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๐ฌ Think about it
Why are most leaves broad, flat, and green? That green colour is a clue. It comes from something that catches sunlight, like a tiny solar panel. Let us find out how a leaf makes food.
Why are leaves called the 'food factories' of plants?
- Plants make starch, their food, in the leaves.
- Leaves are usually broad and flat.
- They are green because of a pigment called chlorophyll.
- Chlorophyll helps capture sunlight well.
Example:
A spinach leaf is broad, flat, and green, so its chlorophyll catches plenty of sunlight to make food.
In this Activity, we will test a leaf for starch to show that leaves store food.
Materials needed
A green leaf, boiling water, a test tube with alcohol, a beaker, a spirit lamp, a plate, and iodine solution.
Procedure
1. Keep a leaf in boiling water for five minutes to soften it.
2. Dip this leaf in a test tube containing alcohol.
3. Place the test tube in a beaker of boiling water until the leaf becomes colourless.
4. Take out the leaf and place it on a plate.
5. Put a few drops of diluted iodine solution on the leaf and wait.
6. If the colour changes to blue-black, starch is present.
2. Dip this leaf in a test tube containing alcohol.
3. Place the test tube in a beaker of boiling water until the leaf becomes colourless.
4. Take out the leaf and place it on a plate.
5. Put a few drops of diluted iodine solution on the leaf and wait.
6. If the colour changes to blue-black, starch is present.
Observation
The decolourised leaf turns blue-black where iodine is added.
Explanation
The blue-black colour shows starch is present in the leaf. We decolourise the leaf first so the colour change is easy to see. So leaves store starch as food.
โ Summary
- Leaf made colourless first
- Iodine added to leaf
- Leaf turned blue-black
- Starch is present
๐งฌ Dive Deeper
- Did you wonder why we decolourise the leaf at the start?
- A green leaf hides the colour change in the iodine test.
- Decolourising lets us easily see the blue-black colour.
- So we can clearly tell that starch is present.
In this Activity, we will compare leaves from sunlight and dark to see where starch is made.
Materials needed
A leaf with green and non-green patches from two similar potted plants, tracing paper, and iodine solution.
Procedure
1. Take a leaf with green and non-green patches from a plant kept in sunlight.
2. Take a similar leaf from a plant kept in the dark for 36 hours.
3. Sketch the green and non-green patches using tracing paper.
4. Perform the iodine test on both leaves as in Activity 10.2.
5. Record the colours before and after the test in Table 10.2.
2. Take a similar leaf from a plant kept in the dark for 36 hours.
3. Sketch the green and non-green patches using tracing paper.
4. Perform the iodine test on both leaves as in Activity 10.2.
5. Record the colours before and after the test in Table 10.2.
Table 10.2 — Presence of starch in green and non-green parts of the leaves of plants
| S.No. | Light conditions for potted plant | Initial colours before iodine test | Final colours after iodine test |
|---|---|---|---|
| 1. | Plant kept in sunlight | Green and non-green patches on the leaf | Green patches of leaf turned blue-black |
| 2. | Plant kept in the dark | Green and non-green patches on the leaf | No change in colour |
Observation
Only the green patches of the sunlight leaf turn blue-black. The dark-kept leaf shows no change, even on green patches.
Explanation
Starch forms only in green patches that got sunlight. So chlorophyll makes starch in the presence of sunlight. Non-green patches may lack enough chlorophyll to be detected.
โ Summary
- Green patches turned blue-black
- Dark leaf, no change
- Sunlight is needed
- Chlorophyll makes starch
๐ Fascinating Facts
- Some leaves look red, violet, or brown, not green.
- They have more coloured pigments than green chlorophyll.
- These pigments hide the green colour.
- An iodine test can still show starch in these leaves.
Important Points
- Leaves are the 'food factories' of plants.
- Chlorophyll gives leaves their green colour.
- Chlorophyll captures sunlight to make starch.
โ Test Yourself
-
Where do plants make their food?
View Answer
In their leaves, the 'food factories' of plants. -
What is chlorophyll?
View Answer
The green pigment in leaves that captures sunlight. -
What colour shows starch in the iodine test?
View Answer
Blue-black. -
Why do we decolourise the leaf first?
View Answer
So the colour change is easy to see. -
Did the dark-kept leaf make starch?
View Answer
No, it showed no blue-black colour.
Important Definitions
- Chlorophyll — the green pigment in leaves that helps capture sunlight to make food.
- Food factory — the leaf, the part of the plant where food is prepared.
๐ NCERT Question 4 — Does the broad and
Does the broad and flat structure of leaves make plants more efficient for photosynthesis? Justify your answer.
View Answer →
๐ NCERT Question 6 — Krishna set-up an experiment
Krishna set-up an experiment with two potted plants, one in sunlight and one in a dark room, to test for starch.
View Answer →
