Nutrition is the process of taking food and using it for obtaining energy, growth and repair of the body. Animals depend on other organisms for getting their food. They cannot make their own food, so they are heterotrophs.
Animals Obtain their Food from Plants and other Animals
Animals need readymade food and therefore they depend on either plants or other animals which they eat. For example, snake eats frogs, insects eat dead bodies of animals, birds eat worms and insects etc.
Modes of Nutrition
The method of obtaining food by an organism is called modes of nutrition. There are two modes of nutrition. They are:
Heterotrophic Mode of Nutrition
All the animals cannot make their own food from simple inorganic material like carbon dioxide and water. They depend on other organisms for food. This is called heterotrophic mode of nutrition. The organisms which depend on other organisms for food are called heterotrophs. For example, man, dog, cat, deer, tiger, cow, non-green plants like yeast are all heterotrophs. They depend on plants or other organisms for their food.
Types of Heterotrophic Nutrition
There are three types of heterotrophic mode of nutrition. They are:
i) Saprotrophic nutrition
ii) Parasitic nutrition
iii) Holozoic nutrition
Sapro means rotten. There are organisms which feed on dead and decaying organic matter for obtaining their food. These organisms feed on rotting wood of dead and decaying trees, rotten leaves, dead animals, rotten bread etc. Such organisms are called saprophytes.
Fungi and many bacteria are saprophytes. These saprophytes break down the complex organic matter from the dead and decaying organic matter into simpler substances outside their body. These simpler substances are then absorbed by saprophytes.
It is that mode of nutrition in which organisms feed on other living organisms, called their host, without killing them. The organisms which obtain food in a manner are called parasites.
Parasites harm the host, which may be a plant or an animal. Parasites cause diseases to mankind, domestic animals, and crops. Fungi, bacteria, a few plants like cuscuta and some animals like plasmodium and roundworms undergo parasitic mode of nutrition.
Holozoic is a mode of nutrition in which organisms eat solid food. The food may be a plant product or animal product. In this process, an organism ingests the complex organic food material into its body and then digests the food which is then absorbed into the body cells. The unabsorbed food is thrown out of the body of the organisms by the process of egestion. Man, cat, dog, bear, giraffe, fog, fish, etc., have holozoic mode of nutrition.
On the basis of food habits animals can be divided into three groups. They are:
Herbivores are those animals which eat only plants like grass, leaves, fruits, bark etc. The examples of herbivorous animals are cow, goat, sheep, horse, camel, deer, etc.
Those animals which eat only other animals are called carnivores. They do not eat plants. Lion, tiger, frog, vulture, wolf, lizard, etc., are examples of carnivorous animals. Carnivores are meat eaters.
Those animals which eat both plants and flesh of other animals are called omnivores. Dog, crow, sparrow, bear, ant, etc., are examples of omnivorous animals.
This figure shows how all living things depend on sun for their food.
Different Steps in the Process of Nutrition in Animals
There are five steps in the process of nutrition in animals. They are:
Ingestion means eating food so as to get energy and carry on the activities of life. Ingestion is the process of taking food into the body.
Food that animals eat consists of large insoluble molecule which cannot be absorbed by their body. So the process which breaks down these insoluble food molecules into small, water soluble molecules is called digestion.
Animals use physical and chemical methods for digestion. Physical method means chewing and grinding food in mouth and chemical method means addition of digestive juices to food by the body. After digestion food can be utilised by the animal body.
After the food molecules become small they pass through the walls of intestine and go in to blood. This process is called absorption.
The absorbed food is now carried to all parts of the body and to each and every cell so as to produce energy and to make materials for the growth and repair of the body. This process is called assimilation.
The process through which undigested food is moved out of the body is called egestion.
In unicellular organisms the process of nutrition is performed by the single cell only.
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