Educational Galaxy

Chapter 9: Life Processes in Animals — EG's own notes, in simple words

Book: Curiosity (CBSE, Class 7)  |  Chapter 9  |  Medium: English

Diagram (labels are printed on the image): A stylised heart with red and blue blood vessels. A general diagram; it is not a scene from the book.

A person jogging and breathing fresh air. A general picture about life processes, not a book scene.

What the chapter is about

All animals carry out life processes to stay alive: nutrition, respiration, circulation, excretion and movement. Nutrition means taking in food and using it. In human beings, digestion breaks food into small, soluble parts. Food is chewed in the mouth, mixed with acid and juices in the stomach, digested further in the small intestine, and the useful parts are absorbed into the blood. Undigested food leaves the body as waste.

Respiration releases energy from food. We breathe in air, oxygen reaches the lungs, and from there it goes into the blood. In the body cells, oxygen breaks down food to release energy, and carbon dioxide is produced as waste and breathed out. Breathing is the physical act of taking air in and out; cellular respiration is the chemical process that releases energy inside cells.

Circulation carries materials around the body. The heart pumps blood through blood vessels: arteries carry blood away from the heart, veins bring it back, and tiny capillaries connect them. Blood carries oxygen, digested food, hormones and waste. Excretion removes waste: the kidneys filter the blood and form urine, which leaves the body. Together these processes keep the body's inner conditions steady.

9.1 Nutrition in animals — the alimentary canal

Life processes such as nutrition, respiration, excretion and reproduction are essential for survival, and collectively they are called life processes. This chapter studies nutrition and respiration in detail. Animals eat very different foods: bees and sunbirds suck nectar, infants of humans and many other animals feed on their mother's milk, a python swallows the animals it preys upon, and some aquatic animals filter tiny food particles floating nearby. Animals obtain energy from food, but complex components such as carbohydrate, protein and fat must be broken down into simpler forms before the body can use them.

That breakdown happens in a long tube called the alimentary canal, starting in the mouth and ending at the anus. As food moves through the canal, digestive juices secreted at different parts break it down into simpler forms, which are absorbed by different parts of the canal and transported to various parts of the body. The human digestive system consists of the alimentary canal — mouth, oesophagus, stomach, small intestine, large intestine and anus — plus its associated parts, the liver and the pancreas.

9.1.1 Digestion in human beings — mouth to anus

In the mouth, teeth crush and chew food into smaller pieces — mechanical digestion. Saliva does chemical work too: chapati or boiled rice chewed for 30–60 seconds begins to taste sweet because saliva breaks down starch, a carbohydrate, into sugar. Activity 9.1 proves it with iodine: boiled rice in test tube A turns blue-black with iodine, showing starch is present, while chewed rice in test tube B shows no change or only a very light blue-black, showing the starch has been broken down into simple sugars. If colour still appears in B, chewing longer is the change to try. Good oral hygiene matters too: brush teeth and clean the tongue twice a day, and rinse the mouth with water after each meal, to prevent tooth decay and bad smell.

Saliva also moistens food, and the tongue mixes it and pushes it into the food pipe, or oesophagus. The walls of the food pipe contract and relax in a wave-like motion to push food down into the stomach, and this wave-like movement continues throughout the alimentary canal. In the stomach, the walls churn the food and mix it with a secretion containing digestive juice, acid and mucus: the digestive juice breaks down proteins into simpler components, the acid helps break down proteins and kills many harmful bacteria, and the mucus protects the stomach lining from the acid. The food becomes a semi-liquid mass, partly digested, ready for the next stage. How scientists learnt this is a famous chance discovery: in 1822 Alexis St. Martin was accidentally shot in the stomach, and his wound never fully healed, leaving a small permanent hole through which Dr William Beaumont could observe digestion as it happened.

The small intestine is the longest part of the alimentary canal — almost 6 metres long, almost twice the height of a classroom — even though it is called small. It receives digestive secretions from three sources: its own inner lining, the liver and the pancreas. The liver secretes bile, which is mildly basic: bile neutralises acids coming down from the stomach and breaks down fats into tiny droplets, making their digestion easier. The pancreas secretes pancreatic juice, also basic, which neutralises acids and breaks down carbohydrates, proteins and fats; the juice from the wall of the small intestine further breaks down fats, proteins and partly digested carbohydrates into simpler forms.

