ncert-textbook-for-class-7-science-chapter-2 (1).pdf

2 Nutrition in Animals Y ou have learnt in Chapter 1 that food into simpler substances is called plants can prepare their own food digestion. by the process of photosynthesis 2.1 DIFFERENT WA...

2 Nutrition in Animals Y ou have learnt in Chapter 1 that food into simpler substances is called plants can prepare their own food digestion. by the process of photosynthesis 2.1 DIFFERENT WAYS OF TAKING FOOD but animals cannot. Animals get their The mode of taking food into the body food from plants, either directly by varies in different organisms. Bees and eating plants or indirectly by eating humming-birds suck the nectar of animals that eat plants. Some animals plants, infants of human and many eat both plants and animals. Recall that other animals feed on mother’s milk. all organisms including humans require Snakes like the python swallow the food for growth, repair and functioning animals they prey upon. Some aquatic of the body. Animal nutrition includes animals filter tiny food particles floating nutrient requirement, mode of intake nearby and feed upon them. of food and its utilisation in the body. You have studied in Class VI that food Activity 2.1 consists of many components. Try to What is the type of food and mode of recall and list them below: feeding of the following animals? Write 1. ______________________ down your observations in the given 2. ______________________ Table. You may find the list of modes of feeding given below the Table helpful. 3. ______________________ Table 2.1 Various modes of feeding 4. ______________________ 5. ______________________ Name of Kind of Mode of animal food feeding 6. ______________________ Snail The components of food such as Ant carbohydrates are complex substances. Eagle These complex substances cannot be Humming-bird utilised as such. So they are broken Lice down into simpler substances. The Mosquito breakdown of complex components of Butterfly House fly (Scraping, chewing, siphoning, capturing Complex substance Simpler substances and swallowing, sponging, sucking etc.) Rationalised 2023-24  Amazing fact of the stomach and the small intestine, and the various glands associated with the Starfish feeds on animals covered by canal such as salivary glands, the liver hard shells of calcium carbonate. and the pancreas secrete digestive juices. After opening the shell, the starfish The digestive juices convert complex 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. Buccal cavity Salivary gland Oesophagus Liver Fig. 2.1 Starfish Stomach Gall bladder 2.2 DIGESTION IN HUMANS Pancreas We take in food through the mouth, Small digest and utilise it. The unused parts intestine of the food are defecated. Have you ever Large wondered what happens to the food intestine inside the body? The food passes Rectum through a continuous canal (Fig. 2.2) Anus which begins at the buccal cavity and Fig. 2.2 Human digestive system ends at the anus. The canal can be divided into various compartments: substances of food into simpler ones. The (1) the buccal cavity, (2) foodpipe or digestive tract and the associated glands oesophagus, (3) stomach, (4) small together constitute the digestive system. intestine, (5) large intestine ending in Now, let us know what happens to the rectum and (6) the anus. Is it not a the food in different parts of the digestive very long path? These parts together tract. form the alimentary canal (digestive tract). The food components gradually The mouth and buccal cavity get digested as food travels through the Food is taken into the body through the various compartments. The inner walls mouth. The process of taking food into 12 SCIENCE Rationalised 2023-24  Milk teeth and permanent teeth Do you remember about falling of your teeth some years ago? 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. The permanent teeth may last throughout life or fall off during old age or due to some dental disease. Molar Boojho is fascinated by the Premolar highly coiled small intestine seen in Fig. 2.2. He wants to know its length. Would you like to make a Canine wild guess? We have given its approximate length on page 16. Just imagine how such a long Incisor structure is accommodated in a small space within our body! the body is called ingestion. We chew Fig. 2.3 Arrangement of teeth and different the food with the teeth and break it type of teeth down mechanically into small pieces. Each tooth is rooted in a separate socket which ones for piercing and tearing? in the gums (Fig. 2.3). Our teeth vary in Also find out the ones that are used for appearance and perform different chewing and grinding? functions. Accordingly they are given Record your observations in Table 2.2 different names (Fig. 2.3). Table 2.2 Activity 2.2 Type of teeth Number of teeth Total Wash your hands. Look into the Lower jaw Upper jaw mirror and count your teeth. Use Cutting and your index finger to feel the teeth. biting teeth How many kinds of teeth could you Piercing and