NCERT Solutions for Class 7 Science Chapter 1
Important NCERT Questions
7th Chapter 1 Nutrition in Plants
NCERT Books for Session 2022-2023
CBSE Board and State Board
Questions No: 3
How would you test the presence of starch in leaves?
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• Take a potted plant with variegated leaves – for example, money plant or crotons.
• Keep the plant in a dark room for three days so that all the starch gets used up.
• Now select a leaf cover its portion with a black strip paper and keep the plant in sunlight for about six hours.
• Pluck the leaf from the plant. Mark the uncovered area in it and trace them on a sheet of paper.
• Dip the leaf in boiling water for a few minutes.
• After this, immerse it in a beaker containing alcohol.
• Carefully place the above beaker in a water-bath and heat till the alcohol begins to boil.
• Now dip the leaf in a dilute solution of iodine for a few minutes.
• Take out the leaf and rinse off the iodine solution.
• You observe that the presence of starch in various areas of the leaf which was uncovered.
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To test for starch presence in leaves, conduct an iodine staining test:
1. Boil Leaves: Boil the leaves to soften them and halt ongoing photosynthesis.
2. Boiling Alcohol: Treat leaves with boiling alcohol to eliminate chlorophyll, allowing better visibility of other leaf components.
3. Rinse and De-Starch: Rinse leaves and keep them in the dark for 24+ hours to deplete stored starch.
4. Iodine Test: Apply iodine solution to leaves.
– Positive Result: If leaves turn blue-black or deep blue, starch is present. Iodine reacts with starch, causing this color change.
– Negative Result: No color change suggests absence of starch.
This iodine staining test confirms the presence or absence of starch in leaves, aiding in the study of plant physiology and photosynthesis in biology research.