Class 7 Science Chapter 9: Life Processes in Animals

Look at What Animals Eat

Watch the animals around you and the variety is striking. Bees and sunbirds suck the nectar of flowers. Human infants and many other young feed on their mother milk. A python swallows its prey whole. Some water animals filter tiny food particles floating nearby.

One Thing They Share

Different as the meals are, the purpose is the same. Animals get the energy for every life process from their food. Without that energy nothing else a body does is possible.

But Food Arrives Complex

Food holds complex components: carbohydrate, protein and fat. The body cannot use any of them in that state. Each has to be broken down into a simpler form first, and that is the whole business of this topic.

Where the Breaking Happens

It happens inside one long tube, the alimentary canal. The tube starts at the mouth and ends at the anus. Food travels down it, and juices secreted at different points along the way take it apart as it goes.

Notice the Design

Nothing is done in one place and nothing is done all at once. The simpler forms are absorbed by parts of the canal and carried off to the rest of the body. So the tube both breaks food down and hands it over.

The First Stage Is Teeth

Your teeth crush and chew what you eat into fine pieces. That initial breaking of food into small pieces is called mechanical digestion. Nothing has been chemically changed yet; the pieces are just smaller, and smaller pieces are easier to work on.

Something Else Is Happening Too

Think of a food you love and your mouth waters, because more saliva gets released. Saliva is not only for wetting food. It carries a digestive juice, and that juice does chemistry.

Chew and Taste

Take a piece of chapati or a little boiled rice and chew it for thirty to sixty seconds. The taste changes. It begins to taste sweet, and that sweetness is the clue.

Why It Turns Sweet

Chapati and rice hold starch, which is a kind of carbohydrate. The juice in saliva breaks starch down into sugar. Chew long enough and you have made sugar in your own mouth, which is why starchy food sweetens as you chew.

Proving It With Iodine

Put boiled rice in tube A and chewed boiled rice in tube B, add a little water to each, then a few drops of iodine. Tube A turns blue-black. Tube B either does not change colour or changes only very slightly.

Reading the Result

Iodine goes blue-black with starch, which you met in Grade 6. So the colour in A says starch is there. No colour in B says the starch has gone, broken into simple sugars by saliva, and a faint colour says only a little is left.

Naming It

That breaking of complex food components into simpler forms inside the body is called digestion. And notice the careful word for the mouth: food there is only PARTIALLY digested. The work has started, not finished.

Down the Food Pipe

Saliva has also softened and moistened the food, which makes it easy to swallow. The tongue mixes it and pushes it into a long flexible tube, the food pipe or oesophagus. But food does not simply fall down it.

A Wave, Not a Drop

the walls of the food pipe gently squeeze and release in a rolling wave, and that wave pushes the food along. The same movement happens the whole length of the alimentary canal. It is why swallowing works even lying down.

Into the Stomach

The stomach walls contract and relax too, and this churns the food. That churned food then takes up a secretion given out by the stomach lining. That secretion has three parts, and each part has its own job.

Juice, Acid and Mucus

The digestive juice breaks the proteins of the food into simpler pieces. The acid helps break proteins as well, and it kills many harmful bacteria that arrived with the meal. The mucus protects the stomach lining from that acid, so the stomach is not damaged by its own defence.

A Lesson From an Accident

Nobody could see inside a working stomach until an accident made it possible. In 1822 Alexis St. Martin took a gunshot wound to the stomach. His doctor, William Beaumont, treated him, but the wound never healed fully and left a small permanent hole. Through it Beaumont watched digestion as it happened, tested how different foods were broken down, and even studied how feelings affected it.

What Leaves the Stomach

The food is partially digested here too, and it leaves as a semi-liquid mass, ready for the next stage. Two stages down, and the meal has been cut small, sweetened, churned and softened. The rest happens further along.

Scenario:

Three pupils test for starch. Sahil puts plain pieces of chapati in tube A. Neha chews her chapati well and puts that in tube B. Santushti takes boiled and mashed potato in tube C. Each adds a few drops of iodine. Sahil tube goes blue-black and Santushti tube goes blue-black as well. Neha tube barely changes at all. Sahil says the test has failed on Neha sample, because chapati obviously contains starch and her tube should look like his. Santushti says the test worked perfectly and Neha result is the most interesting of the three.

