Class 7 Science Chapter 10: Life Processes in Plants

Growth You Can See

Look at any plant you pass often and the changes add up over months. New leaves and branches appear, the height goes up. Its stem thickens. Growth in a plant shows itself in several ways at once, not just one.

The Animal Comparison

In animals growth looks different. As an animal grows, its size and weight usually increase and its body goes through various changes. You know too that animals eat food in order to grow. The question is what the matching story is for a plant.

What Food Supplies

Food provides nutrients: carbohydrates, fats, proteins, vitamins and minerals. Those, together with water, are every one of them needed for growth. So whatever a plant does, it has to end up with the same kind of supply.

Three Guesses

Ask a class and three answers come up. Plants grow better when watered regularly, so water contributes. Sunlight plays some part. And perhaps plants draw ready-made food out of the soil by their roots. All three are reasonable, and guessing settles nothing.

So Test Two of Them

Take three earthen pots of the same size, filled with garden soil. Plant saplings of similar size of a fast-growing plant such as chilli or tomato in each. Label the pots A, B and C, and count the leaves on each sapling before you begin.

Why All That Sameness Matters

Notice how much is being held the same: pot size, soil, kind of plant, size of sapling. That is deliberate. If the pots differed in several ways, a difference in growth could not be traced to any one cause.

What Each Pot Gets

Pot A goes in direct sunlight, and its soil is kept slightly moist with water every day. Pot B goes in direct sunlight too, but gets no water. Pot C is kept in the dark, and its soil is watered every day like A.

Read the Design

A has both things. B is missing water. C is missing sunlight. So each of the two conditions is removed once, separately, while everything else stays as it was. That is what makes the result worth anything.

What to Record, and For How Long

Observe the plants for two weeks. Record the height, the number of leaves, the colour of the leaves, and any other change that appears. Two weeks is long enough for a fast-growing sapling to show a clear difference.

What You Find

The plant in pot A, with direct sunlight and enough water, outgrows the plant in pot C, which had plenty of water but no sunlight. And the sapling in pot B may well be dead altogether, since it received no water even though it had all the sunlight it needed.

Reading Pot B

Pot B is the sharpest result of the three. Sunlight alone was not enough to keep the plant alive. So sunlight cannot be a substitute for water, however much of it a plant is given.

Reading Pot C

Pot C makes the other half of the point. It was watered every day and still grew far less than A. So water alone is not enough either, and the two conditions are not interchangeable.

The Inference

Put both together and the conclusion follows: a plant must have sunlight as well as water in order to grow. Neither on its own will do, which is exactly what two separate pots were needed to show.

What the Activity Did Not Test

The third guess is still untested. Nothing here tells you whether plants take food from the soil through their roots. Keeping track of what an experiment has NOT shown is as useful as reading what it has.

Where Food Comes From

Here is the odd thing about plants. Animals take their food from plants, either by eating them, or indirectly by eating animals that eat plants. But plants, unlike animals, do not eat food at all. How they come by it is the business of the next topic.

Written Down Long Ago

Much of this was worked out by patient watching long before any laboratory existed. An ancient Indian text, the Vrikshayurveda, says that planting a tree does not of itself bring any fruit or flower. Planting is a beginning, not a guarantee. The text records observations about plant growth, about soil and about farming practice. Those observations were built up from practical experience and from patterns noticed over many seasons. They were then written down in an orderly way, so that they could guide farmers. It even gives methods of preparing organic manure, by mixing water, barley and seeds such as green, black and horse grams.

Scenario:

Two cousins each keep four chilli saplings on a balcony. Meera waters hers every morning and keeps them at the open end, where the sun falls for most of the day. Ravi waters his every morning too, but keeps them pushed back against the inner wall, under a solid concrete slab where no direct sun reaches. After three weeks Meera plants are tall and dark green with many new leaves, and Ravi plants are thin, pale and barely taller than when he started. Ravi says the difference must be the soil, since he waters exactly as often as Meera does. His grandmother says he has arranged the classroom experiment without meaning to, and that he already knows the answer.

