Life Processes in Animals
Why This Matters
Think about your morning. You ate some breakfast. Maybe a roti, or some rice, or an egg. Right now, you are breathing in and out without even thinking about it.
But have you ever wondered what happens to that roti after you swallow it? Where does it go? How does it give you the energy to walk to school, play, and even read this page?
And what about your breath? Why can you go a whole day without water, but you cannot stop breathing for more than a minute or two?
Your body is doing amazing work right now, quietly, without asking you. It is breaking down your food. It is taking in oxygen. It is carrying useful things to every corner of your body.
In this chapter, we will follow your food on its journey. We will follow your breath too. By the end, you will understand exactly how your body turns a simple roti into the energy that keeps you alive.
The Big Idea
Every animal must do certain jobs just to stay alive. We call these jobs life processes — for example nutrition (getting and using food), respiration (using oxygen to get energy from food), and circulation (carrying useful things around the body). Food has big, complex parts that the body cannot use directly. So the body breaks them into small, simple parts. This breaking down is called digestion. The simple parts are soaked into the blood. Then oxygen, which we breathe in, helps burn these simple parts to release energy. The blood carries food and oxygen to every part of the body. All animals do these same jobs — but in their own ways, suited to where they live and what they eat.
Let’s Break It Down
We will look at three big jobs your body does: nutrition (digesting food), respiration (getting energy using oxygen), and circulation (transport around the body). Let us start with food.
Why food must be broken down
When you eat, you take in food that has complex parts. “Complex” just means big and made of many pieces joined together. The main ones are carbohydrates (give energy — found in rice and roti), proteins (build and repair the body — found in dal, egg, milk), and fats (store energy — found in ghee, oil, nuts).
Here is the problem. These complex parts are too big for the body to use as they are. They must be broken down into simple forms first. Only then can the body soak them up and use them.
This breaking down of complex food into simple forms is called digestion. It happens inside a long tube in the body called the alimentary canal. This tube starts at the mouth and ends at the anus.
Before we trace the journey of food, let us remind ourselves what a nutrient is, because we will use the word a lot.
Digestion is just one part of a bigger story. Food has to go IN, get broken down, get soaked up, and the leftover waste has to come OUT. These four steps have names. Figure 9.1 below lays them out in order.
Now let us follow the food through the body, one organ at a time.
The human digestive system — the full tube
The alimentary canal is one long tube with several parts. Along the way, juices are added that help break the food down. Figure 9.2 below shows the whole system. Keep coming back to it as we walk through each part.
Let us now visit each part. We will start where eating begins — the mouth.
The mouth — two jobs at once
The journey starts when food enters your mouth. Two things happen here at the same time.
First, your teeth crush and chew the food into small pieces. This is called mechanical digestion — “mechanical” because it is like a machine grinding something. The food only changes in size. No new substance is made.
Second, a liquid called saliva mixes in. Saliva is made in the mouth. It does two useful things. It makes the food soft and wet, so it is easy to swallow. And it starts to break starch into sugar. Starch is a kind of carbohydrate found in rice and roti.
Here is a small test you can try. Chew a piece of plain roti or boiled rice slowly for about a minute. At first it is not sweet. But slowly it begins to taste sweet! Why? Because the saliva has broken some of the starch into sugar, and sugar tastes sweet.
So in the mouth, the teeth do mechanical work, and the saliva starts a chemical change. Figure 9.3 below shows both jobs side by side.
Why does the saliva matter so much? Because the body cannot use big starch directly. Breaking it into sugar early in the mouth gives digestion a head start. The tongue then mixes the soft food and pushes it into the next part.
The food pipe — how food moves down
The soft, chewed food is pushed into a long, flexible tube called the food pipe, also known as the oesophagus. It carries food from the mouth down to the stomach.
But here is a puzzle. The food does not just fall down. So how does it move? You can even swallow while lying down or upside down. The food still reaches your stomach. Why?
