How Plants Make Their Food
Class 7 Science · Chapter 10: Life Processes in Plants · NCERT Curiosity section 10.2
Learn
Plants don't eat like animals. They make food and store it as starch. With a teacher, the friends boil a leaf to soften it, then warm it in alcohol (in a test tube inside hot water, never near a flame) until it turns pale, and add iodine: it turns blue-black, so the leaf contains starch. Leaves are green because of chlorophyll, a pigment that captures sunlight.
Ishaan tests a leaf with green and white patches. From a plant kept in sunlight, only the green parts turn blue-black. From a plant kept in the dark for days, no part does. So chlorophyll and sunlight are both needed to make starch. Next, half of a leaf is sealed in a bottle with caustic soda, which absorbs carbon dioxide: only the half outside makes starch. Carbon dioxide is needed too.
A water plant under a funnel in sunlight gives off bubbles; a glowing splint in the gas flares up: it is oxygen. In the dark, no bubbles. So plants use carbon dioxide and water, with sunlight and chlorophyll, to make glucose and oxygen: photosynthesis. Glucose gives quick energy or is stored as starch. Gases pass in and out through tiny pores on leaves, the stomata.
carbon dioxide + water → glucose + oxygen (sunlight, chlorophyll)
Leaves are broad and flat to catch lots of light: they are the plant's food factories. Other green parts, like green stems, photosynthesise too. Indian botanist R. H. Dastur studied how water, temperature and the colour of light affect photosynthesis.
Explore: where is starch made?
- Test each leaf part and sort it.
Explore: the recipe of photosynthesis.
- Sort each one.
Remember
- Photosynthesis: carbon dioxide + water → glucose + oxygen, using sunlight and chlorophyll.
- The iodine test shows starch only where chlorophyll, light and CO₂ were present; gases move through stomata.
Quick check
Half a leaf inside a bottle of caustic soda makes no starch. What was missing?
- carbon dioxide
- sunlight
- water
- chlorophyll
Answer
carbon dioxide. Caustic soda absorbs carbon dioxide from the air.
Think about it
If every plant on Earth vanished, what would happen to animals?
One answer
Animals would lose their food, directly or through food chains, and the oxygen supply would slowly run down. Most life would not survive.
Practice
Practise with fresh questions at three levels (easy, medium, hard), some answered hands-on, with hints and worked answers. Get 4 of your last 5 right to finish the topic.