Analysis:

Santushti is right. Iodine goes blue-black with starch, so a blue-black tube reports starch present and a colourless one reports starch absent. Sahil tube A held plain chapati, which has its starch still in it, so it goes blue-black. Santushti tube C held mashed potato, which also holds starch, so it goes blue-black too. Neha tube B held chewed chapati, and chewing mixed it with saliva. The digestive juice in saliva broke that starch down into simple sugars, so there was little or no starch left for the iodine to colour. Her tube is not a failed test; it is the one that shows digestion actually happening. If her tube had turned faintly blue-black, a small amount of starch would still have been left, and chewing for longer would reduce it further.

Activity:

Repeat the activity at home with two spoons of cooked rice, chewing one for fifteen seconds and the other for a full minute. Add iodine to each and write down which is darker. Then write one sentence saying what the difference tells you about how long saliva needs.

Title:

Try it yourself: tasting the chemistry

Safety:

Use clean hands and fresh food, and rinse your mouth afterwards.

What You Notice:

The taste turns sweeter the longer you chew. The starch in the food is being broken down into sugar by the digestive juice in your saliva, so the sweetness you taste was made in your own mouth.

Chew rice long enough and it turns sweet, because saliva is already making sugar.

The acid in the stomach kills many harmful bacteria as well as helping with protein.

A hole left by a gunshot wound in 1822 is how digestion was first watched happening.

Juice, acid, mucus. Break the protein, kill the germs, guard the wall.

Teeth do the cutting, saliva does the chemistry. Both happen in the mouth.

A Surprise About the Name

The semi-liquid food leaving the stomach moves into the small intestine. Guess how long that is. It is almost six metres, which is nearly twice the height of your classroom, and it is the longest part of the whole alimentary canal.

So Why Small?

The name does not describe its length. It describes how narrow it is. Hold on to that, because the next organ has the opposite problem with its own name.

Three Sources of Help

The small intestine does not work alone. Three sources feed it digestive secretions: its own inner lining, plus the liver and the pancreas, two structures attached to the canal.

What the Liver Sends

Bile comes from the liver, and it is mildly basic. Recall neutralisation from the chapter on acids and bases. The food arriving from the stomach is acidic, and bile neutralises those acids.

Bile Has a Second Job

It also breaks fats down into tiny droplets. That does not finish digesting the fat, but it makes digesting it much easier, because small droplets are far easier to work on than one large mass.

What the Pancreas Sends

The pancreas secretes pancreatic juice, which is basic as well and helps neutralise the acids too. But it goes further than bile does: pancreatic juice breaks down carbohydrates, proteins and fats, all three.

And the Wall Itself

The juice that the wall of the small intestine gives out finishes the work. It goes on to reduce fats, proteins and part-digested carbohydrates to simpler forms. By now the meal really is simple enough to use.

Handing It Over

The digested nutrients pass from the small intestine into blood, which sits in blood vessels sitting within the walls of that intestine. That passing over is called the absorption of nutrients, and it is the point of all the breaking down.

A Problem of Area

But how do you pass a whole meal into blood through a tube wall? The answer is in the lining. It is thin, and it carries thousands of finger-like projections.

Why Fingers Help

Those projections increase the surface area, which makes absorption efficient. A smooth tube would offer very little area to absorb through; a tube lined with thousands of tiny fingers offers an enormous amount in the same length.

Where the Nutrients Go

Through the projections the nutrients cross into the blood, and it carries them to different parts of the body. There they give energy, back growth and repair, and keep the body work properly.

What Is Left Over

Most nutrients are digested and taken up in the small intestine. So what happens to the food that was never digested? It moves into the large intestine, which is about one and a half metres long.

The Second Odd Name

That makes the large intestine shorter than the small one, by about four times. It is called large because it is wider. So one name is about narrowness and the other about width, and neither is about length at all.

Reclaiming What Is Useful

The large intestine absorbs water and a few salts from the undigested food. That is worth doing: the water would otherwise be lost. Removing it leaves the waste semi-solid, and semi-solid waste is called stool.

Stored, Then Removed

The stool is held in the rectum, the lower end of the large intestine, until the body is ready to be rid of it. Finally it is expelled through the anus, and that process has its own name: egestion.

