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๐ฌ Think about it
We have seen leaves need sunlight and chlorophyll. But is that enough to make food? There is one more invisible ingredient that comes from the air around us. Let us find out which gas it is.
Which gas from the air is needed to make food?
- Air plays a role in making food in plants.
- The gas carbon dioxide from the air is needed.
- Caustic soda can absorb carbon dioxide from the air.
- Without carbon dioxide, a leaf cannot make starch.
Example:
In Activity 10.4, the leaf part inside the bottle gets no carbon dioxide, so it makes no starch.
Leaf inside the bottle
Leaf outside the bottle
No carbon dioxide (absorbed by caustic soda)
Gets carbon dioxide from the air
Makes no starch
Makes starch
Stays pale in the iodine test
Turns blue-black in the iodine test
What is photosynthesis?
- Sunlight, water, chlorophyll, and carbon dioxide are all needed.
- Together they help the plant make food.
- Making food using sunlight and chlorophyll is photosynthesis.
- A leaf is the main site for photosynthesis.
Example:
Other green parts of a plant that have chlorophyll, like a green stem, can also do photosynthesis.
In this Activity, we will test whether carbon dioxide from the air is needed to make starch.
Materials needed
A potted green plant, a wide-mouthed bottle, caustic soda (sodium hydroxide), a split cork, and iodine solution.
Procedure
1. Keep a potted plant in the dark for two to three days to destarch it.
2. Pour some caustic soda into a wide-mouthed bottle.
3. Insert half a destarched leaf into the bottle through a split cork, leaving the other half outside.
4. Place the set-up in sunlight for a few hours.
5. Record water, sunlight, chlorophyll, and carbon dioxide in Table 10.3.
6. Test the leaf for starch using the iodine test.
2. Pour some caustic soda into a wide-mouthed bottle.
3. Insert half a destarched leaf into the bottle through a split cork, leaving the other half outside.
4. Place the set-up in sunlight for a few hours.
5. Record water, sunlight, chlorophyll, and carbon dioxide in Table 10.3.
6. Test the leaf for starch using the iodine test.
Table 10.3 — Role of air in the preparation of starch by plants
| Part of the leaf | Availability of | Starch present (Yes/No) | |||
|---|---|---|---|---|---|
| Water | Sunlight | Chlorophyll | Carbon dioxide | ||
| Part of the leaf inside the bottle | Yes | Yes | Yes | No | No |
| Part of the leaf outside the bottle | Yes | Yes | Yes | Yes | Yes |
Observation
The leaf part outside the bottle turns blue-black. The part inside the bottle does not turn blue-black.
Explanation
The caustic soda absorbed the carbon dioxide inside the bottle, so no food was made there. So carbon dioxide from air is needed to make starch.
โ Summary
- Leaf half inside bottle
- Outside half made starch
- Inside half made none
- Carbon dioxide is needed
In this Activity, we will find out which gas is released by a water plant in sunlight.
Materials needed
Two beakers, two funnels, two inverted test tubes, water plants, and water.
Procedure
1. Set up a water plant under a funnel with an inverted test tube in each beaker.
2. Place set-up A in sunlight and set-up B in the dark.
3. Look for air bubbles in the inverted test tube of each set-up.
4. When enough gas collects in A, close the test tube with a thumb and lift it out.
5. Insert a lit matchstick into the tube and watch the flame.
2. Place set-up A in sunlight and set-up B in the dark.
3. Look for air bubbles in the inverted test tube of each set-up.
4. When enough gas collects in A, close the test tube with a thumb and lift it out.
5. Insert a lit matchstick into the tube and watch the flame.
Observation
Air bubbles collect in the test tube of set-up A in sunlight. The matchstick gives an intense flame. Set-up B in the dark shows no bubbles.
Explanation
The bright flame shows the gas is rich in oxygen. So oxygen is released during photosynthesis, and this happens in the presence of sunlight.
โ Summary
- Bubbles formed in sunlight
- Matchstick flared brightly
- Gas is oxygen
- Oxygen released in photosynthesis
Important Points
- Carbon dioxide from air is needed to make starch.
- Caustic soda absorbs carbon dioxide from the air.
- Oxygen is released during photosynthesis.
โ Test Yourself
-
Which gas from the air is needed to make food?
View Answer
Carbon dioxide. -
What does caustic soda do in Activity 10.4?
View Answer
It absorbs the carbon dioxide from the air inside the bottle. -
Which part of the leaf made starch in Activity 10.4?
View Answer
The part outside the bottle, which got carbon dioxide. -
Which gas is released during photosynthesis?
View Answer
Oxygen. -
What is the main site of photosynthesis?
View Answer
The leaf.
Important Definitions
- Photosynthesis — the process by which plants make food in the presence of sunlight and chlorophyll.
๐ NCERT Question 7 — Vani believes that 'carbon
Vani believes that 'carbon dioxide is essential for photosynthesis'. She sets up an experiment to test her idea.
View Answer →
