You have learnt in Chapter 1 that plants can prepare their own food by the process of photosynthesis but animals cannot. Animals get their food from plants, either directly by eating plants or indirectly by eating animals that eat plants. Some animals eat both plants and animals. Recall that all organisms including humans require food for growth, repair and functioning of the body. Animal nutrition includes nutrient requirement, mode of intake of food and its utilisation in the body.
You have studied in Class VI that food consists of many components. Try to recall and list them below:
The components of food such as carbohydrates are complex substances. These complex substances cannot be utilised as such. So they are broken down into simpler substances. The breakdown of complex components of food into simpler substances is called digestion.
2.1 DIFFERENT WAYS OF TAKING FOOD
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.
Starfish feeds on animals covered by hard shells of calcium carbonate. After opening the shell, thestarfish pops out its stomach through its mouth to eat the soft animal inside the shell. The stomach then goes back into the body and the food is slowly digested.
Fig. 2.1 Starfish
2.2 DIGESTION IN HUMANS
We take in food through the mouth, digest and utilise it. The unused parts of the food are defecated. Have you ever wondered what happens to the food inside the body? The food passes through a continuous canal (Fig. 2.2) which begins at the buccal cavity and ends at the anus. The canal can be divided into various compartments: (1) the buccal cavity, (2) foodpipe or oesophagus, (3) stomach,(4) small intestine, (5) large intestine ending in the rectum and (6) the anus. Is it not a very long path? These parts together form the alimentary canal(digestive tract). The food components gradually get digested as food travels through the various compartments. The inner walls of the stomach and the small intestine, and the various lands such as salivary glands, the liver and the pancreas secrete digestive juices. The digestive juices convert complex
Fig. 2.2 Human digestive system
substances of food into simpler ones. The digestive tract and the associated glands together constitute the digestive system.
Now, let us know what happens to the food in different parts of the digestive tract.
The mouth and buccal cavity
Food is taken into the body through the mouth. The process of taking food into
|Milk teeth and permanent teeth|
Do you remember about falling of your teeth some years ago? The first set of teeth grows during infancy andthey fall off at the age between six to eightyears. These are termed milk teeth. The second set that replaces them are the permanent teeth. The permanent teeth may last throughout life or fall off during old age.
Boojho is fascinated by the highly coiled small intestine seen in Fig. 2.2. He wants to know its length. Would you like to make a wild guess? We have given its approximate length on page 16. Just imagine how such a long structure is accommodated in a small space within our body!
the body is called ingestion. We chew the food with the teeth and break it down mechanically into small pieces. Each tooth is rooted in a separate socket in the gums (Fig. 2.3). Our teeth vary in appearance and perform different functions. Accordingly they are given different names (Fig. 2.3).
Our mouth has the salivary glands which secrete saliva. Do you know the action of saliva on food? Let us find out.
The swallowed food passes into the foodpipe or oesophagus. Look at Fig. 2.2. The foodpipe runs along the neck
Paheli wants to know how food moves in the opposite direction during vomiting.
Fig. 2.7 Movement of the food in the oesophagus of the alimentary canal
and the chest. Food is pushed down by movement of the wall of the foodpipe. Actually this movement takes place throughout the alimentary canal and pushes the food downwards (Fig. 2.7). At times the food is not accepted by our stomach and is vomited out. Recall the instances when you vomited after eating and think of the reason for it. Discuss with your parents and teacher.
The stomach is a thick-walled bag. Its shape is like a flattened U and it is the widest part of the alimentary canal. 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 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. The digestive juices break down the proteins into simpler substances.
The small intestine
The small intestine is highly coiled and is about 7.5 metres long. It receives secretions from the liver and the pancreas. Besides, its wall also secretes juices.
The liver is a reddish brown gland situated in the upper part of the abdomen on the right side. It is the largest gland in the body. It secretes bile juice that is stored in a sac called the gall bladder (Fig. 2.2). The bile plays an important role in the digestion of fats.
The pancreas is a large cream coloured gland located just below the stomach (Fig. 2.2). The pancreatic juice acts on carbohydrates and proteins and changes them into simpler forms. The
The working of the stomach was discovered by a strange accident. In 1822, a man named Alexis St. Martin was badly hit by a shot gun. The bullet had seriously damaged the chest wall and made a hole in his stomach. He was brought to an American army doctor William Beaumont. The doctor saved the patient but he could not close the hole properly (Fig. 2.8). Beaumont took it as a great opportunity to see the inside of the stomach through the hole. He made some wonderful observations.
