Introduction to the Topic

In our previous studies, we learned how plants prepare their own food through photosynthesis. But what about animals? Unlike plants, animals cannot make their own food. They depend directly on plants or indirectly on plant-eating animals for their nourishment. This chapter, Nutrition in Animals from Class VII Science, explores how different animals, including humans, acquire food, break it down, and utilize it for energy, growth, and body repair. Animal nutrition includes nutrient requirement, mode of intake of food, and its utilization in the body.

Key Concepts Explained

The breakdown of complex components of food into simpler substances is called digestion. Different animals have fascinating ways of taking in food. For instance, bees and hummingbirds suck the nectar of plants, infants of humans and many other animals feed on mother's milk, and snakes like the python swallow the animals they prey upon. Let us look closely at how digestion works in humans and other animals.

Digestion in Humans

We take in food through the mouth, digest and utilize it. The unused parts of the food are defecated. The food canal (alimentary canal) can be divided into various compartments: (1) the buccal cavity, (2) food pipe or oesophagus, (3) stomach, (4) small intestine, (5) large intestine ending in the rectum, and (6) the anus. These parts together form the digestive tract.

1. The Mouth and Buccal Cavity: Food is taken into the body through the mouth. The process of taking food into the body is called ingestion. We chew the food with teeth and break it down mechanically. Our mouth has salivary glands which secrete saliva. Saliva breaks down starch into sugars.

2. The Food Pipe (Oesophagus): The swallowed food passes into the food pipe. The food pipe runs along the neck and chest. Food is pushed down by movement of the wall of the food pipe, known as peristalsis.

3. The Stomach: The stomach is a thick-walled bag. Its shape is like a flattened J. It receives food from the food pipe at one end and opens into the small intestine at the other. The inner lining of the stomach secretes mucous, hydrochloric acid, and digestive juices. The acid kills many bacteria that enter along with the food and makes the medium in the stomach acidic so that the digestive juices can act.

4. The Small Intestine: The small intestine is highly coiled and is about 7.5 meters long. It receives secretions from the liver and the pancreas. The liver is a reddish brown gland situated in the upper part of the abdomen on the right side. It secretes bile juice that is stored in a sac called the gall bladder. Bile plays an important role in the digestion of fats. The pancreas is a large cream-colored gland located just below the stomach. The pancreatic juice acts on carbohydrates, fats, and proteins and changes them into simpler forms.

5. Absorption in the Small Intestine: The digested food can now pass into the blood vessels in the wall of the small intestine. This process is called absorption. The inner walls of the small intestine have thousands of finger-like outgrowths called villi. Villi increase the surface area for absorption of the digested food.

6. Large Intestine and Egestion: The food that remains undigested and unabsorbed enters the large intestine. It is wider and shorter than the small intestine (about 1.5 meter in length). Its function is to absorb water and some salts from the undigested food material. The remaining waste passes into the rectum and remains there as semi-solid faeces. The faecal matter is removed through the anus from time to time. This is called egestion.

Digestion in Grass-Eating Animals

Have you observed cows, buffaloes and other grass-eating herbivores chewing continuously even when they are not eating? They quickly swallow grass and store it in a part of the stomach called the rumen. Here the food gets partially digested and is called cud. But later the cud returns to the mouth in small lumps and the animal chews it. This process is called rumination and these animals are called ruminants. Grass is rich in cellulose, a type of carbohydrate. Ruminants have a large sac-like structure called rumen between the oesophagus and the small intestine where the cellulose of the food is digested by the action of certain bacteria which are not present in humans.

Feeding and Digestion in Amoeba

Amoeba is a microscopic single-celled organism found in pond water. Amoeba has a cell membrane, a dense, rounded nucleus and many small bubble-like vacuoles in its cytoplasm. Amoeba constantly changes its shape and position. It pushes out one or more finger-like projections, called pseudopodia (false feet) for movement and capture of food. Amoeba senses food, pushes pseudopodia around it, and engulfs it. The food gets trapped in a food vacuole. Digestive juices are secreted into the food vacuole. They act on the food and break it down into simpler substances. Gradually the digested food is absorbed. The undigested residue of the food is expelled outside by the vacuole.

Summary & Key Takeaways

  • Animal nutrition includes nutrient requirement, mode of intake of food and its utilization in the body.
  • The alimentary canal and associated glands together constitute the digestive system.
  • The main steps of digestion in humans are ingestion, digestion, absorption, assimilation, and egestion.
  • Digestion of carbohydrates begins in the buccal cavity, protein digestion starts in the stomach, and the small intestine is the site where digestion of all components of food is completed.
  • Water and some salts are absorbed in the large intestine.
  • Animals like cows and buffaloes are called ruminants. They swallow grass quickly, store it in the rumen, and later bring it back to the mouth to chew it slowly.
  • Amoeba ingests its food with the help of false feet called pseudopodia and digests it in the food vacuole.