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2Nutrition in AnimalsYou have learnt in Chapter 1 thatplants can prepare their own foodby the process of photosynthesisbut animals cannot. Animals get theirfood from plants, either directly byeating plants or indirectly by eatinganimals that eat plants. Some animalseat both plants and animals. Recall thatall organisms including humans requirefood for growth, repair and functioningof the body. Animal nutrition includesnutrient requirement, mode of intakeof food and its utilisation in the body.You have studied in Class VI that foodconsists of many components. Try torecall and list them below:1.2.3.5.6.The components of food such ascarbohydrates are complex substances.These complex substances cannot beutilised as such. So they are brokendown into simpler substances. Thebreakdown of complex components ofSimpler substances2.1 DIFFERENT WAYS OF TAKING FOODThe mode of taking food into the bodyvaries in different organisms. Bees andhumming-birds suck the nectar ofplants, infants of human and manyother animals feed on mother’s milk.Snakes like the python swallow theanimals they prey upon. Some aquaticanimals filter tiny food particles floatingnearby and feed upon them.Activity 2.1What is the type of food and mode offeeding of the following animals? Writedown your observations in the givenTable. You may find the list of modes offeeding given below the Table helpful.Table 2.1 Various modes of feeding4.Complex substancefood into simpler substances is calleddigestion.Name ofKind ofMode squitoButterflyHouse fly(Scraping, chewing, siphoning, capturingand swallowing, sponging, sucking etc.)2021–22

Amazing factStarfish feeds on animals covered byhard shells of calcium carbonate.After opening the shell, the starfishpops out its stomach through itsmouth to eat the soft animal insidethe shell. The stomach then goes backinto the body and the food is slowlydigested.of the stomach and the small intestine,and the various glands associated with thecanal such as salivary glands, the liverand the pancreas secrete digestive juices.The digestive juices convert complexBuccal cavitySalivary glandOesophagusLiverFig. 2.1 Starfish2.2 DIGESTIONINGall bladderHUMANSStomachPancreasWe take in food through the mouth,digest and utilise it. The unused partsof the food are defecated. Have you everwondered what happens to the foodinside the body? The food passesthrough a continuous canal (Fig. 2.2)which begins at the buccal cavity andends at the anus. The canal can bedivided into various compartments:(1) the buccal cavity, (2) foodpipe oroesophagus, (3) stomach, (4) smallintestine, (5) large intestine ending inthe rectum and (6) the anus. Is it not avery long path? These parts togetherform the alimentary canal (digestivetract). The food components graduallyget digested as food travels through thevarious compartments. The inner wallsSmallintestineLargeintestineRectumAnusFig. 2.2 Human digestive systemsubstances of food into simpler ones. Thedigestive tract and the associated glandstogether constitute the digestive system.Now, let us know what happens tothe food in different parts of the digestivetract.The mouth and buccal cavityFood is taken into the body through themouth. The process of taking food into12SCIENCE2021–22

Milk teeth and permanent teethDo you remember about falling of your teeth some years ago? The first set ofteeth grows during infancy and they fall off at the age between six to eightyears. These are termed milk teeth. The second set that replaces them are thepermanent teeth. The permanent teeth may last throughout life or fall offduring old age or due to some dental disease.MolarBoojho is fascinated by thehighly coiled small intestine seenin Fig. 2.2. He wants to know itslength. Would you like to make awild guess? We have given itsapproximate length on page 16.Just imagine how such a longstructure is accommodated in asmall space within our body!PremolarCanineIncisorthe body is called ingestion. We chewthe food with the teeth and break itdown mechanically into small pieces.Each tooth is rooted in a separate socketin the gums (Fig. 2.3). Our teeth vary inappearance and perform differentfunctions. Accordingly they are givendifferent names (Fig. 2.3).Fig. 2.3 Arrangement of teeth and differenttype of teethwhich ones for piercing and tearing?Also find out the ones that are used forchewing and grinding?Record your observations in Table 2.2Activity 2.2Type of teethWash your hands. Look into themirror and count your teeth. Useyour index finger to feel the teeth.How many kinds of teeth could youfind? Take a piece of an apple orbread and eat it. Which teeth doyou use for biting and cutting, andCutting andbiting teethPiercing andtearing teethChewing andgrinding teethNUTRITION IN ANIMALSTable 2.2Number of teethTotalLower jaw Upper jaw132021–22