Fibre, and Some Helpful Residents

Eating fibre-rich food such as fruit, vegetables and unrefined grains helps the large intestine work, by making the stool easier to pass. The large intestine also holds small living organisms, bacteria among them, which break down undigested food, especially fibre, and produce essential nutrients.

A Note on What to Eat

The Charaka Samhita, an ancient Ayurvedic text, already stressed easily digestible food and the careful use of spices such as ginger, black pepper and cumin. Modern work on nutrition adds much the same advice: eat at proper times, eat mindfully, and avoid overeating. Curd and buttermilk, shrikhand, kanji and pickles, gundruk and poita bhat are all fermented foods, and they suit the digestive system well. So does fibre, which gives the bacteria of the large intestine something to work on.

When the Lining Is Damaged

All of this depends on a healthy lining. In celiac disease the body turns against gluten, a protein present in wheat, barley and rye, and the reaction damages the lining of that intestine, the very place nutrients are absorbed. Jowar, bajra, ragi and other millets carry none of it naturally. Avoiding gluten is the only way to manage the condition, since the intestine cannot then work properly.

Scenario:

A pupil in Grade 7 eats well and eats plenty, but she stays tired, grows slowly and often has stomach pain. Her family is puzzled, because the food is going in. A doctor asks what she eats and finds wheat chapati at nearly every meal, then tests her and finds celiac disease. Her uncle says the problem cannot be the food, since she eats more than her brother does. Her aunt says the amount eaten is not the question at all, and that the trouble is further along. The doctor tells the family to replace wheat with millets such as jowar, bajra and ragi.

Analysis:

The aunt has it right. Eating food and using food are two different things, and the step in between is absorption. In celiac disease the body turns against gluten, a protein present in wheat, barley and rye. That reaction injures the inner lining of that intestine, and the lining is exactly where nutrients are absorbed, through the thousands of finger-like projections that give it its surface area. With the lining damaged the intestine cannot function properly, so nutrients that were digested perfectly well are never taken into the blood. They pass on instead. That explains the tiredness and the slow growth: the nutrients provide energy and support growth and repair, and she is not receiving them. Eating more cannot fix a problem of absorption, which is why the uncle reasoning fails. The only way to manage the condition is to avoid gluten, and millets are a good substitute because they are naturally gluten-free.

Activity:

Look at the labels or the ingredients of five packaged foods in your kitchen and list which of them contain wheat, barley or rye. Then write down two millet-based dishes your family already eats, and one more you could try.

Title:

Try it yourself: measuring six metres

Safety:

Use blunt scissors with help, and keep the string off stairs.

What You Notice:

The 6 metre string reaches nearly twice the height of the room, and the 1.5 metre string fits into it four times. That is the small intestine against the large one, and it shows plainly that the word small was never about length.

The small intestine is about four times the length of the large one.

Thousands of finger-like projections in its lining are what make absorption efficient.

Bacteria living in the large intestine break down fibre and make essential nutrients.

Small is narrow, large is wide. Neither name is about length.

Liver gives bile, pancreas gives juice, and both of them are basic.

A Puzzle in the Field

Watch a cow that is not grazing and not eating anything, and it is still chewing. That looks odd. If the grass has already gone down, what is left in the mouth to chew?

Starting With a Partial Chew

The answer begins with how a cow eats. Grass-eating animals such as cows and buffaloes chew the grass only partially. Then they swallow it down into their stomachs, far less chewed than you would chew a mouthful of food.

A Stay in the Stomach

In the stomach, partial digestion of that grass takes place. Notice the word partial again. Grass is tough, and a quick chew followed by a short stay in the stomach is not nearly enough.

And Then It Comes Back

The partially digested food makes its way back to the mouth. There it is chewed gradually, and thoroughly this time. That is the chewing you see in a cow lying quietly in a field.

Naming It

This bringing back and chewing again is called rumination, and the animals that do it are called ruminants. Cows and buffaloes are the familiar examples, and both of them are grass eaters.

How Much Chewing

It adds up. A cow spends about eight hours a day just chewing its food. That is a third of the day given to one part of digestion, and it tells you how hard grass is to break down.

What Happens Next

Once it has been chewed thoroughly, the food passes down the alimentary canal again for further digestion. So the canal is not bypassed; the food simply makes one extra trip through its first stretch.