Digested nutrients pass from the small intestine into blood vessels in its walls — absorption of nutrients. The inner lining of the small intestine is thin and has thousands of finger-like projections that increase the surface area for efficient absorption, and the blood carries the nutrients to different parts of the body for energy, growth and repair. After most nutrients are absorbed, undigested food moves into the large intestine, about 1.5 metres long: it is shorter than the small intestine but wider, which is why it is called large. It absorbs water and some salts from the undigested food, making the waste semi-solid — stool — which is stored in the rectum, the lower part of the large intestine, until it is expelled through the anus, a process known as egestion. Fibre-rich foods like fruits, vegetables and whole grains help the large intestine work properly by making stool easier to pass. The large intestine also contains small living organisms such as bacteria that help digestion, break down undigested food, especially fibre, and produce essential nutrients — which is why fibre-rich food and fermented foods like curd, buttermilk, shrikhand, kanji, pickles, gundruk and poita bhat are good for a healthy digestive system. Celiac disease shows what happens when absorption fails: the body reacts to gluten, a protein in wheat, barley and rye, damaging the inner lining of the small intestine, so the only management is avoiding gluten foods, with millets like jowar, bajra and ragi as naturally gluten-free alternatives.

9.1.2 Do all animals digest food the same way?

No — animals show variations in the structure and function of the alimentary canal to suit different foods. Grass-eating animals such as cows and buffaloes partially chew grass and swallow it; in the stomach partial digestion takes place, and the partly digested food is brought back to the mouth for gradual chewing — rumination — and these animals are called ruminants. A cow spends about 8 hours a day just chewing its food, and the thoroughly chewed food then passes down the alimentary canal for further digestion. Birds do not have teeth at all, but they have a chamber called a gizzard, whose walls contract and relax to break down food, often with the help of grit — small stones the birds swallow.

9.2 Respiration in animals — breathing is not respiration

Nutrients carried by the blood include sugar, which is broken down inside the body to release energy — that process is respiration. First, breathing: inhaling and exhaling air is called breathing, and without it we usually cannot survive more than a few minutes, though we can live without food for a week and without water for a day or two. The respiratory system begins with a pair of nostrils, through which we inhale and exhale; the inhaled air passes into nasal passages, where tiny hair inside the nostrils, along with mucus, trap dust and dirt — which is why we should breathe through the nose and not through the mouth. From the nasal passages air reaches the lungs through the windpipe, which forms two branches entering the two lungs; inside the lungs these branches divide into smaller and finer branches ending in small balloon-like sacs called alveoli. The lungs are protected by the rib cage.

Activity 9.2 models breathing with a wide transparent bottle, a Y-shaped tube, two deflated balloons and a thin rubber sheet over the open base. Pull the rubber sheet downwards and the balloons inflate; release it upwards and they deflate. In the model the balloons represent the lungs and the rubber sheet represents the diaphragm. In the body, when you breathe in, the chest expands as the ribs move up and outwards and the diaphragm — a dome-shaped muscle below the lungs — moves downwards, increasing the space inside the chest so air enters the lungs. When you breathe out, the ribs move down and inwards and the diaphragm moves upwards, reducing space and pushing air out.

Activity 9.3 shows what we breathe out. Freshly prepared lime water in two test tubes: in tube A air is passed with a syringe or pichkari — the air we inhale — and in tube B exhaled air is blown through a straw. Tube B turns milky (cloudy) but tube A does not, because lime water turns milky when it reacts with carbon dioxide: exhaled air contains more carbon dioxide than inhaled air. Through breathing, fresh air fills the alveoli, whose thin walls are surrounded by fine blood tubes: blood carries carbon dioxide from the body to the alveoli, where it is released into the air, and at the same time oxygen from the alveoli passes into the blood and is transported to all parts of the body. The word equation of respiration is: Glucose + Oxygen → Carbon dioxide + Water + Energy. Inhaled air contains nearly 21% oxygen and nearly 0.04% carbon dioxide, while exhaled air contains nearly 16–17% oxygen and nearly 4–5% carbon dioxide — not all the oxygen is used up. Breathing is a physical process, while respiration is a chemical process that occurs inside the body; both are essential for survival. The circulatory system — heart, blood and blood vessels — then transports nutrients, oxygen and other substances to all parts of the body and carries waste products away. Smoking is extremely harmful here: it damages the lungs, increases the risk of lung cancer and other respiratory illnesses, and causes persistent coughing and frequent infections, while passive smoking endangers non-smokers, especially children, pregnant women and the elderly.