Analysis:

The grandmother is right, and the arrangement is almost exactly pots A and C. Both cousins water daily, both use similar saplings of the same fast-growing plant, and both keep them on the same balcony. The one condition that differs is direct sunlight: Meera plants get it and Ravi plants, under a solid slab, do not. That is what makes the comparison worth anything, and it is why Ravi cannot blame the soil. To blame the soil he would need a case where the soil differed and everything else matched, and he has no such case. What he does have is the standard result. A plant given plenty of water but no sunlight grows far less than one given both, which is precisely what pot C shows against pot A. Plants require both sunlight and water, and daily watering cannot make up for a slab overhead. Ravi fix is not better soil; it is moving the pots to the open end.

Activity:

Find two plants of the same kind near your home, one in open sunlight and one in deep shade, and write down the height and the number of leaves on each. Then list every other difference between the two spots that you can see, and write one sentence saying why that makes your comparison weaker than the classroom one.

Title:

Try it yourself: the two-week comparison

Safety:

Keep the dark cupboard away from food, and wash your hands after handling soil.

What You Notice:

A grows tallest and greenest. C stays short and pale although it was watered. B wilts and may die. The two failures are different, and together they show that sunlight and water are both needed and that neither can stand in for the other.

Vrikshayurveda says trees do not bear fruit merely because somebody planted them.

The activity counts the leaves BEFORE it starts, so the change can be measured later.

Plants do not eat anything, yet every animal food can be traced back to a plant.

A has both. B has no water. C has no sunlight. Both are needed.

Taller, bushier, thicker. Three signs of one plant growing.

Where the Food Is Kept

Plants store their food in the form of starch, which is a kind of carbohydrate. And that starch is produced in the leaves. So if you want to know how a plant comes by its food, the leaf is where to look.

Look at the Shape

Leaves are generally broad and flat, and that is by design rather than by accident. A broad flat surface catches as much light as possible for the amount of leaf there is.

And the Colour

Leaves are mostly green because of a green pigment called chlorophyll. Chlorophyll helps the leaf capture sunlight efficiently, so the colour is not decoration; it is part of the machinery.

Testing a Leaf for Starch

There is a way to ask a leaf whether it holds starch. Keep the leaf in boiling water for five minutes, which softens it. Then dip it into a test tube containing alcohol.

Why the Water Bath

Stand that test tube in a beaker of boiling water, and leave it until the leaf becomes colourless. The alcohol is never heated directly over a flame, because alcohol is highly flammable and can easily lead to fire and burns. The hot water heats it safely instead.

Then the Iodine

Take the colourless leaf out and lay it on a plate. Put a few drops of thinned iodine solution on it with a dropper, leave it a few minutes and watch. If the leaf turns blue-black, starch is present.

Why Remove the Colour First?

That step is easy to skip and the test fails without it. Blue-black is hard to see against green. Decolourising the leaf lets you observe the colour change clearly, and so read the presence of starch.

A Cleverer Question

Knowing a green leaf holds starch is one thing. Bhaskar wanted to know whether sunlight has anything to do with making it. So he needed a comparison, and he built two into one experiment.

His Two Plants

He took two similar potted plants. One he kept in sunlight. The other he kept in the dark for thirty-six hours. Everything else about them was alike, so light was the only difference between the two.

And His Two Patches

From each plant he took a leaf carrying patches both green and not. That gives a second comparison inside a single leaf: chlorophyll against little or no chlorophyll, with the same leaf, the same plant and the same light.

The Tracing Paper

Before testing, he sketched each leaf on tracing paper to record exactly where the green and non-green patches lay. That matters, because the test takes the colour out. Afterwards there would be no way of telling which patch had been which.

What He Found, in Sunlight

On the leaf from the plant kept in sunlight, the green patches turned blue-black, so starch was present there. The non-green patches of the same leaf did not turn blue-black.

What He Found, in the Dark

On the leaf from the plant kept in the dark there was no change in colour at all, not even on its green patches. No starch had been produced in that leaf.

Reading the Non-green Patches

Does that mean there is no chlorophyll in a non-green patch? Not quite. The non-green patches may not hold sufficient chlorophyll to prepare enough starch for the iodine test to detect. The test finds starch, not pigment.