The walls of the food pipe gently squeeze and relax in a wave, one section after another. This wave pushes the food forward, like squeezing toothpaste along a tube. This same wave-like squeezing happens all along the alimentary canal, not just the food pipe. That is how food keeps moving forward the whole way.
An astronaut in space, where things float, can still swallow food and it reaches the stomach. Why does the food not just float back up out of the mouth?
Because food is not pushed down by gravity. The walls of the food pipe squeeze and relax in a wave that pushes the food forward. This wave works in any direction, even in space or when you are upside down. So the food still reaches the stomach.
The stomach — churning and acid
Next, the food reaches the stomach. The stomach is like a stretchy bag. Its walls contract and relax to churn the food — that means to mix and mash it around, like stirring batter.
While churning, the stomach adds a special mix from its inner lining. This mix has three things: a digestive juice, an acid, and a slippery liquid called mucus. Each has a job:
- The digestive juice breaks down proteins into simpler parts.
- The acid also helps break proteins, and it kills many harmful bacteria (tiny germs) that came in with the food.
- The mucus is a protective coat. It stops the acid from harming the stomach’s own wall.
That last point answers a question many students ask. If the acid is strong enough to kill germs, why does it not burn the stomach itself? Because the mucus forms a slippery shield over the stomach wall. The acid touches the mucus, not the wall.
After the stomach, the food has become a soft, semi-liquid paste, partly digested and ready for the next stage.
The small intestine — where most digestion and absorption happen
The paste now moves into the small intestine. Do not let the name fool you. It is called “small” only because it is narrow (thin). It is actually very long — about 6 metres! That is almost twice the height of your classroom, all coiled up to fit inside you.
The small intestine is where digestion is finished and where the simple food is finally soaked up. It gets help from three sources:
- Its own inner lining, which makes a juice that breaks down fats, proteins, and carbohydrates further.
- The liver, which makes a liquid called bile. Bile is mildly basic. It does two things: it cancels out the acid coming from the stomach, and it breaks big fat droplets into tiny droplets so they are easier to digest.
- The pancreas, which makes pancreatic juice. This is also basic, so it cancels acid too, and it helps break down carbohydrates, proteins, and fats.
You met acids and bases in an earlier chapter. Let us refresh that idea, because it explains why bile and pancreatic juice are useful.
Once the food is broken into simple nutrients, those nutrients pass from the small intestine into the blood. This passing of nutrients into the blood is called absorption.
How does the small intestine soak up so much? Its inner lining is not smooth. It is covered with thousands of tiny finger-like projections. These finger-like folds add a huge amount of surface for soaking up nutrients. Think of a towel: it dries you fast because its many tiny loops give it more surface to soak water. The finger-like folds do the same job for food. The nutrients pass into the blood, which carries them everywhere — for energy, growth, and repair.
Let us put numbers to this idea of length with a quick example.
The small intestine is about 6 metres long and the large intestine is about 1.5 metres long. How many times longer is the small intestine than the large intestine?
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We want to compare the two lengths. To find “how many times longer”, we divide the longer length by the shorter one.
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Length of small intestine = 6 metres. Length of large intestine = 1.5 metres.
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Divide: 6 ÷ 1.5 = 4.
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So the small intestine is 4 times longer than the large intestine. This is why the small intestine is the longest part of the alimentary canal, even though it is the narrowest.
The large intestine — saving water, making waste
After the small intestine takes out almost all the useful nutrients, some leftover undigested food remains. This moves into the large intestine.
The large intestine is about 1.5 metres long — shorter than the small intestine, but wider. That is exactly why it is called “large”: not because it is longer, but because it is wider.
Its main job is to absorb water and some salts from the undigested food. Pulling out the water makes the leftover semi-solid. This semi-solid waste is called stool.
The stool is stored in the lower part of the large intestine, called the rectum, until the body is ready to remove it. Finally, the stool leaves the body through the anus. This removal of undigested waste is called egestion.