9.2.2 Do other animals breathe the same way as humans?

No — different animals have different breathing mechanisms suited to their habitats. Birds, elephants, lions, cows, goats, lizards and snakes breathe through lungs, although the structure of their lungs is quite different. Most aquatic animals like fish have specialised structures known as gills, richly supplied with blood vessels, across which oxygen and carbon dioxide are exchanged between the blood and the gases dissolved in water. Amphibians like frogs use different body parts at different stages: tadpoles breathe through gills, while adult frogs use lungs for breathing on land and skin for gas exchange when they stay in water. Earthworms use their moist skin for the exchange of oxygen and carbon dioxide. Apart from the digestive, respiratory and circulatory systems, other systems work in coordination in the body, which you will study in higher grades.

What you should learn

Learn the human life processes in order — digestion, respiration, circulation and excretion — and what each one carries or removes.

• Trace food through the alimentary canal in order, naming what each part and secretion does.
• Explain the saliva–iodine test and the lime-water test, and what each result proves.
• Explain the breathing mechanism with ribs and diaphragm, and gas exchange in the alveoli.
• Contrast rumination, the gizzard, gills, frog skin and earthworm skin as adaptations.

Chart / Table: Human life processes

Table: Journey of food and air

Steps: the saliva–starch iodine test (Activity 9.1)

1. Take two test tubes, A and B. Put one teaspoonful of boiled rice in A, and one teaspoonful of boiled rice chewed for 30–60 seconds in B.
2. Add 3–4 mL of water to both test tubes and note the initial colour of each rice-water mixture.
3. Add 3–4 drops of iodine solution to each test tube with a dropper, mixing the contents of each separately.
4. Observe: A turns blue-black (starch present); B shows no change or only a very light blue-black (starch broken down into sugars by saliva).
5. If colour still appears in B, repeat with a longer chewing time and check whether the colour fades further.

Practice set — try it yourself

1. Where is digested food absorbed into the blood?
   Hint: Think about the small intestine.
2. What gas is breathed out after respiration?
   Hint: Think about the waste gas.
3. Which vessels carry blood away from the heart?
   Hint: Think about arteries.
4. What do the kidneys do?
   Hint: Think about filtering blood.
5. How is breathing different from respiration?
   Hint: Think about air movement versus energy release.

Questions with Answers

1. Describe digestion in human beings in order.
   Food is chewed in the mouth, then mixed with acid and digestive juices in the stomach. In the small intestine digestion is completed and digested food is absorbed into the blood. Undigested food passes to the large intestine and leaves the body.
2. What is respiration, and how is it different from breathing?
   Respiration is the process in cells where food is broken down with oxygen to release energy, producing carbon dioxide and water. Breathing is only the physical movement of air into and out of the lungs.
3. How does blood circulation help the body?
   The heart pumps blood through arteries, veins and capillaries. Blood carries oxygen and digested food to all cells, and carries carbon dioxide and other wastes away for removal.
4. What is excretion? Name the main excretory organs in humans.
   Excretion is the removal of waste products from the body. In humans the kidneys filter the blood and form urine, which is stored in the bladder and passed out.
5. Why do animals need all life processes together?
   No single process is enough. Nutrition gives food, respiration releases its energy, circulation delivers materials to every cell, and excretion removes wastes. Together they keep the animal alive and healthy.

