Explain by an activity :
(1) sunlight is necessary for the process of photosynthesis to take place
(2) Co2
is necessary for
Photosy wthesis
Answers
Answer:
1.
AIM
To show experimentally that light is necessary for photosynthesis.
MATERIALS REQUIRED
A potted plant, black paper strips or Ganongs light screen, clips or cellotape, alcohol, iodine solution, water bath, beaker, burner, water, petri dish, dropper.
THEORY
Light is one of the requirements for photosynthesis. When a part of the leaf is covered from both upper and lower sides with a black paper strip, light will not reach to covered part. Hence, photosynthesis will not take place in covered part of the leaf. On the other hand, as light will be available to uncovered part of the leaf, cells of this region will carry out photosynthesis actively and hence, glucose will be synthesised in the uncovered part of the leaf. In the leaf a part of this glucose is utilised to carry out various metabolic activities and rest of the amount converts into starch. Presence of starch in the leaf, which can be detected by performing iodine test, indicates occurrence of photosynthesis.
PROCEDURE
Select a healthy potted plant and destarch the plant by placing it in complete darkness for at least 48 hours.
After 48 hours, using clips or cellotape cover a part of the leaf of the plant from both upper and lower sides with black paper strips.
Now, keep the potted plant in sunlight for at least 3-4 hours.
After 3-4 hours, pluck the leaf and remove black paper strips.
Boil the leaf in water for about 2 minutes.
Now, to decolourise the leaf, transfer the leaf from boiling water to a beaker containing ethanol and boil the leaf in a water bath.
Remove the leaf from beaker and wash it in water at room temperature.
Place this leaf in petri dish and add few drops of iodine solution over the decolourised leaf.
The area of the leaf that was uncovered performed photosynthesis and will have starch, therefore turned blue-black with iodine solution.
The covered portion of the leaf remains yellow as it does not perform photosynthesis.
[Note: Ganong’s light screen can be used in place of black paper strip]
CBSE Class 10 Science Practical Skills – Light is Necessary for Photosynthesis 2
CBSE Class 10 Science Practical Skills – Light is Necessary for Photosynthesis 3
OBSERVATIONS
The uncovered part of the leaf turns blue-black. This confirms the presence of starch.
The covered portion of the leaf remains yellow showing that no starch synthesis occured in this region.
RESULT
This proves that light is necessary for the photosynthesis.
PRECAUTIONS
Use healthy potted plant.
Plant should be destarched by placing in the dark.
The black paper strip should be attached tightly on both sides of leaf so that no sunlight enters in covered portion of leaf.
Always use a water bath for boiling leaf in alcohol as alcohol is inflammable.
Wash the leaf before performing iodine test.
2.
Aim : To prove that carbon dioxide is essential for photosynthesis by Mohl's half leaf experiment. Apparatus : Wide mouthed bottle, KOH solution, potted plant, vertically split cork, Iodine solution, wax.
Procedure : Arrange the apparatus as shown in the figure.
(1) Take a potted plant and keep it in the dark for nearly 3 – 4 days for the removal of starch from the leaves.
(2) Take a wide mouthed transparent bottle.
(3) Pour potassium hydroxide solution in the bottle.
(4) Potassium hydroxide absorbs carbon dioxide in the bottle.
(5) Insert splitted cork in the mouth of the bottle. (6) Insert one of the leaves of destarched plant through a split cork into transparent bottle containing potassium hydroxide pellets or potassium hydroxide solution.
(7) Apply wax or grease to split cork so that, CO2 will not enter into the bottle.
(8) Leave the plant in sunlight.
(9) After a few hours, test this leaf and other leaf of this plant for starch.
Observation :
(1) The part of the leaf which is exposed to the air becomes bluish - black.
(2) The remaining part which is inside the flask containing KOH does not turn to bluishblack. Result : This experiment proves that CO2 is necessary for photosynthesis.
Precautions :
(1) The part of the leaf kept inside the bottle should not touch potassium hydroxide solution. (2) The apparatus should be kept air tight by applying grease or vaseline.
Explanation:
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