Plants make their food by the process of photosynthesis, but animals cannot make their food themselves. Animals get their food from plants. Some animals eat plants directly while some animals eat plant eating animals. Thus, animals get their food from plants either directly or indirectly.
All organisms require food for energy and growth. Requirement of nutrients, mode of intake of food and its utilization in body are collectively known as nutrition.
Steps of Nutrition
Nutrition in complex animals involves following steps:
The intake of food is called ingestion. Method of ingestion, i.e. taking of food, varies from one animal to another. Crocodiles swallow large chunks of food, because crocodiles cannot chew food. Similarly, birds swallow their food in big chunks because they cannot chew. Humans, on the other hand, chew the food to break it into smaller pieces before swallowing it. Lizards use their sticky tongue to catch prey.
The process of breaking down of complex component of food into simpler substances is called digestion. The process of digestion is different in human, grass eating animals, amoeba, etc. Various enzymes help in converting complex food into simple substances.
The process of passing of digested food into blood vessels in the intestine is called the absorption. It is only after absorption that the food goes to different parts of the body for suitable utilisation.
The conversion of absorbed food in complex substances such as proteins and vitamins required by body is called assimilation. In other words, assimilation is the conversion of absorbed food (nutrients) into useful substances for living tissues. Some of these substances are utilised for growth and repair. Carbohydrate is utilised for producing energy.
Removal of waste materials from the body is called egestion. The faecal matter is removed through the anus from time-to-time. Since the waste of food left after digestion is also called faeces, hence the process of egestion is also known as defecation. If we do not clear garbage from our home then it will result in filth all around and will eventually cause diseases. Similarly, if faecal material is not removed from the body, it will make us sick.
Different Ways of Taking Food
Different organism takes food in different ways.
- A humming bird sucks nectar of plants.
- Human beings use their hands to put food into their mouth and swallow the food after chewing.
- Infants of human and many other animals feed upon their mother’s milk by sucking them.
- A snake swallows the animals they prey upon without chewing them.
- A frog captures prey with its sticky tongue.
- An earthworm uses its muscular pharynx to swallow its food.
- Spiders weave sticky web in which small insects get stuck.
- Some aquatic animals filter tiny particles floating nearby and feed upon them.
- Amoeba, a unicellular animal, engulfs tiny particles of food by using pseudopodia. Amoeba surrounds the food by pseudopodia and then makes a food vacuole to engulf the food.
- In multicellular organisms; like hydra there are numerous tentacles around their mouth. Hydra uses tentacles to surround its prey and kill them with its stinging cells. Then the food is pushed inside the body cavity.