Life Processes in Plants
Why This Matters
Think about what you ate today. Maybe some rice, dal, a banana, or a glass of milk.
Now ask a simple question. Where did all that food come from?
The rice came from a rice plant. The banana grew on a banana tree. Even the milk came from a cow — and the cow eats grass, which is a plant. So in the end, almost all of our food comes from plants.
But here is the surprising part. Plants do not eat anything. A goat eats grass. A lion eats a goat. But the grass itself does not “eat” at all. So how does a plant get the food it needs to grow tall and green?
The answer is amazing. Plants make their own food. They are like little kitchens that run on sunlight. In this chapter, we will find out exactly how they do it. We will also see how water and food travel inside a plant, and whether plants breathe like we do.
By the end, you will understand something huge — green plants are the starting point of nearly all the food on Earth.
The Big Idea
A green plant does not eat food — it makes its own food. It takes in carbon dioxide from the air and water from the soil. Then, using sunlight and a green colour inside its leaves called chlorophyll, it joins them together to make glucose (a sugar that is food) and lets out oxygen. This food-making process is called photosynthesis. The leaf is the main kitchen, water and food travel through tiny tubes inside the plant, and plants also respire (break down food to get energy) just like we do.
Let’s Break It Down
We will go step by step. First, what does a plant need to grow? Then, how does it actually make food? Then, how does food and water move around inside it? And finally, do plants breathe?
What does a plant need to grow?
Before we talk about food, let us remember something from Grade 6.
Scientists tested what a plant needs by a simple experiment. They took three small plants of the same size, in three pots:
- Pot A — kept in sunlight, watered every day.
- Pot B — kept in sunlight, but given no water.
- Pot C — kept in the dark, watered every day.
After two weeks, Pot A grew the best — green and tall. Pot B dried up and died (no water). Pot C stayed alive but grew poorly and looked weak (no sunlight).
What does this tell us? A plant needs both water and sunlight to grow well. Water alone is not enough. Sunlight alone is not enough. It needs both.
This is our first big clue. Sunlight is doing something important for the plant. Let us find out what.
How a plant makes its own food: photosynthesis
Here is the heart of the chapter. Let us build it up slowly.
You know that plants store food. When you eat a potato or a slice of bread, you are eating starch. Starch is a type of carbohydrate — a food that gives energy. This starch is made in the leaves of the plant.
Why leaves? Because leaves are usually broad and flat, and they are green. The green colour comes from a substance called chlorophyll.
Chlorophyll is a green colouring (a pigment) inside the leaf. Its special job is to catch sunlight. Think of chlorophyll like a tiny solar panel that traps the Sun’s energy.
So now we know the leaf has chlorophyll to catch sunlight. But to make food, the plant needs raw materials. Where do they come from?
Scientists found that a plant needs four things to make food:
- Sunlight — the energy source (caught by chlorophyll).
- Chlorophyll — the green pigment that catches the sunlight.
- Carbon dioxide (CO₂) — a gas taken in from the air.
- Water — taken in from the soil by the roots.
When all four are present, the leaf joins carbon dioxide and water together to make glucose (a sugar, which is food) and lets out oxygen (O₂) into the air. This whole food-making process is called photosynthesis.
The picture below puts the whole story in one place. Figure 10.1 shows how a single leaf works as a food kitchen.
We can write photosynthesis as a short word equation. It is a simple way to show what goes in and what comes out:
carbon dioxide + water → glucose + oxygen
(this happens only in sunlight, using chlorophyll)
Read it like a sentence: “Carbon dioxide and water become glucose and oxygen, using sunlight and chlorophyll.” The arrow → means “turn into” or “become”.
Now let us slow down and look at the four raw materials, where each one comes from, and the two products. Figure 10.2 lines them up clearly.
Let us check you have understood why each raw material matters.
A plant is kept in a dark cupboard with plenty of water and air. Will it be able to make food? Why or why not?
No, it will not make food. Photosynthesis needs sunlight, because chlorophyll uses sunlight’s energy to join carbon dioxide and water. In the dark there is no sunlight, so no food is made — no matter how much water and air the plant gets. (This is why the plant in Pot C, kept in the dark, grew so poorly.)
Let us put the idea to work on a simple problem.
