Welcome dear students! Today we are going to learn about Nutrition in Animals from Class 7 Science. You have learnt in Chapter One 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 studied in Class Six that food consists of many components. Try to recall and list them. They are carbohydrates, proteins, fats, vitamins, minerals, and dietary fibre. These complex substances cannot be utilised as they are, so they are broken down into simpler substances. The breakdown of complex components of food into simpler substances is called digestion.
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Now let us explore section two point one, Different Ways of Taking Food. The mode of taking food into the body varies in different organisms. Bees and hummingbirds suck the nectar of plants, while infants of humans 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. Let us look at Activity two point one. What is the type of food and mode of feeding of the following animals? Write down your observations in the given table. You may find the list of modes of feeding helpful. The animals are the snail, ant, eagle, hummingbird, lice, mosquito, butterfly, and housefly. The modes include scraping, chewing, siphoning, capturing and swallowing, sponging, and sucking. For example, a snail scrapes its food, an eagle captures and swallows its prey, and a butterfly uses siphoning to drink nectar.
Here is an amazing fact about the starfish. The starfish feeds on animals covered by hard shells of calcium carbonate. After opening the shell, the starfish 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.
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Now we move to section two point two, 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 which begins at the buccal cavity and ends at the anus. This canal is divided into various compartments: the buccal cavity, the foodpipe or oesophagus, the stomach, the small intestine, the large intestine ending in the rectum, and the anus. Is it not a very long path? These parts together form the alimentary canal, also known as the digestive tract. The food components gradually get digested as food travels through these compartments. The inner walls of the stomach and the small intestine, and various glands associated with the canal such as the salivary glands, the liver, and the pancreas, secrete digestive juices. These juices convert complex food substances 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, starting with 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 our teeth and break it down mechanically into small pieces. Each tooth is rooted in a separate socket in the gums. Our teeth vary in appearance and perform different functions, so they are given different names: incisors, canines, premolars, and molars.
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Let us talk about milk teeth and permanent teeth. Do you remember about the falling of your teeth some years ago? The first set of teeth grows during infancy and falls off between the ages of 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. Let us perform Activity two point two. Wash your hands, look into a mirror, and count your teeth. Use your index finger to feel them. How many kinds of teeth could you find? Take a piece of apple or bread and eat it. Notice which teeth you use for biting and cutting, which for piercing and tearing, and which for chewing and grinding. Record your observations in Table two point two, noting the number of incisors, canines, premolars, and molars in both the lower and upper jaws.
Our mouth has salivary glands which secrete saliva. Do you know the action of saliva on food? Let us find out with Activity two point three. Take two test tubes and label them A and B. In test tube A, put one teaspoonful of boiled rice. In test tube B, keep one teaspoonful of boiled rice after chewing it for three to five minutes. Add three to four millilitres of water in both test tubes. Now pour two to three drops of iodine solution in each test tube and observe. Why is there a change in colour in the test tubes? Discuss the results with your classmates and your teacher. The textbook explains that the saliva breaks down the starch into sugars.
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Let us discuss sweets and tooth decay. Normally, bacteria are present in our mouth but are not harmful. However, if we do not clean our teeth and mouth after eating, harmful bacteria grow and break down leftover sugars, releasing acids. These acids gradually damage the teeth, a condition called tooth decay. If untreated, it causes severe toothache and can lead to tooth loss. Chocolates, sweets, and soft drinks are major culprits. Therefore, clean your teeth with a brush or datun and dental floss at least twice a day, and rinse your mouth after every meal. Never put dirty fingers or unwashed objects in your mouth.
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 move in all directions. We use it for talking, mixing saliva with food during chewing, and swallowing. We also taste food with our tongue, which has taste buds to detect different tastes. In Figure two point six, we see the regions of the tongue for different tastes. We can find out the exact position of these taste buds by performing Activity two point four. First, prepare separate samples of sugar solution, common salt solution, lemon juice, and juice of crushed neem leaf or bitter gourd. Second, blindfold a classmate and ask them to take out their tongue and keep it straight and flat. Third, use a clean toothpick to place each sample on different areas of the tongue as shown in the figure, using a new toothpick for each sample. Fourth, ask your classmate which areas detected the sweet, salty, sour, and bitter tastes. Finally, write down your observations and label Figure two point six. Repeat this with other classmates.