find? Take a piece of an apple or tearing teeth bread and eat it. Which teeth do Chewing and you use for biting and cutting, and grinding teeth NUTRITION IN ANIMALS 13 Rationalised 2023-24  Our mouth has the salivary glands of boiled rice; in test tube ‘B’ keep one which secrete saliva. Do you know the teaspoonful of boiled rice after chewing action of saliva on food? Let us find out. it for 3 to 5 minutes. Add 3–4 mL of Activity 2.3 water in both the test tubes (Fig. 2.4). Now pour 2–3 drops of iodine solution Take two test tubes. Label them ‘A’ and in each test tube and observe. Why is ‘B’. In test tube ‘A’ put one teaspoonful there a change in colour in the test tubes? Discuss the results with your classmates and your teacher. The saliva breaks down the starch into sugars. 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. Do Iodine solution you know the functions of the tongue? We use our tongue for talking. Besides, Water it mixes saliva with the food during Boiled rice chewing and helps in swallowing food. We also taste food with our tongue. It Boiled and chewed rice A B has taste buds that detect different Fig. 2.4 Effect of saliva on starch tastes of food. We can find out the Sweets and tooth decay 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, (a) 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 (see Chapter 4 to know what an acid is). The acids gradually damage the teeth (Fig. 2.5). This is called tooth decay. If it is not (b) treated in time, it causes severe toothache and in extreme cases results in tooth loss. Chocolates, sweets, soft drinks and other sugar products are the major culprits of tooth decay. Therefore, one should clean the teeth with a brush or datun and (c) dental floss (a special strong thread which is moved between two teeth to take out trapped food particles) at least twice a day and rinse the mouth after every meal. Also, one should not put dirty fingers or any unwashed object in the mouth. (d) Fig. 2.5 Gradual decay of tooth 14 SCIENCE Rationalised 2023-24  Sometimes when you eat in a hurry, talk or laugh while eating, you may cough, get hiccups or a choking sensation. This happens when food particles enter the windpipe. The windpipe carries air from the nostrils to the lungs. It runs adjacent to the foodpipe. But inside the throat, air and food share a common passage. Then how is food prevented from entering the windpipe? During the act of swallowing a flap-like valve closes the passage of the windpipe and guides the food into the foodpipe. If, by chance, food particles enter the windpipe, we feel choked, get hiccups or cough. 5. Now write down your observations and label Fig. 2.6. Repeat this activity with other classmates. Fig. 2.6 Regions of the The foodpipe/oesophagus tongue for different tastes The swallowed food passes into the foodpipe or oesophagus. Look at Fig. 2.2. The foodpipe runs along the neck position of taste buds by the following activity. Paheli wants to know how Activity 2.4 food moves in the opposite direction during vomiting. 1. Prepare a separate sample each of (i) sugar solution, (ii) common salt solution, (iii) lemon juice and (iv) juice of crushed neem leaf or bitter gourd. Food 2. Blindfold one of your classmates and ask her/him to take out the tongue and keep it in straight and flat position. Oesophagus 3. Use a clean toothpick to put the above samples one by one on different areas of the tongue as shown in Fig. 2.6. Use a new Stomach toothpick for each sample. 4. Ask the classmate which areas of the tongue could detect the sweet, salty, Fig. 2.7 Movement of the food in the oesophagus sour and bitter substances. of the alimentary canal NUTRITION IN ANIMALS 15 Rationalised 2023-24 and the chest. Food is pushed down by food and makes the medium in the movement of the wall of the foodpipe. stomach acidic and helps the digestive Actually this movement takes place juices to act. The digestive juices break throughout the alimentary canal and down the proteins into simpler pushes the food downwards (Fig. 2.7). substances. At times the food is not accepted by our The small intestine stomach and is vomited out. Recall the instances when you vomited after eating The small intestine is highly coiled and and think of the reason for it. Discuss is about 7.5 metres long. It receives with your parents and teacher. secretions from the liver and the pancreas. Besides, its wall also secretes juices. The stomach The liver is a reddish brown gland The stomach is a thick-walled bag. Its situated in the upper part of the shape is like a flattened J and it is the abdomen on the right side. It is the widest part of the alimentary canal. It largest gland in the body. It secretes bile receives food from the food pipe at one juice that is stored in a sac called the end and opens into the small intestine gall bladder (Fig. 2.2). The bile plays at the other. an important role in the digestion of fats. The inner lining of the stomach The pancreas is a large cream secretes mucous, hydrochloric acid and coloured gland located just below the digestive juices. The mucous protects stomach (Fig. 2.2). The pancreatic juice the lining of the stomach. The acid kills acts on carbohydrates, fats and proteins many bacteria that enter along with the and changes them into simpler forms. 