A potted plant is kept in bright sunlight, but a thick coat of grease (oily cream) is rubbed all over the underside of its leaves so that no gas can pass in or out of the leaf. Will this leaf be able to make food? Explain.
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List what photosynthesis needs: sunlight, chlorophyll, water, and carbon dioxide gas from the air.
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The plant has sunlight (it is in bright light), it has chlorophyll (green leaves), and it can still get water from the soil through its roots.
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But carbon dioxide gas enters the leaf through tiny pores on its surface. If grease blocks the underside of the leaf, the gas cannot get in. One key raw material — carbon dioxide — is now missing.
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A kitchen cannot cook a dish if one main ingredient is missing. In the same way, with no carbon dioxide entering, the leaf cannot make food. So the greased leaf will not be able to carry out photosynthesis.
The leaf is the food factory — and stomata are its tiny doors
We keep saying the leaf takes in carbon dioxide and lets out oxygen. But how does a gas get in and out of a leaf? A leaf looks solid and smooth.
The answer is that a leaf has thousands of tiny pores (very small holes) on its surface, mostly on the lower side. Each tiny pore is called a stoma (say it “STOH-ma”). More than one are called stomata (say “STOH-ma-ta”).
You cannot see a single stoma with your eyes. It is far too small. You need a microscope. When scientists peel a thin layer from the underside of a leaf and look at it under a microscope, they see many tiny mouth-shaped openings — the stomata.
These stomata are the leaf’s doors for gases. Carbon dioxide comes in through them for photosynthesis. Oxygen goes out through them. (Later we will see oxygen also goes in and carbon dioxide goes out during respiration — the same doors are used.)
Figure 10.3 shows the leaf as a food factory, and zooms in on these tiny doors.
Because the leaf has chlorophyll to catch sunlight, and stomata to let gases in and out, and a flat broad shape to soak up plenty of light, it is perfectly built for making food. That is why leaves are called the food factories of the plant.
Why is it useful for a leaf to be broad and flat rather than thin like a needle?
A broad, flat leaf has a large surface facing the Sun. So it can catch much more sunlight than a thin, needle-shaped leaf. More sunlight caught means more energy for chlorophyll, which means the leaf can make more food. The flat shape also gives lots of room for stomata to take in carbon dioxide.
Why green plants are the base of all food
We started this chapter with a question: where does our food come from? Now we can give a deep answer.
Only green plants can make their own food from sunlight, air and water. Animals cannot do this. You cannot do this — you cannot stand in the Sun and make a meal inside your body. So animals and humans must eat to get food.
What do they eat? Some animals eat plants directly — a cow eats grass, a goat eats leaves. These are plant eaters. Other animals eat those plant-eating animals — a lion eats a goat. These are meat eaters. And we eat both plants and animals.
But trace it back far enough, and every meal starts at the same place: a green plant that made food from sunlight. Without green plants, the plant eaters would have nothing to eat, and then the meat eaters would have nothing to eat either. The whole chain would collapse.
Figure 10.4 shows this idea as a simple web of “who eats whom”.
This is also why plants are so important for the air. Every time a plant makes food, it releases oxygen — the gas we need to breathe. So plants feed us and give us clean air to breathe.
How water and food travel inside a plant
A plant can be very tall. A tree can be many metres high. The roots are down in the soil, but the leaves that make food are up high. So two journeys must happen inside every plant:
- Water (taken in by the roots) must travel up to the leaves, because leaves need water for photosynthesis.
- Food (made in the leaves) must travel down and around to every other part — the stem, the roots, the seeds — because all parts of the plant need food.
How does this happen? Inside the plant are two sets of thin tube-like pipes that run all the way from root to leaf:
- Xylem (say “ZY-lem”) — these tubes carry water and minerals upward, from the roots up to the leaves. (Minerals are useful nutrients dissolved in the soil water, taken in by the roots.)
- Phloem (say “FLOH-em”) — these tubes carry food made in the leaves to all the other parts of the plant.
Here is an easy way to remember which is which: Xylem = water (up), Phloem = Food. Both “phloem” and “food” start with an “f” sound.
Scientists showed water travels up the xylem with a neat experiment. They put a plant’s cut stem into water mixed with red ink. After a day, the red colour appeared in the stem, leaves and even the flowers — proof that water (the red water) had travelled up through the tubes inside the stem.