Putting the Four Results Together

Green and lit gives starch. Green and dark gives none. Non-green and lit gives none. So neither chlorophyll alone nor sunlight alone will do. Chlorophyll helps prepare starch IN THE PRESENCE OF sunlight, and it is essential for preparing it.

Hence the Name

A leaf takes in light, holds the pigment, and turns out starch. That is why leaves are called the food factories of plants, and why the first thing a plant scientist does with a leaf is test it.

Why Starch and Not Sugar

One more thing the test settles quietly. It finds STARCH, which is the stored form of the food. A store is what a plant builds when it has made more than it needs at once. So a blue-black patch is not only evidence of work done; it is evidence of work done in surplus.

When a Leaf Is Not Green

Some leaves look red, violet or brown. They hold more of those coloured pigments than they hold green chlorophyll, and the colour hides the green. Some of those pigments also help in photosynthesis. An iodine test on such a leaf can still turn blue-black, which shows that photosynthesis has indeed taken place there.

Scenario:

Two pupils run the starch test on leaves from the same healthy money plant on a sunny windowsill. Aarti softens her leaf in boiling water, stands it in alcohol over a water bath until it goes pale, lays it on a plate and adds iodine. Hers goes blue-black. Nikhil is in a hurry. He skips the alcohol, drops iodine straight onto his bright green leaf, sees no clear change, and writes down that the plant contains no starch. He then argues that money plants must get their food some other way, since two leaves of one plant cannot differ. Their teacher says his reasoning is sound and his conclusion is still wrong.

Analysis:

The teacher is right on both counts. Nikhil reasoning is sound: two leaves from one healthy plant in the same light should indeed hold starch alike, so a real difference between the results would be strange. That is exactly why he should have suspected his method rather than the plant. Iodine turns blue-black where starch is present, but blue-black is extremely hard to see against a bright green background. The alcohol step exists for that one reason: decolourising the leaf lets the colour change, and so the presence of starch, be observed easily. Skipping it does not stop the iodine working; it stops the result being readable. So Nikhil has not shown that his leaf lacks starch. He has shown nothing at all, which is a different and less useful outcome. His conclusion about money plants getting food some other way does not follow, and Aarti result from the same plant settles the matter.

Activity:

Take two leaves from the same plant and test only one of them properly, decolourising it first, while dropping iodine straight onto the other. Place the two side by side and write two sentences on which result you could actually read, and why.

Title:

Try it yourself: reading a patchy leaf

Safety:

Press gently so the leaf is not torn, and do not pick leaves from a neighbour garden.

What You Notice:

Your prediction should mark the green areas and leave the pale ones blank. Starch is produced where chlorophyll is present. The tracing is exactly what Bhaskar needed. A real test takes the colour out, leaving no way to tell the patches apart.

The leaf has to lose its green before the test can show anything at all.

Bhaskar traced the patches on paper first, because the test destroys the colours.

A red or violet leaf can still pass the starch test, so it is photosynthesising.

Soften, decolourise, then iodine. Colour out first, colour in after.

Green patch in sunlight is the only one that turns blue-black.

One Ingredient Still Missing

So far sunlight, water and chlorophyll are all known to matter. But air has been left out, and air is where the next ingredient comes from. The question is which gas in it a plant needs.

First, Empty the Plant

put a potted green plant away in the dark for two or three days. That lets it destarch, which means lose any starch it had stored. Only then pick the leaf you will use.

Why That Step Comes First

Without destarching, a leaf would already hold starch from before the experiment began. The iodine test cannot tell old starch from new. So the plant has to be emptied, or the result means nothing.

The Bottle and the Chemical

fetch a wide-mouthed bottle and pour in some caustic soda into it, which is sodium hydroxide. Caustic soda takes carbon dioxide out of the air, so the air inside that bottle is left with almost none.

A Word of Warning

Caustic soda is a harsh chemical, well able to burn skin. Only a teacher should handle it. This is one of the activities to watch rather than to run yourself.

Half In, Half Out

Now the clever part. Push half the destarched leaf inside the bottle, held by a split cork, with the other half of the same leaf outside. Then place the apparatus in sunlight for some hours.