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 and left it bandaged (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 Fig. 2.8 Alexis St. Martin’s shotgun wound 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. 16 SCIENCE Rationalised 2023-24 The partly digested food now reaches the proteins required by the body. This is lower part of the small intestine where called assimilation. In the cells, glucose the intestinal juice completes the breaks down with the help of oxygen digestion of all components of the food. into carbon dioxide and water, and The carbohydrates get broken into energy is released. The food that simple sugars such as glucose, fats into remains undigested and unabsorbed fatty acids and glycerol, and proteins enters into the large intestine. into amino acids. Large intestine Absorption in the small The large intestine is wider and shorter intestine than small intestine. It is about 1.5 metre The digested food can now pass into the in length. Its function is to absorb water blood vessels in the wall of the intestine. and some salts from the undigested food This process is called absorption. The material. The remaining waste passes inner walls of the small intestine have into the rectum and remains there as thousands of finger-like outgrowths. semi-solid faeces. The faecal matter is These are called villi (singular villus). removed through the anus from Can you guess what the role of villi could time-to-time. This is called egestion. be in the intestine? The villi increase the surface area for absorption of the 2.3 DIGESTION IN GRASS-EATING digested food. Each villus has a network ANIMALS of thin and small blood vessels close to Have you observed cows, buffaloes its surface. The surface of the villi and other grass-eating animals absorbs the digested food materials. The chewing continuously even when absorbed substances are transported they are not eating? Actually, they via the blood vessels to different organs quickly swallow the grass and store of the body where they are used to build it in a part of the stomach called complex substances such as the rumen (Fig. 2.9). Here the food gets Diarrhoea 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). NUTRITION IN ANIMALS 17 Rationalised 2023-24  helps in digestion of cellulose. Many animals, including humans, cannot Paheli wants to know why digest cellulose. these animals cannot chew Animals like horses, rabbit, etc., have food properly at the time they a large sac-like structure called Caecum take it in? between the oesophagus and the small intestine (Fig. 2.9). The cellulose of the food is digested here by the action of certain bacteria which are not present in Boojho wants to know why humans. we cannot digest cellulose So far you have learnt about animals like the cattle do. 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 partially digested and is called cud. food? In the section below you will learn But later the cud returns to the mouth another interesting way of food intake. in small lumps and the animal chews it. This process is called rumination and 2.4 FEEDING AND DIGESTION IN these animals are called ruminants. AMOEBA The grass is rich in cellulose, a type Amoeba is a microscopic single-celled of carbohydrate. In ruminants like cattle, organism found in pond water. Amoeba deer, etc., bacteria present in rumen has a cell membrane, a rounded, dense nucleus and many small bubble-like Small Intestine Oesophagus vacuoles (Fig. 2.10) in its cytoplasm. Rumen 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. Nucleus Pseudopodium Food particle (ingestion) Food vacuole Stomach Large Intestine Caecum Egested waste Fig. 2.9 Digestive system of ruminant Fig. 2.10 Amoeba 18 SCIENCE Rationalised 2023-24  Amoeba feeds on some microscopic The absorbed substances are used for organisms. When it senses food, it growth, maintenance and multiplication. pushes out pseudopodia around the The undigested residue of the food is food particle and engulfs it. The food expelled outside by the vacuole. becomes trapped in a food vacuole The basic process of digestion of [Fig. 2.10). food and release of energy is the same Digestive juices are secreted into the in all animals. In a later chapter you food vacuole. They act on the food and will learn about the transport of food break it down into simpler substances. absorbed by the intestine to the various Gradually the digested food is absorbed. parts of the body. Keywords Absorption Fatty acid Oesophagus Amino acid Food vacuole Pancreas Amoeba Gall bladder Premolar Assimilation Glycerol Pseudopodia Bile Incisor Rumen Buccal cavity Ingestion Ruminant Canine Liver Rumination Cellulose Milk teeth Salivary glands Digestion Molar Villi Egestion Permanent teeth Saliva What you have learnt Animal nutrition includes nutrient requirement, mode of intake of food and its utilisation in the body. 