Now a Different Problem

Birds face the opposite difficulty. A bird has no teeth at all. Whatever it swallows arrives unchewed, because there is nothing in its mouth to chew with.

The Gizzard

A bird has a chamber called the gizzard. Food is broken down there by the contraction and relaxation of the gizzard walls, which squeeze and release against whatever is inside.

The Stones Make the Difference

That squeezing is often helped by grit, which is small stones the bird has swallowed. The stones are not eaten by accident. Inside the gizzard they grind against the food, doing the job that teeth would have done.

Compare the Three

Put the three side by side and the pattern shows. You chew in your mouth, once. A cow chews in its mouth twice, with a stomach stay between the rounds. A bird does not chew in its mouth at all, and the breaking happens lower down in a chamber with stones in it.

The Same Job, Moved

In every case food has to be broken into fine pieces before the chemistry can work on it. What differs is WHERE that happens and WHAT does it. The requirement is fixed; the arrangement is not.

Why the Differences Exist

All of this shows that animals have variations in how the alimentary canal is built and in what it does. Those variations let them digest different kinds of food in different ways. Grass needs two chewings; seeds need stones.

Not Faults, But Fits

It would be a mistake to read these as better or worse designs. A cow digestive system would be useless to a bird in flight, and a gizzard full of gravel would not help an animal that lives on grass. Each arrangement fits a diet.

A Day in the Life of a Cow

Put the number against a whole day and it is striking. Eight hours of chewing leaves sixteen for everything else: grazing, drinking, walking and resting. No animal would give a third of its life to one task unless the task were necessary. Grass simply will not yield to a single quick chew.

What the Grit Tells You

The swallowed stones are worth one more thought. They are not part of the bird body at all. A bird takes in something from outside itself, uses it as a tool inside the gizzard, and in time passes it on and swallows more. That is a solution no amount of growing teeth could have provided, and replacing a worn stone costs the bird nothing.

Where the Nutrients Go

Whatever the route, the end is the same. Nutrients out of the digested food travel to different parts of the body. Some of them build and repair the body itself.

And What the Rest Are For

Others, such as sugar get taken apart inside the body so that energy is set free. The process that turns a nutrient into energy the body can spend is called respiration, and that is where this chapter goes next.

Scenario:

A family keeps four hens and two buffaloes. The hens are kept on a clean cemented floor and fed only soft ground grain. The owner thinks stones in the feed are dirty and dangerous. The buffaloes are given chopped fodder. The owner complains that they lie about chewing for hours instead of eating properly. He wants to feed them something they can finish quickly. A neighbour who keeps the same animals says both ideas are mistakes. In each case, she says, he is working against the animal own digestive arrangement.

Analysis:

The neighbour is right on both counts. Take the hens first. A bird has no teeth, so the breaking of food cannot happen in its mouth. It happens in the gizzard, where the walls contract and relax, and that grinding is often helped by grit, which is small stones the bird swallows deliberately. A bird kept on a bare cemented floor with only soft feed has no access to grit at all, so the one tool its gizzard relies on has been taken away. Now the buffaloes. They are ruminants, like cows. They chew fodder only partially, swallow it for partial digestion in the stomach, and then bring it back to the mouth to chew gradually and thoroughly before it travels down the canal again. The hours of chewing the owner objects to ARE the digestion, not a delay before it. A cow spends about eight hours a day on exactly this. In both cases the lesson is the same: animals vary in how the alimentary canal is built and in what it does, and the keeper has to feed the arrangement the animal actually has.

Activity:

Watch a cow or buffalo for ten minutes and count how many seconds it spends chewing without taking any new fodder. Then look for a hen or pigeon pecking at the ground and note whether it picks up anything that is clearly not food. Write two sentences on what each animal was doing and why.

Title:

Try it yourself: grinding without teeth

Safety:

Keep the lids tight and wash the pebbles and your hands first.

What You Notice:

The grain shaken with pebbles is noticeably finer. The jar stands for a gizzard and the shaking for its walls contracting and relaxing, and the difference the pebbles make is why a bird swallows grit on purpose.

A cow spends about eight hours of every day simply chewing its food.

Birds swallow small stones on purpose, because the stones do the grinding.

Rumination means the same mouthful is chewed twice, with a stomach stay between.