Eating fibre-rich foods like fruits, vegetables, and whole grains helps the large intestine work well and makes the stool easier to pass.
The small intestine is 6 metres long and the large intestine is only 1.5 metres long. So why is the shorter one called the LARGE intestine?
Because “large” here means wider, not longer. The large intestine is shorter than the small intestine but it is wider across. The small intestine is longer but narrow (thin). So the names come from the width, not the length.
Do all animals digest food the same way?
Animals eat very different foods, so their digestive systems vary a little. Two examples are in the textbook.
Grass-eating animals like cows and buffaloes. Have you seen a cow chewing even when it is not eating anything fresh? Grass is tough and hard to break down. So a cow first quickly chews the grass a little and swallows it into a part of its stomach (the rumen). There it gets partly digested. Later, the cow brings this partly digested food back up to the mouth and chews it slowly again. This is called rumination, and such animals are called ruminants. A cow may spend about 8 hours a day just chewing!
Birds. Birds have no teeth. So they cannot chew. Instead, they have a muscular chamber called a gizzard. The gizzard’s walls squeeze the food to grind it. Many birds even swallow small stones (grit) that sit in the gizzard and help grind the food, like tiny grinding stones.
This shows a beautiful idea: animals have slightly different alimentary canals to suit the kind of food they eat and the way they live.
| Animal type | Special feature | Why it helps |
|---|---|---|
| Humans | Teeth to chew, long alimentary canal | Can eat many kinds of food |
| Cows, buffaloes (ruminants) | Bring food back to mouth to chew again (rumination) | Tough grass is hard to digest, so chewing it twice helps |
| Birds | A grinding gizzard, often with swallowed grit | They have no teeth, so the gizzard does the grinding |
Respiration — turning food into energy using oxygen
We have seen how food is digested and carried to the body in the blood. But food alone is not energy. The body still has to release the energy locked inside the food. To do that, it needs oxygen.
The process by which the body uses oxygen to break down simple food (like sugar) and release energy is called respiration. Let us first see how we get that oxygen — by breathing.
How we breathe — the respiratory system
Breathing is taking air in (inhaling) and pushing air out (exhaling). We do it to take in oxygen and remove carbon dioxide.
The air follows a fixed path through a set of parts called the respiratory system. Figure 9.4 below shows the whole system. Let us trace the path of the air.
Here is the path, step by step. Air enters through a pair of openings called nostrils. Inside the nose are tiny hairs and mucus that trap dust and dirt — like a filter. This is why we should breathe through the nose, not the mouth. The cleaned air then goes down the windpipe. The windpipe splits into two branches, one into each lung. Inside the lungs, the branches keep dividing into smaller and smaller tubes that end in tiny balloon-like sacs called alveoli (say it: al-VEE-oh-lie). The lungs are protected by the rib cage, and below them sits a muscle called the diaphragm.
What makes air go in and out — the breathing mechanism
Why does air rush into the lungs and then get pushed out? The secret is the size of the chest space.
When you breathe in (inhale): your ribs move up and outwards, and the diaphragm moves down. This makes the space inside your chest bigger. Air rushes in to fill the extra space.
When you breathe out (exhale): your ribs move down and inwards, and the diaphragm moves up. This makes the chest space smaller, so air is squeezed out.
Figure 9.5 below shows both, side by side.
There is a neat way to picture this. Imagine a balloon inside a bottle. If you pull the bottom of the bottle to make more room, the balloon fills with air. If you push it back, the balloon empties. Your diaphragm is like that bottom, and your lungs are like the balloons.
What happens to the oxygen — gas exchange in the alveoli
Air reaches the tiny alveoli at the end of the journey. The alveoli are where the real exchange happens.
Each alveolus has a very thin wall, and it is wrapped by tiny blood vessels carrying blood. Because the wall is so thin, gases can pass through it easily. Two things happen at the same time:
- Oxygen from the air in the alveolus passes into the blood. The blood carries it to every part of the body.
