NCERT Solutions for Class 7th Science Chapter 10 .2.2 Role of air in the preparation of food — Activity 10.4: Let us experiment (demonstration activity)
Book page 143 & 14410 Updated on2026-09-19
Q1.
Observe and record the availability of water, sunlight, chlorophyll, and carbon dioxide in Table 10.3. Remove the leaf and test it for starch using the iodine test, as was done in Activity 10.2. Record your observations in Table 10.3.
Fig. 10.3(a), page 143 — the set-up for Activity 10.4.
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
Part of the leaf outside the bottle
Table 10.3, page 144 — the blank recording table for Activity 10.4.
Answer
Half of a destarched leaf is sealed inside a wide-mouthed bottle containing caustic soda through a split cork; the other half stays outside in the air (Fig. 10.3a). The whole set-up is kept in sunlight for a few hours. Here is Table 10.3 completed.
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 (absorbed by caustic soda)
No
Part of the leaf outside the bottle
Yes
Yes
Yes
Yes
Yes
After the iodine test (Fig. 10.3b), the half that was outside the bottle turns blue-black; the half that was inside shows no colour change.
Caution: caustic soda is a strong chemical that can cause skin burns; only teachers should handle it.
Why the plant is destarched first: if the plant already had stored starch, both halves of the leaf would turn blue-black and the experiment would prove nothing. Keeping the plant in the dark for two to three days empties this store, so any starch found afterwards must have been made during the experiment.
Q2.
What does this experiment show?
Answer
This experiment shows that carbon dioxide present in the air is essential for plants to prepare starch.
The two halves are part of the same leaf on the same plant, in the same sunlight, with the same water and the same chlorophyll. Only one factor differs — the air around the inside half has had its carbon dioxide removed by the caustic soda. That half makes no starch; the other half does.
Why one leaf, and not two plants: using two halves of one leaf makes every other condition automatically identical. It is the cleanest possible way to test a single factor — a good example of a fair test.
Q3.
Based on Activities 10.3 and 10.4, what do you conclude?
Answer
Together, the two activities identify all four requirements for making food in plants.
Activity
Factor removed
Starch formed?
Conclusion
10.3 — plant kept in the dark
Sunlight
No
Sunlight is essential
10.3 — non-green patches of leaf
Chlorophyll (insufficient)
No
Chlorophyll is essential
10.4 — leaf-half inside the bottle
Carbon dioxide
No
Carbon dioxide is essential
10.1 — pot without water
Water
Plant wilts and dies
Water is essential
Conclusion: sunlight, water, chlorophyll and carbon dioxide are all essential for the synthesis of food in plants. This process, by which plants prepare food in the presence of sunlight and chlorophyll, is called photosynthesis.
What goes into photosynthesis and what comes out of it. Activities 10.1, 10.3 and 10.4 each remove one input and show that starch is then not formed.
Q4.
Which part of the plant is involved in the synthesis of starch?
Answer
Mainly the leaf. A leaf is the primary site for photosynthesis — that is why leaves are called the 'food factories' of plants.
Leaves are suited to the job because they are:
broad and flat — a large surface to catch sunlight;
green — full of chlorophyll to capture that light;
dotted with stomata — pores through which carbon dioxide enters;
supplied by xylem — thin tubes bringing water from the roots.
Q5.
Do other green parts of the plant also perform photosynthesis?
Answer
Yes. Other parts of the plant which have chlorophyll also perform photosynthesis.
Green stems of young plants, and of cactus, where the stem does the work because the leaves are reduced to spines.
Green sepals of a flower and the green skin of unripe fruits such as raw mango or lauki.
Green parts of leaf stalks and tendrils.
Check it yourself: destarch a money plant, keep it in sunlight for a day, and do the iodine test on a piece of its green stem. It too turns blue-black.