Why One Leaf and Not Two

Think about what is the same for both halves. The same plant, the same leaf, the same water reaching it, the same sunlight falling on it, and the same chlorophyll. Only one thing differs: the half inside the bottle has no carbon dioxide around it.

Test It and Read It

Remove the leaf and run the iodine test on the whole of it. The part that had been outside the bottle darkens to blue-black, so starch is present there. The part that was inside does not turn blue-black at all, so no food was made in it.

The Conclusion

The only difference between the two halves was the carbon dioxide, so that must be the cause. The experiment shows that the carbon dioxide of the air is essential if a plant is to make starch.

Naming the Whole Process

Four things are now known to be necessary: sunlight, water, chlorophyll and carbon dioxide. The process in which a plant makes its food, using sunlight and chlorophyll has the name photosynthesis. A leaf is its primary site, and other parts of the plant that hold chlorophyll carry it out as well.

But Does Anything Come Out?

Everything so far is about what goes in. Barkha didi asked the other question. She set up two beakers, each with a funnel over a water plant and an inverted test tube above the funnel. She put set-up A in sunlight and set-up B in the dark.

The Bubbles

In set-up A, air bubbles emerge from the plant, rise, and gather in the upturned test tube of set-up B, kept in the dark, they do not. Something is being produced, and only where there is light.

Testing the Gas

When enough gas had collected, she put her thumb over the mouth of the test tube while lifting it off the set-up, so the gas could not escape. Then she put a lit matchstick in at once, and it produced an intense flame.

What the Flame Means

A flame that leaps up like that is burning in gas rich in oxygen. So oxygen is released during photosynthesis. And since the bubbles came only from the set-up in sunlight, the experiment confirms that photosynthesis happens in the presence of sunlight.

What Is Actually Made

Now the food itself. In photosynthesis the food that really appears is glucose, which is a simple carbohydrate. Glucose serves as an instant source of energy, and later it gets converted into starch for storage.

That reorders what the earlier topic suggested. The iodine test finds starch, and starch is real, but it is the stored form. What photosynthesis makes first is glucose, and the starch comes afterwards.

The Word Equation

Put it all in one line. Carbon dioxide plus water gives glucose plus oxygen, with sunlight and chlorophyll written above the arrow. The two on the left are the raw materials, the two on the right are the products, and the two above the arrow are what the process needs without being used up as ingredients.

Each Experiment in the Equation

Every experiment of this chapter now has a place in that line. The three pots gave sunlight and water. The patches gave chlorophyll. The split leaf gave carbon dioxide. The water plant gave oxygen. The equation is the summary of four experiments, not a fact to learn on its own.

An Indian Who Studied It

Many scientists worked out this picture between them. In India, Rustom Hormusji Dastur lived from 1896 to 1961. A plant scientist, he led the Botany Department at what was then the Royal Institute of Science, Bombay, and is now the Institute of Science, Mumbai. He held that post from 1921 to 1935. He studied the effects of the amount of water and of temperature on photosynthesis, and examined how water, temperature and even the colour of the light matter to it.

Scenario:

A pupil wants to prove that a plant needs air. She destarches a potted plant properly, then shuts one whole leaf inside a clear jar of caustic soda, leaving no part of it outside, and seals the jar with a split cork. She takes a second leaf from a DIFFERENT plant on a shelf across the room as her comparison. After four hours in sunlight she tests both and finds the jarred leaf colourless and the other blue-black. She writes that carbon dioxide is essential. Her partner says the result is probably right but the experiment cannot show it, and that she has thrown away the one feature that made the textbook version work.

Analysis:

The partner is right. The conclusion is true. But this experiment does not establish it, and the reason matters more than the conclusion. In the textbook version, HALF of one leaf goes into the bottle and the other half of the SAME leaf stays outside. The plant, the leaf, the water reaching it, the sunlight on it and its chlorophyll are then identical for the two halves. Carbon dioxide is the only thing that differs. So any difference in the iodine result has one possible cause. Here she has compared a leaf on one plant with a leaf on another plant across the room. Those two leaves may differ in the sunlight they received. They may differ in how well watered their plants were, in their chlorophyll, and in whether the second plant was destarched. With several differences at once, a difference in the result cannot be pinned on any one of them. Her fix is simple: use one leaf, half in and half out.