- Carbon dioxide (a waste gas) passes from the blood into the alveolus, ready to be breathed out.
Figure 9.6 below shows this swap.
Now we can answer the big question: how does food give you energy? The answer needs both food and oxygen together. Inside the body cells, oxygen helps break down sugar (glucose) from your food, and this releases energy. The waste gases made are carbon dioxide and water. We can write this as a word equation:
Glucose + Oxygen → Carbon dioxide + Water + Energy
This released energy is what lets you walk, run, play, and even think.
So here is an important difference. Breathing is a physical process — just air moving in and out. Respiration is a chemical process inside the body, where oxygen breaks down food to release energy. Breathing brings in the oxygen; respiration uses it.
A friend says 'breathing and respiration are the same thing.' What one fact can you tell them to show they are different?
You can tell them: breathing is only the movement of air in and out of the lungs (a physical process), while respiration is the chemical process inside the body where oxygen breaks down food to release energy. Breathing happens in the lungs; respiration releases the actual energy. So they are not the same — breathing brings in the oxygen that respiration needs.
We also know exhaled air carries more carbon dioxide. You can show this with lime water. Lime water turns milky when carbon dioxide passes through it. If you blow air through a straw into lime water, it turns milky. This proves your exhaled air has more carbon dioxide than the air you breathed in.
Do other animals breathe the same way?
All animals need oxygen, but they get it in different ways to suit where they live.
- Animals with lungs: birds, elephants, lions, cows, goats, lizards, and snakes all breathe using lungs, like us (though the lungs are shaped differently).
- Fish and many water animals: they use gills. Gills are special organs full of blood vessels. They take oxygen from the oxygen dissolved in the water.
- Frogs (amphibians): they live both on land and in water. A young frog (tadpole) uses gills. An adult frog uses lungs on land and its moist skin to exchange gases when in water.
- Earthworms: they breathe through their moist skin.
The idea is the same everywhere: get oxygen in and carbon dioxide out. Only the body part used changes, to suit the habitat.
Circulation — the transport system
We keep saying “the blood carries it.” But what moves the blood around? That job belongs to the circulatory system.
The circulatory system has three main parts: the heart, the blood, and the blood vessels (the tubes the blood flows through). The heart is a pump. It pushes blood through the blood vessels to every part of the body.
The blood is like a delivery service. It carries oxygen and nutrients to all body parts, and it carries away waste like carbon dioxide. This is how the food you absorbed and the oxygen you breathed actually reach your fingers, your brain, and your toes.
So the three systems work as a team: digestion gets the nutrients, respiration provides the energy using oxygen, and circulation does the delivery.
Common Mistakes
Let us clear up a few ideas that students often get mixed up.
The small intestine is called 'small' and the large intestine 'large' because of how long they are.
It feels obvious that 'large' should mean longer, because in everyday talk a larger thing is usually the bigger, longer one.
The names come from the width, not the length. The large intestine is wider but shorter (about 1.5 m). The small intestine is narrower but much longer (about 6 m) — in fact it is the longest part of the alimentary canal.
Breathing and respiration are just two words for the same thing.
It is tempting because we use both words loosely in daily life, and breathing and respiration always seem to happen together.
Breathing is a physical process — air simply moving in and out of the lungs. Respiration is a chemical process inside the body, where oxygen breaks down food to release energy. Breathing supplies the oxygen; respiration uses it to make energy.
Food moves down the food pipe to the stomach because gravity pulls it down.
It seems right because when we stand, food does travel downwards, so it looks like it is simply falling.
Food is pushed by a wave of squeezing and relaxing in the walls of the food pipe. That is why you can swallow even while lying down or upside down. The same wave moves food all along the alimentary canal.
When we breathe, all of the oxygen in the air gets used up by the body.
It feels right because we breathe in for the oxygen, so it seems the body should grab every bit of it.