Cow chews twice. Bird chews never, and swallows stones instead.

Teeth, a second chewing, or a bag of pebbles. Three ways to one end.

Something You Cannot Stop For Long

You breathe in and you breathe out, without ever deciding to. Try holding it and the limit is obvious. A body can manage a week without food and a day or two without water, but without breathing we usually last no more than a few minutes.

Not Only Us

All living beings respire, which you met in Grade 6. Plants breathe, and so do other animals. So the question is not whether a living thing breathes but how its body is arranged to do it.

A System With a Route

Just as food follows a particular pathway through the digestive system, air follows a particular pathway too. The parts that carry it are together called the respiratory system, and the route runs the same way every time.

Stop One: the Nostrils

The system starts at a pair of nasal openings, the nostrils. Air is both inhaled and exhaled through them, so the same opening serves both directions.

Why the Nose and Not the Mouth

Look inside a nostril and there is tiny hair. That hair, together with mucus, traps dust and dirt out of the air you breathe in. A mouth has no such filter, which is exactly why we should breathe through the nose.

Stop Two: the Nasal Passages

From the nostrils the inhaled air moves into a pair of narrow ways, the nasal passages. From there it heads downwards towards the lungs.

Stop Three: the Windpipe

The air reaches the lungs through the windpipe. The windpipe does not enter one lung; it forms two branches, and one branch enters each of the two lungs.

Stop Four: Branch After Branch

Inside each lung those branches divide again into smaller and finer branches. It is a tree that keeps splitting, and every split leaves the passages narrower than before.

Stop Five: the Alveoli

Each of the finest branches ends in a small balloon-like sac called an alveolus, and there are a great many of them. The alveoli are where the route finally stops, and the next topic shows why they matter so much.

What Surrounds the Lungs

Two other parts belong to the picture. The lungs are protected by the rib cage, a frame of bone around them. And below the lungs lies the diaphragm, a dome-shaped muscle.

Building a Model

The best way to see how breathing works is to make it. Take a wide transparent bottle and cut away its bottom. Then pierce a hole in the lid and push the straight end of a Y-shaped hollow tube through it, sealing the lid with clay. On the forked end fix two deflated balloons, tied airtight with rubber bands. Over the open base stretch a thin rubber sheet, held with a large band.

Working the Model

Now pull the rubber sheet downwards from the centre and watch. The balloons inflate. Release the sheet upwards and the balloons deflate. Nothing was blown into them; the only thing you touched was the sheet at the bottom.

Reading the Model

In the model, the balloons stand for the lungs and the rubber sheet stands for the diaphragm. Pulling the sheet down made space inside the bottle, and air moved into the balloons to fill it. That is the whole mechanism in one object.

Inhaling, in the Body

When you breathe in, your chest expands, because the ribs move up and outwards. At the same time the diaphragm moves downwards. Together those two increase the room inside the chest, so air moves into the lungs.

Exhaling, in the Body

When you breathe out, both movements reverse. The ribs move down and in while the diaphragm rises. That reduces the space inside the chest and pushes the air out of the lungs.

What That Tells You

Notice what is doing the work. The lungs are not muscles and they do not suck. The chest around them changes size, and the air follows. Which is why the diaphragm, a muscle you never think about, matters more to a breath than the lungs themselves.

Why Any of This Can Fail

A route this fine can be interfered with. During the COVID-19 pandemic, a virus called SARS-CoV-2 struck at the respiratory system, leading to breathing difficulties and often to serious lung problems. Much dust is filtered out on the way in, but small infectious particles can still get through to the lungs.

Looking After It

Breathing exercises have been practised for centuries. Pranayama has a name for improving respiratory health, relaxing the mind and helping concentration. In Ladakh people practise Tummo breathing, which builds lung function and helps hold the body warm in cold weather.

Scenario:

Four people wait at a bus stop. One of them smokes, standing under the shelter with the others. A woman carrying a baby moves away and asks him to step out from under the roof. The smoker says the choice is his own and affects nobody else, since he is the only one putting anything in his mouth. An older man agrees with him and adds that the smoke rises away anyway. A college student says both of them have the biology wrong, and that the woman with the baby had the strongest reason of anyone there to move.