Our mouth has the salivary glandswhich secrete saliva. Do you know theaction of saliva on food? Let us find out.Activity 2.3Take two test tubes. Label them ‘A’ and‘B’. In test tube ‘A’ put one teaspoonfulIodine solutionWaterBoiled riceABoiled and chewed riceBFig. 2.4 Effect of saliva on starchof boiled rice; in test tube ‘B’ keep oneteaspoonful of boiled rice after chewingit for 3 to 5 minutes. Add 3–4 mL ofwater in both the test tubes (Fig. 2.4).Now pour 2–3 drops of iodine solutionin each test tube and observe. Why isthere a change in colour in the testtubes? Discuss the results with yourclassmates and your teacher. The salivabreaks down the starch into sugars.The tongue is a fleshy muscularorgan attached at the back to the floorof the buccal cavity. It is free at the frontand can be moved in all directions. Doyou know the functions of the tongue?We use our tongue for talking. Besides,it mixes saliva with the food duringchewing and helps in swallowing food.We also taste food with our tongue. Ithas taste buds that detect differenttastes of food. We can find out theSweets and tooth decayNormally bacteria are present in our mouth but they are not harmfulto 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 bacteriabreak down the sugars present from the leftover food and releaseacids (see Chapter 5 to know what an acid is). The acids graduallydamage the teeth (Fig. 2.5). This is called tooth decay. If it is nottreated in time, it causes severe toothache and in extreme casesresults in tooth loss. Chocolates, sweets, soft drinks and other sugarproducts are the major culprits of tooth decay.Therefore, one should clean the teeth with a brush or datun anddental floss (a special strong thread which is moved between twoteeth to take out trapped food particles) at least twice a day andrinse the mouth after every meal. Also, one should not put dirtyfingers or any unwashed object in the mouth.(a)(b)(c)(d)Fig. 2.5 Gradual decay of tooth14SCIENCE2021–22

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 thewindpipe. The windpipe carries air from the nostrils to the lungs. It runs adjacentto 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 ofswallowing a flap-like valve closes the passage of the windpipe and guides thefood into the foodpipe. If, by chance, food particles enter the windpipe, we feelchoked, get hiccups or cough.5. Now write down your observationsand label Fig. 2.6.Repeat this activity with otherclassmates.The foodpipe/oesophagusFig. 2.6 Regions of thetongue for different tastesThe swallowed food passes into thefoodpipe or oesophagus. Look at Fig. 2.2.The foodpipe runs along the neckposition of taste buds by the followingactivity.Paheli wants to know howfood moves in the oppositedirection during vomiting.Activity 2.41. Prepare a separate sample each of(i) sugar solution, (ii) common saltsolution, (iii) lemon juice and (iv) juiceof crushed neem leaf or bitter gourd.2. Blindfold one of your classmates andask her/him to take out the tongueand keep it in straight and flat position.3. Use a clean toothpick to put theabove samples one by one ondifferent areas of the tongue asshown in Fig. 2.6. Use a newtoothpick for each sample.4. Ask the classmate which areas of thetongue could detect the sweet, salty,sour and bitter substances.FoodOesophagusStomachFig. 2.7 Movement of thefood in the oesophagusof the alimentary canalNUTRITION IN ANIMALS152021–22

and the chest. Food is pushed down bymovement of the wall of the foodpipe.Actually this movement takes placethroughout the alimentary canal andpushes the food downwards (Fig. 2.7).At times the food is not accepted by ourstomach and is vomited out. Recall theinstances when you vomited after eatingand think of the reason for it. Discusswith your parents and teacher.The stomachThe stomach is a thick-walled bag. Itsshape is like a flattened J and it is thewidest part of the alimentary canal. Itreceives food from the food pipe at oneend and opens into the small intestineat the other.The inner lining of the stomachsecretes mucous, hydrochloric acid anddigestive juices. The mucous protectsthe lining of the stomach. The acid killsmany bacteria that enter along with thefood and makes the medium in thestomach acidic and helps the digestivejuices to act. The digestive juices breakdown the proteins into simplersubstances.The small intestineThe small intestine is highly coiled andis about 7.5 metres long. It receivessecretions from the liver and the pancreas.Besides, its wall also secretes juices.The liver is a reddish brown glandsituated in the upper part of theabdomen on the right side. It is thelargest gland in the body. It secretes bilejuice that is stored in a sac called thegall bladder (Fig. 2.2). The bile playsan important role in the digestion of fats.The pancreas is a large creamcoloured gland located just below thestomach (Fig. 2.2). The pancreatic juiceacts on carbohydrates, fats and proteinsand changes them into simpler forms.The working of the stomach was discovered by astrange accident. In 1822, a man named Alexis St.Martin was badly hit by a shot gun. The bullet hadseriously damaged the chest wall and made a holein his stomach. He was brought to an Americanarmy doctor William Beaumont. The doctor savedthe patient but he could not close the hole properlyand left it bandaged (Fig. 2.8). Beaumont took it asa great opportunity to see the inside of the stomachthrough the hole. He made some wonderfulobservations.Beaumont found that the stomach was churning Fig. 2.8 Alexis St. Martin’sshotgun woundfood. Its wall secreted a fluid which could digestthe food. He also observed that the end of the stomach opens into the intestineonly after the digestion of the food inside the stomach is completed.16SCIENCE2021–22