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Now let us look at the foodpipe or oesophagus. Swallowed food passes into the foodpipe, which runs along the neck and chest. In Figure two point seven, we see the movement of food in the oesophagus. Food is pushed down by the wave-like movement of the foodpipe wall. This movement actually takes place throughout the alimentary canal to push food downwards. 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. Sometimes when you eat in a hurry, talk, or laugh while eating, you may cough or choke. This happens when food enters the windpipe. The windpipe carries air from the nostrils to the lungs and runs adjacent to the foodpipe. Inside the throat, air and food share a common passage. During swallowing, a flap-like valve closes the windpipe passage and guides food into the foodpipe. If food enters the windpipe, we cough or get hiccups.
The stomach is a thick-walled, J-shaped bag and the widest part of the alimentary canal. It receives food from the foodpipe and opens into the small intestine. Its inner lining secretes mucous, hydrochloric acid, and digestive juices. The mucous protects the stomach lining, the acid kills bacteria and creates an acidic medium, and the digestive juices break down proteins into simpler substances. The working of the stomach was discovered by accident in eighteen twenty two, when a man named Alexis St. Martin survived a shotgun wound that left a permanent hole in his stomach. Doctor William Beaumont observed through this hole that the stomach churns food, secretes digestive fluid, and only opens into the intestine after digestion inside the stomach is complete.
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Next is the small intestine. The small intestine is highly coiled. Boojho is fascinated by this coiling and wants to know its length. The textbook mentions its approximate length is about seven point five metres. Just imagine how such a long structure is accommodated in the small space within our body! It receives secretions from the liver and pancreas, and its own wall also secretes juices. The liver is a reddish-brown gland on the upper right side of the abdomen and is the largest gland in the body. It secretes bile juice, stored in the gall bladder, which plays an important role in fat digestion. The pancreas is a large, cream-coloured gland below the stomach. Its juice acts on carbohydrates, fats, and proteins, changing them into simpler forms.
Sometimes you may need to pass watery stool frequently, a condition known as diarrhoea. It can be caused by infection, food poisoning, or indigestion. Severe diarrhoea can be fatal due to excessive loss of water and salts. Before consulting a doctor, give the patient plenty of boiled and cooled water with a pinch of salt and sugar. This is called Oral Rehydration Solution, or ORS. The partly digested food reaches the lower small intestine, where intestinal juice completes digestion. Carbohydrates break into glucose, fats into fatty acids and glycerol, and proteins into amino acids.
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Now let us understand absorption in the small intestine. Digested food passes into blood vessels in the intestinal wall. This process is called absorption. The inner walls have thousands of finger-like outgrowths called villi, singular villus. The villi increase the surface area for absorption. Each villus has a network of thin blood vessels that absorb digested food materials. These substances are transported via blood to different organs to build complex substances like proteins. This is called assimilation. In cells, glucose breaks down with oxygen into carbon dioxide and water, releasing energy. Undigested and unabsorbed food enters the large intestine.
The large intestine is wider and shorter than the small intestine, about one point five metres long. It absorbs water and some salts from the undigested material. The remaining waste passes into the rectum as semi-solid faeces. Faecal matter is removed through the anus periodically. This is called egestion.
Let us move to section two point three, Digestion in Grass-Eating Animals. Have you observed cows and buffaloes chewing continuously even when not eating? They quickly swallow grass and store it in a stomach part called the rumen. Here, food gets partially digested and is called cud. Later, the cud returns to the mouth in small lumps for chewing. This process is rumination, and these animals are ruminants. Paheli wants to know why these animals cannot chew food properly when they first take it in. Grass is rich in cellulose. Bacteria in the rumen help digest cellulose. Many animals, including humans, cannot digest cellulose. Animals like horses and rabbits have a large sac called the caecum between the oesophagus and small intestine, where bacteria digest cellulose. Humans lack these bacteria.
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So far, you have learnt about animals with digestive systems. But many small organisms lack a mouth and digestive tract. How do they acquire and digest food? This brings us to section two point four, Feeding and Digestion in Amoeba. Amoeba is a microscopic, single-celled organism in pond water. It has a cell membrane, a dense nucleus, and small bubble-like vacuoles. It constantly changes shape and pushes out finger-like projections called pseudopodia or false feet for movement and food capture. When it senses food, it pushes pseudopodia around the particle and engulfs it. The food gets trapped in a food vacuole. Digestive juices are secreted into the vacuole, breaking food into simpler substances. Digested food is absorbed for growth and maintenance. Undigested residue is expelled outside. The basic process of digestion and energy release is the same in all animals.