Activity:

Draw her set-up and the textbook set-up side by side. Under each, list every condition that is the same for the two parts being compared. Then write one sentence saying which drawing has fewer differences, and why that matters.

Title:

Try it yourself: bubbles from a water plant

Safety:

Do not test any collected gas with a flame; leave that to your teacher.

What You Notice:

Gas collects in the tube on the sunny windowsill. None collects in the one kept in the dark. That gas is rich in oxygen, released during photosynthesis. And only the lit set-up produced any, which confirms that the process needs sunlight.

Glucose is what is really made; starch is what it is turned into for storage.

The gas collected over a water plant makes a lit matchstick flare up brightly.

R. H. Dastur studied how the very colour of light affects photosynthesis.

Carbon dioxide and water in. Glucose and oxygen out. Sunlight and chlorophyll above the arrow.

Half in, half out. Same leaf, same light, one gas missing.

A Gap in the Story

Photosynthesis takes in carbon dioxide and gives out oxygen. So gases have to get into a leaf and out of it. But a leaf looks solid. Which part of the plant lets the gases cross?

Taking a Leaf Apart

The answer is too small to see by eye, so a microscope is needed. Take a leaf from a plant such as coleus, hibiscus, onion, money plant, rhoeo or any grass. Stand it in a beaker filled with water.

The Peel

Carefully strip a thin layer off the underside of the leaf. Place that peel in a watch glass of water so it does not dry or curl. Everything here is kept wet, which matters for a layer that thin.

Mounting It

Put a drop of water on a microscope slide. Using forceps, transfer the peel from the watch glass to the slide. Add a drop of ink on the peel with a dropper, cover it with a coverslip, and look through the microscope.

What the Ink Is For

The ink is not decoration. A peel is almost colourless, and the ink collects in a way that makes the structures stand out. Without it there is very little to see.

What You See

Tiny pores appear across the peel. Those pores are called stomata, and they are present on the surface of leaves. Stomata help in the exchange of gases, which closes the gap in the story: carbon dioxide in, oxygen out.

How Small

The figure in the book is printed with a scale bar of a tenth of a millimetre. That is why nobody noticed stomata before microscopes, and why a whole leaf surface can hold a great many of them.

Now the Other Problem

Water is needed for photosynthesis, and water is taken up by the roots. The roots also take up the minerals present in the soil, which are important nutrients for growth. But the leaves are at the top of the plant. How does any of it get up there?

Following the Water

You cannot watch water move inside a stem, so you have to make it visible. Take two glass tumblers, mark them A and B, and fill each a third full with water, and put a few drops of red ink into tumbler B only.

Choosing the Plants

Take twigs of two similar tender plants, preferably ones with white flowers, such as white sadabahar or balsam. The white flowers are chosen deliberately: red ink would hardly show in a red petal, and in a white one it shows at once.

The Careful Cut

Cut the stems of both plants obliquely at the base, holding them under the water, and then at once put one plant in each tumbler. Cutting under water keeps air out of the tubes, and a slanting cut leaves a wider surface to draw up through.

The Next Day

Look at both plants the day after. In the plant from tumbler B, red colour is visible in the stem, in the leaves and in the flowers. The plant in tumbler A, which stood in plain water, shows nothing.

Looking Inside

Then cut the stem at a point above the water line, which was never immersed in the coloured water, and look at the cut end through a magnifying glass. The red colour is there inside the stem, which proves the ink travelled up through the plant rather than merely wetting the outside.

Naming the Route

The ink rises through thin tube-like structures called xylem, which are present in the stem, in the branches and in the leaves. And just as the red ink moved, minerals dissolved in water move up the stem through the xylem too.

Why the Ink Is Trustworthy

The ink does not change how the plant works; it simply goes where the water goes. That is what makes it useful. Anything that marks a path without altering it will do the same job.