Analysis:

The student is right. Smoking is extremely harmful to the smoker himself. It damages the lungs and raises the risk of serious diseases, lung cancer among them. It also leads to persistent coughing and frequent infections. But the claim that it affects nobody else is simply false. Smoking releases toxic chemicals into the air, so everyone sharing that air is at risk. When non-smokers breathe in that polluted air they experience passive smoking. And here the route of air matters. The nostrils trap dust and dirt with their hair and mucus. But that filter is for particles. It does not clean out toxic chemicals mixed into the air itself, which travel down the windpipe into the alveoli with every breath. Passive smoking is most dangerous of all for children, for pregnant women and for the elderly. That is exactly why the woman carrying a baby was right to move away and to ask him to step out.

Activity:

Find out whether there is a no-smoking sign at your nearest railway station, bus stand or hospital, and write down where it is placed. Then write two sentences explaining, in terms of shared air, why the rule covers the whole waiting area and not just one corner of it.

Title:

Try it yourself: feel the mechanism

Safety:

Breathe normally and do not hold your breath or breathe very fast.

What You Notice:

The ribs move up and outwards as you breathe in and down and inwards as you breathe out, and the chest measurement is larger on the way in. That difference is the extra space that air moves into.

A body survives a week without food, days without water, but only minutes without air.

The tiny hair in your nostrils works with mucus to trap dust before it reaches the lungs.

In the bottle model, the rubber sheet you pull is standing in for your diaphragm.

Breathing in: ribs up and out, diaphragm down, more room inside.

Nostrils, passages, windpipe, branches, alveoli. One route, five stops.

A Question You Can Test

You breathe air in and you breathe air out. Is the air that comes out the same as the air that went in? That is not something to settle by arguing. It is something to test.

The Lime Water Test

Take equal amounts of lime water freshly made up in two test tubes, A and B. Into tube A pass air with a syringe or a pichkari, which is the same air you inhale. Into tube B blow air repeatedly through a straw, which is the air you exhale.

What You See

The lime water in tube B turns milky, or cloudy. The lime water in tube A does not. Both tubes held the same liquid, so the difference must be in the air.

Reading the Result

Lime water goes milky on meeting carbon dioxide. So the milkiness in B reports carbon dioxide, and its absence in A reports much less of it. The air breathed out carries more carbon dioxide than the air we take in.

Where the Swap Happens

Something inside you is adding that carbon dioxide. Fresh air travels down the pathway and fills the alveoli, the balloon-like sacs at the ends of the finest branches. Those sacs have thin walls, and the walls are surrounded by fine tubes containing blood.

Two Things at Once

Blood arrives carrying carbon dioxide from the body, and releases it into the air in the alveoli. At the same moment oxygen from that air passes into the blood. One swap, in both directions, across a thin wall.

And Then It Travels

The blood that has taken up oxygen is carried away to every part of the body. So the air you breathed in reaches a finger or a brain not as air, but as oxygen carried in blood.

Now the Real Question

Why does the body want oxygen at all? The answer joins this topic to the first half of the chapter. When you eat, the body breaks the food down into simple substances such as sugar, which is glucose.

Oxygen Meets Glucose

Oxygen helps break that glucose down, and breaking it down releases energy. That process is called respiration. So food supplies the fuel and breathing supplies the oxygen, and neither is any use without the other.

The Word Equation

Written out, respiration is this: glucose plus oxygen gives carbon dioxide plus water plus energy. Read the right-hand side and the chapter joins up. The carbon dioxide is what the lime water detected, and the energy is the point of the whole thing.

What the Energy Is For

That energy lets you walk, run, play and even think. Thinking is on the list deliberately. There is no activity of a body, however quiet, that does not run on energy released this way.

Counting the Oxygen

Now look at the numbers. Inhaled air is nearly 21 per cent oxygen. Exhaled air is nearly 16 to 17 per cent oxygen. So you use only about four or five parts in a hundred, and most of the oxygen goes straight back out again.

Counting the Carbon Dioxide

The other gas moves the other way, and far more sharply. Inhaled air holds nearly 0.04 per cent carbon dioxide. Exhaled air holds nearly 4 to 5 per cent. That is about a hundred times as much, which is why the lime water test is so easy to see.

An Honest Admission

Not all the oxygen is used up, then. And certain other animals draw on a larger share of the oxygen during respiration than we manage. Our arrangement works; it is not the most thorough one there is.

