction is chemically called hydrolysis - breakdown of a bond by the addition of water molecule. The enzymes that catalyze these reactions are therefore, called hydrolases. Digestion converts the food into soluble form. This is done in order to absorb the food into the cells. Food 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. In lower organisms like the Hydra, the ingestion and the egestion takes place through the same opening. Features like ingestion and egestion having different openings and each portion of the canal having specific enzymes targeting specific type of food increase the efficiency of the digestive system. |
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| Absorption |
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| 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 e.g., 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. |
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| For e.g., absorption of glucose and sodium ions. |
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| Assimilation |
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| It involves the Utilisation of the nutrients for the various functions of the body. |
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| Egestion |
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| The removal of undigested food or waste matter from the body is called egestion. In lower unicellular organisms like the Amoeba, egestion is 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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| Longitudinal section of Hydra showing gastrovascular cavity |
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