The Second Pipeline

One problem is left. Leaves are the primary site for photosynthesis, so the food is made at the top. It has to reach the roots, the stem and everything else. That cannot be the xylem job, since the xylem is busy carrying water the other way.

The Phloem

the food a plant makes in its leaves travels to every part of it by a second set of thin tube-like structures, the phloem. So a plant runs two pipelines at once, in opposite directions.

And Some of It Is Stored

the food so carried may also be put away in other parts of the plant, such as the seeds and the roots. That is worth remembering next time you eat a potato or a handful of grain: the food in it was made in a leaf and carried there through the phloem.

Scenario:

A florist sells white roses that have turned pale blue, and she will not say how. A customer guesses that she paints the petals. A second customer says she must have grown a new kind of rose. A school pupil watching says neither is needed, that the florist has done the classroom ink experiment with blue dye instead of red, and that the proof would be in a cut stem. The florist laughs and cuts one open, and a ring of blue shows inside well above where the stem had stood in the water.

Analysis:

The pupil is right, and the cut stem is the evidence that settles it. Suppose the petals had been painted. The colour would sit on the outside of the petal, and the inside of the stem would be clean. Suppose instead the rose were a new kind. The colour would be in the petals from the start, and again the stem would show nothing unusual. What the cut actually shows is blue inside the stem, at a height the water never reached. That can only have arrived by travelling up. The route is the xylem, the thin tube-like structures of the stem, the branches and the leaves. They carry water, and the minerals dissolved in it, upward from the roots. A dye dissolved in that water is drawn up the same way, which is exactly what the red ink does in the classroom. The dye is a tracer. It goes where the water goes without changing how the plant works. It ends up visible in the stem, the leaves and the flowers, which is why a white petal turns colour.

Activity:

Stand a stick of celery or a white flower in water coloured with any food dye for a day. Then cut the stem across at two heights, one near the bottom and one near the top, and draw what you see at each. Write one sentence on which of the two drawings is the better evidence of transport.

Title:

Try it yourself: following the water

Safety:

An adult must do all the cutting, and keep the food colouring away from clothes.

What You Notice:

The stem from the coloured glass shows coloured specks inside its cut end, and the petals or ribs higher up have taken the colour. The plain water stem shows nothing. The colour rose through the xylem, exactly as water and dissolved minerals do.

The stomata in the book figure are drawn beside a scale bar of a tenth of a millimetre.

The stems are cut under water, so that no air gets into the tubes.

White-flowered plants are chosen on purpose, because red ink shows up in white petals.

Xylem carries water up. Phloem carries food out. Two pipes, two directions.

Peel the lower surface, add ink, and the pores show up.

A Question Worth Asking

All living beings respire, which you met in Grade 6. Plants are living beings. So plants ought to respire, and the sensible thing is to look for the evidence rather than assume it.

What to Look For

Respiration in animals gives out carbon dioxide, and lime water turns milky with carbon dioxide. So if a plant respires, a closed space around it should end up with more carbon dioxide than it started with.

Preparing the Seeds

leave some moong bean seeds in water overnight. Lay a bed of cotton in a conical flask and damp the cotton so that it stays wet. Set the soaked seeds down on that wet cotton inside the flask.

Why Germinating Seeds

Soaked seeds are beginning to germinate, and a germinating seed is busy. It is also not green. That matters, because a green plant in light would be taking carbon dioxide in as well as giving it out.

Closing the Flask

Cap the mouth of the flask with a cork pierced by two holes, and slide tubes A and B into them. Then leave the flask undisturbed for twenty-four hours in the dark.

Why in the Dark

Think about what light would allow. Photosynthesis happens only in the presence of sunlight, and it uses carbon dioxide up. Keeping the flask dark removes that possibility, so any carbon dioxide found at the end cannot have been affected by it.

The Two Test Tubes

Now take two test tubes and fill them both with lime water. Close the mouth of one with a cork having a single hole, dip a glass tube through that hole, and connect the flask to that test tube with a rubber pipe. The second test tube is joined to nothing.

What the Second Tube Is For

That second tube is the whole reason the result can be trusted. Without it, somebody could say lime water turns milky by itself after a day. With it, the two tubes differ in exactly one way: one is joined to the flask of seeds and the other is not.