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 stomach wall and the wall of the small intestine also secrete digestive juices. The modes of feeding vary in different organisms. Nutrition is a complex process involving: (i) ingestion, (ii) digestion, (iii) absorption, (iv) assimilation and (v) egestion. NUTRITION IN ANIMALS 19 Rationalised 2023-24  Digestion of carbohydrates, like starch, begins in the buccal cavity. The digestion of protein starts in the stomach. The bile secreted from the liver, the pancreatic juice from the pancreas and the digestive juice from the intestinal wall complete the digestion of all components of food in the small intestine. The digested food is absorbed in the blood vessels from the small intestine. The absorbed substances are transported to different parts of the body. Water and some salts are absorbed from the undigested food in the large intestine. The undigested and unabsorbed residues are expelled out of the body as faeces through the anus. The grazing animals like cows, buffaloes and deer are known as ruminants. They quickly ingest, swallow their leafy food and store it in the rumen. Later, the food returns to the mouth and the animal chews it peacefully. Amoeba ingests its food with the help of its false feet or pseudopodia. The food is digested in the food vacuole. Exercises 1. Fill in the blanks: (a) The main steps of nutrition in humans are __________, __________, __________, _________ and __________. (b) The largest gland in the human body is __________. (c) The stomach releases hydrochloric acid and ___________ juices which act on food. (d) The inner wall of the small intestine has many finger-like outgrowths called _________. (e) Amoeba digests its food in the ____________. 2. Mark ‘T’ if the statement is true and ‘F’ if it is false: (a) Digestion of starch starts in the stomach. (T/F) (b) The tongue helps in mixing food with saliva. (T/F) (c) The gall bladder temporarily stores bile. (T/F) (d) The ruminants bring back swallowed grass into their mouth and chew it for some time. (T/F) 3. Tick () mark the correct answer in each of the following: (a) Fat is completely digested in the (i) stomach (ii) mouth (iii) small intestine (iv) large intestine 20 SCIENCE Rationalised 2023-24  (b) Water from the undigested food is absorbed mainly in the (i) stomach (ii) foodpipe (iii) small intestine (iv) large intestine 4. Match the items of Column I with those given in Column II: Column I Column II Food components Product(s) of digestion Carbohydrates Fatty acids and glycerol Proteins Sugar Fats Amino acids 5. What are villi? What is their location and function? 6. Where is the bile produced? Which component of the food does it help to digest? 7. Name the type of carbohydrate that can be digested by ruminants but not by humans. Give the reason also. 8. Why do we get instant energy from glucose? 9. Which part of the digestive canal is involved in: (i) absorption of food ________________. (ii) chewing of food ________________. (iii) killing of bacteria ________________. (iv) complete digestion of food ________________. (v) formation of faeces ________________. 10. Write one similarity and one difference between the nutrition in amoeba and human beings. 11. Match the items of Column I with suitable items in Column II Column I Column II (a) Salivary gland (i) Bile juice secretion (b) Stomach (ii) Storage of undigested food (c) Liver (iii) Saliva secretion (d) Rectum (iv) Acid release (e) Small intestine (v) Digestion is completed (f ) Large intestine (vi) Absorption of water (vii) Release of faeces NUTRITION IN ANIMALS 21 Rationalised 2023-24  12. Label Fig. 2.11 of the digestive system. Fig. 2.11 A part of human digestive system 13. Can we survive only on raw, leafy vegetables/grass? Discuss. Extended Learning — Activities and Project 1. Visit a doctor and find out: (i) Under what conditions does a patient need to be on a drip of glucose? (ii) Till when does a patient need to be given glucose? (iii) How does glucose help the patient recover? Write the answers in your notebook. 2. Find out what vitamins are and get the following information. (i) Why are vitamins necessary in the diet? (ii) Which fruits or vegetables should be eaten regularly to get vitamins? Write a one-page note on the information collected by you. You may take help of a doctor, a dietician, your teacher or any other person, or from any other source. 22 SCIENCE Rationalised 2023-24  3. 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: S. No. Age at which Age at which No. of teeth No. of teeth first tooth fell last tooth fell lost replaced 1. 2. 3. 4. 5. 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. Did you know? Fats in goat’s milk are much simpler than those in cow’s milk. Therefore, the goat’s milk is much easier to digest than the cow’s milk. NUTRITION IN ANIMALS 23 Rationalised 2023-24