Two Words That Are Not the Same

Here is the distinction the chapter most wants you to keep. Breathing fetches oxygen in and carries carbon dioxide away. Respiration uses that oxygen to take food apart and set its energy free. One is transport; the other is the reaction transport exists for.

Physical, and Chemical

Say it another way. Breathing is a physical process, because air is moved about and nothing is turned into anything else. Respiration is a chemical process that happens inside the body, because glucose and oxygen become carbon dioxide and water. Both are essential for survival.

Who Does the Carrying

One system is left to name. The heart, the blood and the blood vessels together make up the circulatory system, which the body uses to move nutrients, oxygen and other substances about. The heart pumps blood through the vessels, delivering oxygen and nutrients to every part while waste products are carried away.

Scenario:

Three pupils are arguing after the lime water demonstration. Anu says that we inhale air. Shanu says that we inhale oxygen. Tanu says that we inhale air rich in oxygen. Their friend Anil, listening in, says the three statements all mean the same thing. He adds that breathing and respiration are two words for one process, since both are about oxygen. The teacher asks the class to settle both questions using the figures for inhaled and exhaled air that they have just written down.

Analysis:

The figures settle both arguments. Take the three claims first. Anu is not wrong, but she is vague: we do inhale air. Shanu is wrong, because air is a mixture and oxygen is only part of it; inhaled air is nearly 21 per cent oxygen, so we are plainly not inhaling oxygen on its own. Tanu is the most accurate of the three, because air rich in oxygen says both things at once, that a mixture goes in and that oxygen is the part of it we need. So the three do not mean the same thing. Now Anil second claim. Breathing fetches oxygen in and carries carbon dioxide away, which is movement of air and nothing more, so it is a physical process. Respiration uses that oxygen to break down glucose, giving carbon dioxide, water and energy, so new substances are formed and it is a chemical process. The figures show the gap between them: exhaled air still holds nearly 16 to 17 per cent oxygen, so breathing has delivered a great deal more oxygen than respiration has consumed.

Activity:

Write the two percentages for oxygen and the two for carbon dioxide in a small table of your own. Then work out the difference for oxygen and the number of times carbon dioxide has multiplied, and write one sentence saying which of the two changes is the larger and why that matters for the lime water test.

Title:

Try it yourself: counting breaths before and after

Safety:

Stop at once if you feel dizzy, and do not run on a slippery floor.

What You Notice:

The count is far higher after running. Working muscles need more energy, so more glucose must be broken down, so more oxygen is needed and more carbon dioxide has to be removed. Breathing faster is how the body supplies the respiration it is doing.

Exhaled air still holds about 16 to 17% oxygen, so not all of it is used.

Carbon dioxide in exhaled air is about a hundred times what came in.

Certain other animals draw on a larger share of the oxygen they breathe than we can.

Breathing moves the air. Respiration burns the sugar. Two words, two jobs.

Glucose and oxygen in; carbon dioxide, water and energy out.

Same Need, Different Places

Every animal has to take oxygen in and get carbon dioxide out. But animals live in very different habitats. A bird flies and a fish swims, and the arrangement that works in air will not work under water.

The Familiar Case

A great many animals breathe through lungs, as you do. Lizards, snakes, birds, cows, goats, lions and elephants all use lungs. That is a list with very little else in common, which makes the point nicely.

But Lungs Are Not All Alike

Here is a detail worth stopping on. Although all of those animals have lungs, those lungs are built quite differently. Having the same organ is not the same as having the same organ built the same way.

Now Go Under Water

Most aquatic animals, including fish, do not have lungs. They have specialised structures known as gills. A fish has gills on both sides of its head, and water passes over them as the fish swims.

What Gills Are Like

Gills are richly supplied with blood vessels. That matters, because there is no point having a surface for exchange if no blood arrives to take the oxygen away.

What a Fish Actually Breathes

Now a point that is often got wrong. A fish does not breathe water. The exchange of oxygen and carbon dioxide happens between the blood and the gases held in solution in the water, and it takes place across the gills. Water is the carrier, not the thing taken in.

The In-Between Animals

Amphibians, like frogs, are at home on land and in water alike. That creates a problem no fish or elephant has to solve. The same animal needs to breathe in two very different places.