More questions with answers — extension

1. Complete the journey of food through the alimentary canal by filling the missing parts: Food → Mouth → ? → Stomach → ? → ? → Anus.
   Food → Mouth → Oesophagus → Stomach → Small intestine → Large intestine → Rectum → Anus. The rectum is the lower part of the large intestine where stool is stored before egestion.
2. Sahil put chapati pieces in test tube A, Neha put chewed chapati in test tube B, and Santushti put boiled and mashed potato in test tube C. All added iodine solution. What would each observe, and why?
   A turns blue-black: unchewed chapati still contains starch. B shows no change or only a very light blue-black: chewing let saliva break the starch into sugars. C turns blue-black: boiled mashed potato still contains starch, since it was not chewed and saliva never acted on it.
3. What is the role of the diaphragm in breathing? (i) To filter the air (ii) To produce sound (iii) To help in inhalation and exhalation (iv) To absorb oxygen.
   (iii) To help in inhalation and exhalation. The diaphragm moves downwards during inhalation, increasing chest space so air enters, and moves upwards during exhalation, reducing space and pushing air out. Filtering is done by nose hair and mucus, not by the diaphragm.
4. Match the parts with their functions: (i) Nostrils (ii) Nasal passages (iii) Windpipe (iv) Alveoli (v) Ribcage — with (a) fresh air from outside enters (b) exchange of gases occurs (c) protects lungs (d) tiny hair and mucus trap dust and dirt (e) air reaches the lungs through this part.
   (i)–(a), (ii)–(d), (iii)–(e), (iv)–(b), (v)–(c). Fresh air enters through the nostrils; the nasal passages trap dust with hair and mucus; the windpipe carries air to the lungs; gas exchange happens in the alveoli; the ribcage protects the lungs.
5. Anil claims that respiration and breathing are the same process. What questions can Sanvi ask him to show he is not correct?
   Sanvi can ask: Is breathing a physical or a chemical process, and where does respiration happen? Does breathing release energy from food, or does it only move air in and out? Can breathing happen without glucose being broken down — and can respiration happen in a cell without lungs? The answers separate them: breathing moves air, respiration breaks down glucose with oxygen in cells to release energy.
6. We often sneeze when we inhale a lot of dust-laden air. What can be possible explanations for this?
   Dust and dirt irritate the nasal passages, even though nose hair and mucus trap much of it. Sneezing is the body's forceful way of expelling the irritating particles from the nose and airways, protecting the lungs from dust reaching deeper inside.
7. Paridhi and Anusha started running for their morning workout. After running, Anusha was breathing faster than Paridhi. Give at least two possible explanations.
   First, Anusha may have run harder or faster, so her muscles needed more energy and therefore more oxygen, with more carbon dioxide to remove. Second, Anusha may be less used to running, so her body has to breathe faster to meet the same demand, while Paridhi's fitter body supplies oxygen more efficiently. Either way, faster breathing supplies more oxygen for respiration and removes carbon dioxide faster.
8. Yadu added rice flour and water to two test tubes, added a few drops of saliva to test tube B only, left both for 35–45 minutes, then added iodine solution. What do you think he wants to test, and what would the results show?
   He wants to test whether saliva breaks down starch. Test tube A (no saliva) will turn blue-black, showing starch is still present. Test tube B (with saliva) will show no change or only a very light blue-black, showing saliva has broken the starch into sugars during the waiting time.
9. Rakshita filled two test tubes with lime water. In tube A she passed the surrounding air we inhale, and in tube B she blew exhaled air. What is she trying to investigate, and how can she confirm her findings?
   She is investigating whether exhaled air contains more carbon dioxide than inhaled air. She can confirm it because only tube B turns milky: lime water turns milky when it reacts with carbon dioxide, so the milky result in B — and no change in A — shows exhaled air carries more carbon dioxide.
10. Why is the small intestine called “small” even though it is almost 6 metres long, and why is the large intestine called “large” even though it is only about 1.5 metres long?
    The names refer to width, not length. The small intestine is narrow but very long (about 6 metres), giving a large surface for absorption through its finger-like projections. The large intestine is much wider, even though it is shorter (about 1.5 metres), because its job is to hold and process semi-solid waste, not to absorb nutrients over a long tube.

How to use this page: Read the summary, say the Key Terms in your own words, then try the practice set without looking. Use the Questions with Answers to check yourself afterwards. Ask your teacher or parent to check your answers.