The Result

Compare the two tubes. The lime water in the tube connected to the flask turns milky. The lime water in the other tube does not. Only one thing differed, so the flask is responsible.

Where the Gas Came From

Lime water turns milky because of the extra carbon dioxide in the flask. But carbon dioxide is naturally present in air only in very small quantities. The additional carbon dioxide was produced by the seeds as they respired.

Naming the Process

So plants do respire. During respiration, glucose is taken apart in the presence of oxygen, and the breaking down releases carbon dioxide, water and energy.

Setting It Down as an Equation

Written out: glucose plus oxygen gives carbon dioxide plus water plus energy. Compare that with the equation from the last topic and the resemblance is not an accident. The raw materials of one are the products of the other.

What the Energy Is For

The carbon dioxide and the water are what comes off; the energy is the point. The energy that respiring releases goes to the growth and development of the plant. Making food was never the goal in itself.

Where It Happens

Photosynthesis needs chlorophyll, so it happens in the green parts. Respiration has no such requirement. Every part of a plant respires, green or not, which is why a root buried in the soil respires as much as a leaf.

Putting the Two Side by Side

Photosynthesis takes in carbon dioxide and water and gives out glucose and oxygen, needs sunlight and chlorophyll, and happens in green parts in the light. Respiration takes in glucose and oxygen and gives out carbon dioxide, water and energy, needs neither light nor chlorophyll, and happens in every part at all times.

One Stores, the Other Spends

That is the cleanest way to hold the pair. Photosynthesis puts energy away into food. Respiration takes it back out again for the plant to use. A plant that only photosynthesised would have food and no usable energy at all.

Why the Pair Matters Beyond Plants

The same two equations describe you. Your food came from a plant that photosynthesised, whether you ate the plant or ate an animal that did. And your own respiration breaks glucose down with oxygen to release energy, exactly as a root does. The one difference is that you cannot make the food for yourself.

The Whole Chapter in One Line

Plants therefore have different mechanisms for synthesising food, for transporting it, and for utilising it to get energy. Photosynthesis makes it, the xylem and phloem move it and the water it needs, and respiration spends it.

Scenario:

A family argues about a money plant kept in a bedroom. An uncle says it must be moved out at night. At night, he says, a plant stops making oxygen and gives out carbon dioxide instead, so it competes with sleeping people for air. An aunt says the plant gives out carbon dioxide all day as well, so moving it at night makes no sense. A cousin in Grade 7 says they are each half right and each half wrong. The chapter, she says, answers it in two lines.

Analysis:

The cousin is right. Take the uncle first. He is correct that photosynthesis stops in the dark, because photosynthesis happens only in the presence of sunlight and chlorophyll. He is also correct that the plant gives out carbon dioxide, since respiration releases carbon dioxide, water and energy. Where he is wrong is in supposing the carbon dioxide begins at night. Now the aunt. She is correct that the plant respires and gives out carbon dioxide during the day too, because every part of a plant respires, green or not at all times. Where she is wrong is in saying nothing changes at night, because during the day the plant is also photosynthesising and taking carbon dioxide IN. So the true picture is that respiration runs all the time and photosynthesis runs only in the light. At night only one of the two is working, which is the grain of truth in the uncle worry, though a single houseplant gives out very little.

Activity:

Write the two word equations one under the other and mark with arrows which gas each process takes in and which it gives out. Then write one sentence saying what happens to a plant gas exchange at sunset, using both equations.

Title:

Try it yourself: the breath of a seed

Safety:

Ask your teacher to prepare and handle the lime water, and wash the jars afterwards.

What You Notice:

The lime water from the jar with seeds clouds, and the other stays clear. The seeds have been respiring. They break glucose down with oxygen and give out carbon dioxide. The empty jar shows that the jar and the cotton are not responsible.

Germinating seeds are not green, and they respire all the same.

Carbon dioxide is present in air only in very small quantities, which is why the increase shows.

The equation for respiration is the one for photosynthesis read the other way round.

Photosynthesis stores the energy as food. Respiration spends it again.

Milky in the joined tube, clear in the other. The seeds made the difference.