Figure 10.5 shows both pipelines at work.
Let us test this with a small problem.
A gardener removes a thin ring of bark (the outer layer that contains the phloem tubes) all the way around the trunk of a tree, but leaves the inner wood (which contains the xylem) untouched. The leaves above the ring stay alive and green for a while, but the roots below slowly weaken. Why?
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Recall the two pipelines. Xylem carries water and minerals from the roots up to the leaves. Phloem carries food from the leaves down to the rest of the plant.
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The gardener removed the phloem (in the bark) but left the xylem (the inner wood). So water can still travel up to the leaves through the xylem.
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That is why the leaves stay alive and green for a while — they still get water, and they can still make food using sunlight.
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But the food made in the leaves cannot travel down to the roots, because the phloem pipeline below the ring is cut. The roots stop receiving food, so they slowly run out of energy and weaken. This shows the phloem is the tube that carries food downward.
Do plants respire?
We have learnt that plants make food and let out oxygen. So you might think plants only give out oxygen and never take it in. But that is not the full story.
Let us recall a key idea from Grade 6.
Yes — plants respire as well. Inside the plant, glucose (the food it made) is broken down using oxygen. This releases carbon dioxide, water, and most importantly energy. The plant uses that energy to grow and carry out its life processes.
The word equation for respiration is:
glucose + oxygen → carbon dioxide + water + energy
Notice something interesting. Respiration is almost the reverse of photosynthesis:
- In photosynthesis, the plant takes in carbon dioxide and gives out oxygen, and it makes food (it stores energy).
- In respiration, the plant takes in oxygen and gives out carbon dioxide, and it uses up food (it releases energy).
Scientists showed plants respire using germinating (sprouting) seeds. They placed soaked moong seeds in a sealed flask in the dark. After a day, the air in the flask had extra carbon dioxide — which turned lime water milky. The seeds were alive, not photosynthesising (it was dark), and yet they were giving out carbon dioxide. That carbon dioxide came from the seeds respiring.
Here is the important point: all parts of the plant respire — leaves, stem, roots, even non-green parts. And plants respire all the time, day and night. Photosynthesis only happens when there is sunlight, but respiration never stops.
Let us compare the two processes side by side, because students mix them up a lot.
| Feature | Photosynthesis | Respiration |
|---|---|---|
| What it does | Makes food (glucose) | Breaks down food to release energy |
| Gas taken IN | Carbon dioxide | Oxygen |
| Gas given OUT | Oxygen | Carbon dioxide |
| Needs sunlight? | Yes — only in light | No — happens day and night |
| Energy | Stores energy in food | Releases energy for the plant to use |
| Where | Green parts (mainly leaves) | All parts of the plant |
During the day, a green leaf is doing both photosynthesis AND respiration at the same time. On a sunny afternoon, which process is faster — and what gas does the plant give out overall?
On a bright sunny afternoon, photosynthesis is much faster than respiration. So overall the plant takes in more carbon dioxide than it gives out, and gives out more oxygen than it takes in. The net result is that the plant releases oxygen into the air during the sunny day. (At night there is no sunlight, so photosynthesis stops, but respiration continues — so at night the plant only gives out carbon dioxide.)
Common Mistakes
These are the slip-ups students make most often. Read each one and see why it is tempting but wrong.
Plants take in their food ready-made from the soil through their roots.
It feels right because we water plants and add manure to the soil, and the plant clearly grows better — so it looks like the soil is feeding the plant its food, just like we feed a pet.
Roots take in WATER and dissolved MINERALS from the soil, not ready-made food. The plant MAKES its own food (glucose) in its green leaves by photosynthesis, using sunlight, carbon dioxide and that water. Soil and manure only supply water and minerals — they are raw materials, not the food itself.
Plants only release carbon dioxide; they never take it in.
It seems right because we are taught that 'plants give out oxygen and we give out carbon dioxide', so it is easy to flip it and assume plants only deal with carbon dioxide one way — by releasing it.
During photosynthesis a plant TAKES IN carbon dioxide (and gives out oxygen) to make food. It is during respiration that it gives out carbon dioxide. In strong sunlight the take-in is greater, so overall a plant takes in carbon dioxide during a sunny day.