Not all the oxygen is used. Inhaled air has about 21% oxygen; exhaled air still has about 16 to 17% oxygen. The body uses only part of it. That leftover oxygen in exhaled air is why mouth-to-mouth rescue breathing can still help someone.
Quick Check
Time for a quick check. Pick the best answer and read the explanation either way.
In which part of the digestive system are most nutrients absorbed into the blood?
What is the main job of the large intestine?
During inhalation (breathing in), what does the diaphragm do?
Which statement about respiration is correct?
Practice Problems
Easy
Complete the journey of food in order by naming the missing parts: Mouth → ______ → Stomach → Small intestine → ______ → Anus.
The full path in order is: Mouth → Food pipe (oesophagus) → Stomach → Small intestine → Large intestine → Anus.
So the first missing part is the food pipe (oesophagus), and the second missing part is the large intestine.
What is the role of saliva in the mouth? Give two things it does.
Saliva does two useful things:
- It makes the food soft and wet, so it is easy to swallow.
- It breaks starch into sugar. This is why a roti starts to taste sweet if you chew it for a long time.
Name the two helper organs that are joined to the small intestine and add juices to help digestion.
The two helper organs are the liver (which makes bile) and the pancreas (which makes pancreatic juice). Both juices are basic, so they cancel the acid coming from the stomach, and they help break down food further.
Medium
Sahil put plain chapati in test tube A. Neha put well-chewed chapati in test tube B. Both added a few drops of iodine solution. (Iodine turns blue-black when starch is present.) What colour will each test tube show, and why?
Test tube A (plain chapati): It will turn blue-black. The plain chapati still has its starch, and iodine turns blue-black when starch is present.
Test tube B (chewed chapati): It will show no colour change, or only a very light blue-black. While chewing, the saliva broke the starch into sugar. With little or no starch left, the iodine has little to react with.
So the experiment shows that saliva breaks down starch — a chemical change that begins right in the mouth.
Anil says, 'Respiration and breathing are the same process.' What question or example could you give to show him he is wrong?
You could ask Anil: “If breathing is just air moving in and out, then where does the energy from our food actually get released?”
This shows the difference:
- Breathing is only a physical process — air moving into and out of the lungs.
- Respiration is a chemical process inside the body, where oxygen breaks down food (glucose) to release energy, giving out carbon dioxide and water.
So breathing brings the oxygen in, but respiration is the step that actually releases energy. They are not the same.
Why do we have tiny hairs and mucus inside our nose, and why does this mean we should breathe through the nose rather than the mouth?
The tiny hairs and mucus inside the nose act like a filter. They trap dust and dirt from the air we breathe in, so cleaner air reaches the lungs.
If we breathe through the mouth, the air skips this filter, and more dust and germs can reach the lungs. That is why breathing through the nose is healthier — it cleans the air before it goes in.
Challenge
Paridhi and Anusha went running together. Afterwards, Anusha was breathing much faster than Paridhi. Give two possible reasons why Anusha was breathing faster.
When we exercise, our muscles work hard and need more energy. To release more energy, the body needs more oxygen and must remove more carbon dioxide. So we breathe faster during and after exercise.
Two possible reasons Anusha was breathing faster than Paridhi:
- Anusha may have run faster or harder, so her muscles needed more oxygen than Paridhi’s, and she had to breathe faster to supply it.
- Paridhi may be more used to running (fitter), so her body uses oxygen more efficiently and recovers faster, while Anusha (less used to it) needs to breathe faster to catch up on oxygen.
(Other sensible reasons, like Anusha carrying more weight or being more tired, are also acceptable — the key idea is that harder work or lower fitness means a greater need for oxygen, so faster breathing.)
A cow chews its food, swallows it, and yet you often see it chewing again much later when it is not eating anything fresh. Explain what is happening and why this helps the cow.
This is called rumination, and animals that do it (like cows and buffaloes) are called ruminants.
Here is what happens:
- The cow quickly chews the grass a little and swallows it into a part of its stomach (the rumen).
- There, the grass gets partly digested.