Beaumont found that the stomach was churning food. Its wall secreted a fluid which could digest the food. He also observed that the end of the stomach opens into the intestine only after the digestion of the food inside the stomach is completed.
Fig. 2.8 Alexis St. Martin’s shotgun wound
partly digested food now reaches the lower part of the small intestine where the intestinal juice completes the digestion of all components of the food. The carbohydrates get broken into simple sugars such as glucose, fats into fatty acids and glycerol, and proteins into amino acids.
Absorption in the small intestine
The digested food can now pass into the blood vessels in the wall of the intestine. This process is called absorption. The inner walls of the small intestine have thousands of finger-like outgrowths. These are called villi (singular villus). Can you guess what the role of villi could be in the intestine? 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. This is called assimilation. In the cells, glucose breaks down with the help of oxygen into carbon dioxide and water, and energy is released. The food that remains undigested and unabsorbed then enters into the large intestine.
The large intestine is wider and shorter than small intestine. It is about 1.5 metre 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.
2.3 DIGESTION IN GRASS-EATING ANIMALS
Have you observed cows, buffaloes and other grass-eating animals chewing continuously even when they are not eating grass? Actually, they quickly swallow the grass and store it in a separate part of the stomach called rumen (Fig. 2.9). Here the food gets
Sometime you may have experienced the need to pass watery stool frequently. This condition is known as diarrhoea. It may be caused by an infection, food poisoning or indigestion. It is very common in India, particularly among children. Under severe conditions it can be fatal. This is because of the excessive loss of water and salts from the body. Diarrhoea should not be neglected. Even before a doctor is consulted the patient should be given plenty of boiled and cooled water with a pinch of salt and sugar dissolved in it. This is called Oral Rehydration Solution (ORS).
Paheli wants to know why these animals cannot chew food properly at the time they take it in?
Boojho wants to know why we cannot digest cellulose like the cattle do.
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.
The grass is rich in cellulose, a type of carbohydrate. Many animals, including humans, cannot digest cellulose.
Fig. 2.9 Digestive system of ruminant
Ruminants have a large sac-like structure between the small intestine and large intestine (Fig. 2.9). The cellulose of the food is digested here by the action of certain bacteria which are not present in humans.
So far you have learnt about animals which possess the digestive system. But there are many small organisms which do not have a mouth and a digestive system. Then, how do they acquire and digest food? In the section below you will learn another interesting way of food intake.
2.4 FEEDING AND DIGESTION IN AMOEBA
Amoeba is a microscopic single-celled organism found in pond water. Amoebahas a cell membrane, a rounded, dense nucleus and many small bubble-like vacuoles (Fig. 2.10) 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.
Amoeba feeds on some microscopic organisms. When it senses food, it
Fig. 2.10 Amoeba
pushes out pseudopodia around the food particle and engulfs it. The food becomes trapped in a food vacuole [Fig. 2.10).
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 absorbed substances are used for growth, maintenance and multiplication. The undigested residue of the food is expelled outside by the vacuole.
The basic process of digestion of food and release of energy is the same in all animals. In a later chapter you will learn about the transport of food absorbed by the intestine to the various parts of the body.
- Amino acid
- Buccal cavity
- Fatty acid
- Food vacuole
- Gall bladder
- Milk teeth
- Permanent teeth
- Salivary glands
Extended Learning — Activities and Projects
- Visit a doctor and find out:
- Under what conditions does a patient need to be on a drip of glucose?
- Till when does a patient need to be given glucose?
- How does glucose help the patient recover?
- Find out what vitamins are and get the following information.
- Why are vitamins necessary in the diet?
- Which fruits or vegetables should be eaten regularly to get vitamins?
- Collect data from your friends, neighbours and classmates to know more about “milk teeth”.
Tabulate your data. One way of doing it is given below:
Find out from at least twenty children and find the average age at which children lose the milk teeth. You may take help of your friends.
S.No. Age at which first tooth fell Age at which last tooth fell No. of teeth lost No. of teeth replaced 1. 2. 3. 4. 5.