Class 7 Science Chapter 2 Important Questions

Animals get their food from plants, either directly by eating plants or indirectly by eating animals that eat plants. Animal nutrition includes nutrient requirement, mode of intake of food and its utilisation in the body.

The mode of taking food into the body varies in different organisms. Bees and humming-birds suck the nectar of plants, infants of human and many other animals feed on mother’s milk. Snakes like the python swallow the animals they prey upon. Some aquatic animals filter tiny food particles floating nearby and feed upon them.

The food passes through a continuous canal which begins at the buccal cavity and ends at the anus. The 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.

The human digestive system consists of the alimentary canal and secretory glands. It consists of the (i) buccal cavity, (ii) oesophagus, (iii) stomach, (iv) small intestine, (v) large intestine ending in rectum and (vi) anus. The main digestive glands which secrete digestive juices are (i) the salivary glands, (ii) the liver and (iii) the pancreas.
The inner walls of the stomach and the small intestine, and the various glands associated with the canal such as salivary glands, the liver and the pancreas secrete digestive juices. The digestive juices convert complex substances of food into simpler ones. The digestive tract and the associated glands together constitute the digestive system.

The first set of teeth grows during infancy and they fall off at the age between six to eight years. These are termed milk teeth. The second set that replaces them are the permanent teeth.

Incisors (number 8) for cutting and biting. Canines (number 4) for piercing and tearing. Premolars (number 8) and molars (number 12) for chewing and grinding.

The tongue is a fleshy muscular organ attached at the back to the floor of the buccal cavity. It is free at the front and can be moved in all directions. We use our tongue for talking. Besides, it mixes saliva with the food during chewing and helps in swallowing food. We also taste food with our tongue. It has taste buds that detect different tastes of food.

Normally bacteria are present in our mouth but they are not harmful to us. However, if we do not clean our teeth and mouth after eating, many harmful bacteria also begin to live and grow in it. These bacteria break down the sugars present from the leftover food and release acids. The acids gradually damage the teeth. This is called tooth decay. If it is not treated in time, it causes severe toothache and in extreme cases results in tooth loss.

The swallowed food passes into the food-pipe or oesophagus. The food-pipe runs along the neck and the chest. Food is pushed down by movement of the wall of the food-pipe. Actually, this movement takes place throughout the alimentary canal and pushes the food downwards.

The inner walls of the small intestine have thousands of finger-like outgrowths. These are called villi (singular villus).
The villi increase the surface area for absorption of the digested food. Each villus has a network of thin and small blood vessels close to its surface. The surface of the villi absorbs the digested food materials. The absorbed substances are transported via the blood vessels to different organs of the body where they are used to build complex substances such as the proteins required by the body.

The stomach is a U-shaped, thick-walled bag. The inner lining of the stomach secretes mucous, hydrochloric acid and digestive juices. The mucous protects the lining of the stomach. The acid kills many bacteria that enter along with the food and makes the medium in the stomach acidic and helps the digestive juices to act. The digestive juices break down the proteins into simpler substances.

The faecal matter is removed through the anus from time-to-time. This is called egestion.

Grass-eating animals (cows, buffaloes etc.) chewing continuously even when they are not eating. Actually, they quickly swallow the grass and store it in a part of the stomach called 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. So, these animals are called ruminants.

The cellulose of the food is digested by the action of certain bacteria which are not present in humans.

Amoeba is a microscopic single-celled organism found in pond water. Amoeba has a cell membrane, a rounded, dense 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 or false feet for movement and capture of food.

The inner lining of stomach secretes mucus, hydrochloric acid and digestive juices. Out of these HCL kills any bacteria that may enter the stomach with food and makes the medium acidic.
Mucus protects the stomach lining from HCL and digestive juices break down proteins in food into simpler substances.

Raw leafy vegetables and grass are rich in cellulose, a type of carbohydrate. Many animals, including humans, cannot digest cellulose, because they do not have enzymes and certain types of bacteria which can digest cellulose. We cannot survive by taking vegetables in raw form. That’s why we boil or cook vegetables which breaks down cellulose into simple carbohydrates. We can only survive by taking in vegetables in boiled or cooked form.

Ruminants such as cows and buffaloes swallow their food hurriedly and store it in a part of the stomach called rumen. The cellulose of the food is digested here by the action of certain bacteria which are not present in humans.
Later, this partially digested food (cud) is returned to the buccal cavity of the animals in small lumps and animal it sitting restfully to complete the process of digestion. This process is called rumination.
The thoroughly chewed cud is swallowed again to other compartment of stomach and then into small intestine for complete digestion and absorption.

The salivary gland present in mouth secretes saliva. It contains the enzyme amylase (ptyalin) that breaks down into simple sugars. Only about 30% starch digestion takes place in the mouth.