- Later, the cow brings this partly digested food back up to the mouth and chews it slowly and thoroughly again.
- The well-chewed food then goes back down for further digestion.
Why it helps: Grass is tough and hard to break down. By chewing it twice — once quickly, then slowly later — the cow breaks the tough grass into much finer pieces. This makes it far easier to digest and lets the cow get more nutrients out of a hard food. A cow may spend about 8 hours a day just chewing!
Summary
- Life processes are the jobs an animal must do to stay alive, such as nutrition, respiration, and circulation.
- Food has complex parts that must be broken into simple forms before the body can use them. This breaking down is called digestion, and it happens in a long tube called the alimentary canal.
- The path of food is: mouth → food pipe (oesophagus) → stomach → small intestine → large intestine → anus. The four steps are ingestion, digestion, absorption, egestion.
- In the mouth, teeth chew food (mechanical) and saliva starts breaking starch into sugar. The stomach churns food and adds digestive juice, acid, and mucus.
- The small intestine is the longest part (about 6 m); most digestion is finished here, and nutrients are absorbed into the blood through its finger-like folds. The liver (bile) and pancreas help. The large intestine (wider but shorter) absorbs water and forms stool.
- Breathing moves air in and out: ribs and the diaphragm make the chest bigger to inhale and smaller to exhale. Gas exchange happens in the tiny alveoli — oxygen goes into the blood, carbon dioxide comes out.
- Respiration is the chemical process where oxygen breaks down food to release energy (Glucose + Oxygen → Carbon dioxide + Water + Energy). Breathing is physical; respiration is chemical.
- Different animals are adapted to their food and habitat — ruminants chew twice, birds use a gizzard, fish use gills, frogs use gills, lungs and skin, and earthworms use moist skin. The circulatory system (heart, blood, blood vessels) carries nutrients and oxygen everywhere.
What’s Next
You now know how animals get food and energy. But what about plants? Plants do not eat or breathe the way we do, yet they are very much alive. So how does a plant make its food? How does it breathe? How does water travel from the roots all the way up to the leaves?
In the next chapter, Life Processes in Plants, we will explore how plants carry out their own life processes — including the amazing way they make their own food using sunlight. Get ready to see the green world in a whole new way.
Frequently Asked Questions
What is digestion and why does food need to be digested?
Digestion is the process of breaking down complex food (like proteins and carbohydrates) into simple, tiny molecules that the body can absorb and use. Food must be digested because the large, complex molecules in raw food are too big to pass through the walls of the intestine into the blood. Only the simple forms can be absorbed and carried to every cell.
What is the path food takes through the human digestive system?
Food travels in this order: mouth (chewing and saliva break it down) — oesophagus (pushes it to the stomach) — stomach (churns it and adds acid) — small intestine (most digestion and absorption happen here) — large intestine (water is absorbed) — rectum and anus (undigested waste is removed). This whole tube is called the alimentary canal.
What is the difference between breathing and respiration?
Breathing is the physical act of moving air in and out of the lungs — it is a mechanical process. Respiration is the chemical process inside cells where oxygen is used to break down glucose to release energy. You breathe to get oxygen into the lungs, but it is respiration at the cell level that actually releases the energy your body needs.
What are alveoli and why are there so many of them in the lungs?
Alveoli are tiny air sacs at the end of the airways inside the lungs. Oxygen from inhaled air passes into the blood through the thin walls of the alveoli, and carbon dioxide passes out. There are millions of alveoli in each lung so that together they have a huge surface area — this large area allows a lot of gas to be exchanged very quickly, giving the body enough oxygen.
What is rumination and which animals are ruminants?
Rumination is a special way of digesting food. A ruminant swallows grass quickly, stores it in the first part of its stomach (the rumen), and then brings it back up into the mouth as cud to chew it again slowly and thoroughly. This is called 'chewing the cud'. Cows, buffaloes, goats, sheep and deer are ruminants. Humans are not.