Let us review what you have learnt. Animal nutrition includes nutrient requirement, intake mode, and utilisation. The human digestive system consists of the alimentary canal and secretory glands. The canal includes the buccal cavity, oesophagus, stomach, small intestine, large intestine ending in the rectum, and anus. Main digestive glands are the salivary glands, liver, and pancreas. Nutrition involves ingestion, digestion, absorption, assimilation, and egestion. Carbohydrate digestion begins in the buccal cavity, protein digestion starts in the stomach, and complete digestion occurs in the small intestine. Digested food is absorbed in the small intestine. Water and salts are absorbed in the large intestine. Faeces are expelled through the anus. Ruminants store food in the rumen and chew cud later. Amoeba uses pseudopodia and digests food in a food vacuole.
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Now let us solve the exercises together. Exercise one: Fill in the blanks. Part a: The main steps of nutrition in humans are ingestion, digestion, absorption, assimilation, and egestion. Part b: The largest gland in the human body is the liver. Part c: The stomach releases hydrochloric acid and digestive juices which act on food. Part d: The inner wall of the small intestine has many finger-like outgrowths called villi. Part e: Amoeba digests its food in the food vacuole. Exercise two: Mark T or F. Part a: Digestion of starch starts in the stomach. False. Part b: The tongue helps in mixing food with saliva. True. Part c: The gall bladder temporarily stores bile. True. Part d: Ruminants bring back swallowed grass into their mouth and chew it. True. Exercise three: Tick the correct answer. Part a: Fat is completely digested in the small intestine. Part b: Water from undigested food is absorbed mainly in the large intestine. Exercise four: Match the items. Carbohydrates break into sugar. Proteins break into amino acids. Fats break into fatty acids and glycerol.
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Exercise five: What are villi? What is their location and function? Villi are thousands of finger-like outgrowths on the inner walls of the small intestine. Their function is to increase the surface area for absorption of digested food. Exercise six: Where is bile produced? Which component does it help digest? Bile is produced by the liver and helps digest fats. Exercise seven: Name the carbohydrate digested by ruminants but not humans. Give the reason. It is cellulose. Ruminants can digest it due to bacteria in their rumen, which humans lack. Exercise eight: Why do we get instant energy from glucose? Glucose is a simple sugar that is directly absorbed into the blood and broken down in cells with oxygen to release energy immediately, without needing further digestion. Exercise nine: Which part is involved in absorption of food? Small intestine. Chewing of food? Buccal cavity. Killing of bacteria? Stomach. Complete digestion of food? Small intestine. Formation of faeces? Large intestine.
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Exercise ten: Write one similarity and one difference between nutrition in amoeba and humans. Similarity: Both involve ingestion, digestion, absorption, assimilation, and egestion. Difference: Humans have a complex digestive system with specialized organs, while amoeba uses temporary structures like pseudopodia and food vacuoles. Exercise eleven: Match the columns. Salivary gland matches with saliva secretion. Stomach matches with acid release. Liver matches with bile juice secretion. Rectum matches with storage of undigested food and release of faeces. Small intestine matches with digestion is completed. Large intestine matches with absorption of water. Exercise twelve: Label Figure two point one one of the digestive system. Exercise thirteen: Can we survive only on raw, leafy vegetables or grass? Discuss.
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Now let us look at the Extended Learning Activities and Projects. Activity one: Visit a doctor and find out under what conditions a patient needs to be on a drip of glucose, till when a patient needs to be given glucose, and how glucose helps the patient recover. Write the answers in your notebook. Activity two: Find out what vitamins are, why they are necessary in the diet, and which fruits or vegetables should be eaten regularly to get them. 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 source. Activity three: Collect data from your friends, neighbours, and classmates to know more about milk teeth. Tabulate your data, noting the age at which the first and last tooth fell, and the number of teeth lost and replaced. 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.
Finally, here is a Did you know fact. Fats in goat's milk are much simpler than those in cow's milk. Therefore, goat's milk is much easier to digest than cow's milk.
Thank you for listening! Keep revising and practicing. Goodbye! [CHAPTER_COMPLETE]