Page 8 - Science Class 07
P. 8
Why does photosynthesis take place only in
green plants?
Aim : To test a leaf for starch.
Leaf is the site of photosynthesis. Only green Materials Required : A green plant, test
plants are called autotrophs because photo- tube, alcohol, water, iodine solu on and
synthesis takes place in green plants. Green breaker.
plants contain chlorophyll in their leaves.
Method : Take a green leaf from the plant
Chlorophyll is an important element that traps
and keep it in a test tube containing alcohol.
sunlight. Besides, leaves have small pores Now, place the test tube in a beaker
called stomata, plants take in carbon dioxide containing water. Now, heat up the beaker
through these pores. un l alcohol starts boiling.
Water and other minerals are absorbed by
Note : You should heat the beaker to an
roots from the soil. When all the raw materials extent that water does not boil.
are present in the green plant, the process of
photosynthesis begins. The leaves synthesise The alcohol will dissolve chlorophyll present
food in the form of glucose and oxygen is in the leaf, and the leaf loses its green colour.
Now, take out the leaf from the test tube
released. In most of the plants, glucose is
and wash it in warm water. Now, spread the
converted into starch and stored in the leaves,
leaf on a wooden table and pour a few drops
stems, roots and fruits. We can test the of iodine solu on on the leaf.
presence of starch in the leaves by an easy
ac vity. The presence of starch also tells us that A B C D
photosynthesis takes place in the leaves of the Alcohol
plants. Water Iodine
White le
You will observe that the colour of the
leaf changes to blue-black which
The leaves of the plants look green due to confirms the presence of starch in the
presence of chlorophyll. They reflect green leaf.
colour and absorb other colours of the light
spectrum.
Aim : To check carbon dioxide is necess-
ary for photosynthesis.
Materials Required : A po ed plant, a
flask and potassium hydroxide (KOH).
Method : Take a destarched po ed plant.
Take a conical flask and put some
When a plant is kept in the dark for 2-3 days potassium hydroxide into it. Potassium
the plant is distracted. It means the starch hydroxide absorbs the carbon dioxide.
present in the leaves is used up by the plant Insert one leaf into this conical flask
and no starch is le in the leaves.
8 Science-7