One Life, Three Surfaces

A frog solves it by using different body parts at different stages of its life. A tadpole breathes through gills. An adult frog uses lungs for breathing on land. And when the adult stays in water, it uses its skin for gas exchange.

Think About What That Means

The organ changes as the animal grows, and then changes again depending on where the adult is. That is three breathing surfaces across one life, which is as neat an example of adaptation as the chapter offers.

Why It Helps

This adaptation helps a frog survive in both water and on land. It shows how animals have fitted themselves over time to different surroundings, rather than being stuck with a single arrangement.

The Simplest Case of All

Earthworms go further still. They have no lungs and no gills. Earthworms use their moist skin to take in oxygen and give out carbon dioxide, and that alone is enough for them.

Notice the Word Moist

The word moist is doing real work in that sentence. A dry skin would not serve. This is the same reason an adult frog can use its skin in water and not while sitting dry in the sun.

What All of Them Share

Line the cases up and a pattern appears. Your alveoli are thin-walled and surrounded by fine tubes containing blood. Gills are richly supplied with blood vessels. An earthworm skin is moist. Every breathing surface is thin, kept wet, and well served with blood.

So the Differences Are Not Random

Different animals have different breathing mechanisms to suit their unique habitats. The requirement is fixed, as it was with digestion. What varies is the structure that meets it, and where on the body that structure sits.

Why a Gill Must Sit Outside

Notice where each surface is placed. Your alveoli lie deep inside a chest, protected by a rib cage, because the air they need is easy to bring in to them. A gill has to sit where water flows across it, since what it uses is dissolved in that water and must be carried past. So the position of a breathing surface follows from what it exchanges with, and so does the protection it can be given.

The Skin Cases Are Not Odd

It is tempting to treat skin breathing as a makeshift. It is not. An earthworm manages on nothing else at all, and an adult frog chooses it over its own lungs whenever it stays in water. A surface that is thin, moist and well supplied with blood will serve, wherever on the animal it happens to lie.

A Closing Reminder

This chapter has described three systems: the digestive, the respiratory and the circulatory systems. There are other systems besides, and they all work in coordination with each other to sustain life. You will meet them in higher grades.

Scenario:

A class keeps a small aquarium and a garden bed. In winter a pupil decides to help both. She lifts two fish out of the tank into a dry tray so they can "get some fresh air", since she has read that air holds more oxygen than water. At the same time she spreads dry sand over the garden bed to keep the earthworms from getting muddy. Within minutes the fish are in distress, and over the next day the worms move away or die. She is upset and says she had the biology right, because air really does hold more oxygen than water does.

Analysis:

Her fact about air is true, and it is beside the point, because neither animal can use oxygen in that form. Take the fish. Most aquatic animals have gills rather than lungs, and gills are richly supplied with blood vessels so that blood can collect what crosses them. What crosses them is oxygen and carbon dioxide dissolved in water, exchanged with the blood. A gill in open air has nothing flowing over it that it can take dissolved oxygen from, so the surface is useless however much oxygen the air around it holds. Now the earthworms. An earthworm has no lungs and no gills; it uses its moist skin to take in oxygen and give out carbon dioxide. Covering the bed with dry sand dries that surface, and a dry skin cannot exchange gases at all. In both cases she removed the condition the surface depends on. Different animals have different breathing mechanisms to suit their own habitats, and moving an animal out of its habitat takes its breathing with it.

Activity:

Look at a damp patch of garden soil early in the morning and then the same patch in the afternoon sun, and note whether you can find any earthworms near the surface at each time. Write two sentences linking what you saw to the need for a moist skin.

Title:

Try it yourself: why a surface must stay wet

Safety:

Keep water away from any electrical fan socket, and ask before using a fan.

What You Notice:

The exposed square dries far sooner. An earthworm skin faces the same problem, and so does the skin of a frog. Gas exchange needs a wet surface. A drying surface is a failing one, which is why these animals keep to damp places.

A fish does not breathe water; it uses the oxygen dissolved in the water.

A frog breathes through three different parts over the course of one life.

All these animals have lungs, yet the structure of those lungs is quite different.

Thin, wet, full of blood vessels. Every breathing surface, without exception.

Tadpole gills, frog lungs, frog skin. Three surfaces in one life.