Plants do not respire — only animals breathe and respire.
It looks right because we never see a plant breathing in and out, it has no nose or lungs, and we mainly hear that plants give out oxygen — the opposite of breathing.
Every living thing respires, including plants. Plants break down glucose using oxygen to release energy, giving out carbon dioxide and water. This happens in ALL parts of the plant, day and night — there is just no obvious breathing movement to watch.
A leaf is green because of the green light it reflects, and the colour has no real job.
It is tempting because colour usually feels like just decoration — a flower is red, a leaf is green, and that seems to be the end of it.
The green colour comes from chlorophyll, and chlorophyll has a vital job: it CATCHES sunlight so the leaf can make food. No chlorophyll means no green colour AND no photosynthesis — which is exactly why the non-green parts of a leaf do not make starch.
Quick Check
Try these short questions to check your understanding.
Which of these is NOT a raw material that a plant needs for photosynthesis?
The tiny pores on the surface of a leaf, through which gases move in and out, are called:
Which tube carries food made in the leaves to the rest of the plant?
During respiration, a plant takes in ____ and gives out ____.
Practice Problems
Try each one yourself first. Then tap to see the full answer.
Easy
Fill in the blanks: Photosynthesis needs four things — sunlight, water, ____ from the air, and the green pigment ____ in the leaf.
The two blanks are carbon dioxide and chlorophyll.
So the full sentence is: Photosynthesis needs sunlight, water, carbon dioxide from the air, and the green pigment chlorophyll in the leaf.
Write the word equation for photosynthesis.
The word equation is:
carbon dioxide + water → glucose + oxygen
This happens in sunlight, using chlorophyll. The carbon dioxide and water are the raw materials. The glucose (food) and oxygen are the products.
Name the two types of tubes that transport things in a plant, and say what each one carries.
The two tubes are xylem and phloem.
- Xylem carries water and minerals upward, from the roots to the leaves.
- Phloem carries food (made in the leaves) to all the other parts of the plant.
Medium
A leaf is tested for starch using iodine solution. The green parts of the leaf turn blue-black, but the non-green (pale) patches do not change colour. What does this tell us?
Iodine turns blue-black when starch is present. So the result shows:
- Green parts have starch → they made food. This is because the green parts contain chlorophyll, which catches sunlight to carry out photosynthesis.
- Non-green parts have no starch (or very little) → they did not make food, because they lack enough chlorophyll.
The conclusion: chlorophyll is needed for photosynthesis. Only the parts with chlorophyll can make food (starch).
On a bright sunny day, is a green plant giving out mostly oxygen or mostly carbon dioxide? Explain, remembering that BOTH photosynthesis and respiration are happening.
On a bright sunny day, the plant is doing both at once:
- Photosynthesis (takes in carbon dioxide, gives out oxygen) — and in strong sunlight this is happening fast.
- Respiration (takes in oxygen, gives out carbon dioxide) — this happens steadily, but slower than photosynthesis in bright light.
Because photosynthesis is faster, the plant gives out mostly oxygen overall during a sunny day. It takes in more carbon dioxide than it releases, and releases more oxygen than it takes in.
(At night, photosynthesis stops because there is no sunlight, but respiration continues — so at night the plant gives out only carbon dioxide.)
A student keeps a healthy potted plant in a completely dark room for a week but waters it daily. The leaves turn pale and the plant becomes weak. Explain why, using the idea of photosynthesis.
Even though the plant has water and air, it is missing one key raw material: sunlight.
Without sunlight, chlorophyll cannot catch any light energy, so photosynthesis stops. The plant cannot make new food (glucose).
But the plant is still alive, so it keeps respiring — using up its stored food for energy. Slowly, its food runs out. With no food being made and the stored food being used up, the plant becomes weak and pale.
This matches the earlier experiment: the plant kept in the dark (Pot C) grew the worst, because it could not make food.
Challenge
A potato is not a leaf and is not green, yet a slice of potato turns blue-black with iodine — meaning it is full of starch (food). Where did this food come from, and how did it reach the potato?
A potato cannot make its own food, because it is underground, has no sunlight, and has no chlorophyll (it is not green). So the food was not made in the potato itself.
The food was made far away, in the plant’s green leaves, by photosynthesis. The leaves made glucose, which was changed into starch for storage.
This food then travelled through the phloem tubes — the pipeline that carries food from the leaves to other parts of the plant. The potato is a special underground part where the plant stores extra food as starch.
So the chain is: leaves make food (photosynthesis) → food travels through the phloem → food is stored as starch in the potato. That stored starch is why the potato turns blue-black with iodine.
Imagine that, suddenly, every plant that can photosynthesise disappeared from the Earth. Describe two serious effects this would have on animals and humans.
Two serious effects:
1. Food would run out. Green plants are the base of nearly all food. Plant eaters (cows, goats, insects) would have nothing to eat and would die. Then the meat eaters that depend on them would also have nothing to eat. Humans, who eat both plants and animals, would lose almost all their food too. The whole food web would collapse.
2. Oxygen would run low. Photosynthesis is the main process that releases oxygen into the air. With no plants making oxygen, the oxygen in the air would slowly get used up by all the breathing and respiring animals — but nothing would be replacing it. Living things need oxygen to respire and stay alive, so this would be deadly.
In short, without photosynthesising plants there would be almost no food and a falling supply of oxygen — life as we know it could not survive.
Summary
- A green plant does not eat food — it makes its own food. This food-making process is called photosynthesis.
- Photosynthesis needs four things: sunlight, chlorophyll, carbon dioxide (from the air) and water (from the soil). It produces glucose (food, stored as starch) and oxygen.
- Word equation: carbon dioxide + water → glucose + oxygen (in sunlight, using chlorophyll).
- Chlorophyll is the green pigment that catches sunlight; this is why leaves are green, and why only green parts make food. Leaves are the plant’s food factories.
- Stomata are tiny pores (mostly on the lower side of the leaf) through which gases like carbon dioxide and oxygen move in and out.
- Xylem carries water and minerals upward from roots to leaves; phloem carries food from leaves to the rest of the plant.
- Plants also respire: they break down glucose using oxygen to release energy, giving out carbon dioxide and water. Respiration happens in all parts of the plant, day and night.
- Green plants are the base of almost all food on Earth and they keep our air rich in oxygen.
What’s Next
You have just seen how plants quietly turn sunlight into food — one of the most important processes on our planet.
Next, in Chapter 11 — Light: Shadows and Reflections, we turn to light itself. You will explore how light travels in straight lines, how shadows form, and how shiny surfaces like mirrors and still water bounce light back to make reflections. The very same sunlight that feeds plants in this chapter is about to reveal some beautiful tricks of its own.
Frequently Asked Questions
How do plants make their own food and what is photosynthesis?
Plants make their own food through a process called photosynthesis. A plant takes in carbon dioxide from the air through tiny holes in its leaves called stomata, and water from the soil through its roots. Using sunlight and a green substance called chlorophyll in the leaves, it combines the water and carbon dioxide to make glucose (a sugar that is food) and releases oxygen as a by-product.
What is chlorophyll and why are most plant leaves green?
Chlorophyll is the green-coloured substance found inside leaf cells. It captures energy from sunlight and uses it to power photosynthesis. Leaves are green because chlorophyll is green — it reflects green light back to your eyes while absorbing red and blue light to drive the food-making reaction.
What are stomata and what do they do?
Stomata are tiny pores (holes) on the surface of leaves, mostly on the underside. They open to let carbon dioxide in from the air (needed for photosynthesis) and to let out oxygen and water vapour. Each stoma is surrounded by two guard cells that control when it opens and closes.
What is the difference between xylem and phloem in plants?
Xylem and phloem are the two types of transport tubes inside a plant. Xylem carries water and dissolved minerals upward from the roots to the leaves. Phloem carries the food made by photosynthesis (glucose) from the leaves to all other parts of the plant — roots, stem, flowers and fruits — wherever it is needed.
Do plants also respire or do they only do photosynthesis?
Plants do both. Photosynthesis makes food using sunlight and happens mainly during the day. Respiration is a separate process that happens all the time, day and night, where the plant uses oxygen to break down glucose and release the energy it needs to live and grow. During the day plants produce more oxygen than they use, but at night they only respire and take in oxygen like other living things.