Life Processes

  • Question 57

    How will you prove that chlorophyll is essential for photosynthesis?


    The following activity can prove that chlorophyll is essential for photosynthesis.


    1. Take a potted plant with variegated leaves (leaves partly green and partly non-green), for example colens, croton, money plant.
    2. Keep the plant in a dark place for 3 days. So that all the starch of leaves gets used
    3. Place the plant for 6 hours in sunlight.
    4. Pluck a leaf from the plant. Mark the green areas of the leaf and trace them on a sheet of paper.
    5. Boil the leaf first in water for a few minutes and then replace the leaf in a beaker containing alcohol. Place this beaker in a bigger beaker containing enough water. Heat till the alcohol begins to boil on a burner.
    (Note: If you use a hot plate to heat the leaf in alcohol, there is no need to use water bath. Boil the leaf directly in alcohol on a hot plate. In case, you use burner, vapour of alcohol can catch fire from flame of the burner).
    6. Boiling of the leaf in alcohol decolourizes the leaf.
    7. Place decolourlized leaf in a petridish and pour .iodine solution on it.

     To show that chlorophyll is necessary for photosynthesis

    8. Take out the leaf and rinse off the iodine solution. Observe the colour of the leaf and compare this with the tracing of the leaf done in the beginning iodine solution on it. The portion of the leaf which was previously green has turned into dark blue colour while the non-green portion turns yellow (colour of iodine solution). The dark blue colour indicates the presence of starch (starch with idone turns blue black) while the yellow portion does not show any starch.
    This proves that only chlorophyll containing cells synthesize starch. Hence chlorophyll is necessary for photosynthesis.

    Question 58

    Explain the structure of stomata and function of guard cells.


    Structure of stomata-
    The epidermis of the young shoot and leaves contains numerous minute pores called stomata. Each stomatal opening is surrounded by two cells known as the guard cells. The guard cells are living and contain chloroplasts. Their inner walls (wall towards opening) are thicker and outer walls thinner. The guard cells are surrounded by subsidiary cells or epidermal cells

    Fig. 6.4. Structure of stomata. (A) Stomatal pour is open. (B) Stomatal opening is closed

    Function of guard cells: The guard cells regulate the opening and closing of the stomatal pore. The unequal thickening of the outer and the inner layer of the guard cells allow the stomatal pore  The guard cells swell when water flows into them causing the stomatal pore to open. Because inflow of water in guard cells causes the stretching and bulging of the walls leads to opening of the pore. When there is outflow of water from guard cells, the outer thin walls come to their original position resulting in closure of stomatal pore.

    Question 59

    Write major functions of stomata present in the epidermis.


    Major functions of stomata
    (i) Stomata are essential for exchange of gases between the plant and the atmosphere— oxygen and carbon dioxide.
    (ii) Normally, plants eliminate excessive water in the form of vapour through stomatal openings. The process is called transpiration.
    (iii) Stomata regulate water loss.

    Question 60

    Devise an activity to show that carbon dioxide is essential for photosynthesis.


    The following activity proves that CO2 is essential for photosynthesis.
    1. Two potted plants of same species and of approximately of same size are taken . They are placed in a safe dark place for three days.
    2. Each potted plant is placed on separate glass plates and a watch-glass/petridish containing KOH (potassium hydroxide solution) is kept by the side of one of the plants. The potassium hydroxide solution is used to absorb carbon dioxide.
    3. Separate bell-jars are placed to cover both plants . The set-up is made air tight by applying vaseline to seal the bottom of the jars to the glass plates.
    4. The plants are then placed in sunlight for 2–3 days.
    5. After 2–3 hours, a leaf is plucked from each plant and boiledin water and then in alcohol separately to decolourize them.
    6. Each decolourized leaf is placed in separate petridish and iodine solution is poured on each leaf.
    7. The leaf of plant of set-up B which did not has potassium hydroxide turned deep blue black showing presence of enough starch. The leaf taken from set-up A either turned slightly blue or remains yellowish showing presence of a little amount of starch or no starch.

    This activity proved that CO2 is essential for photosynthesis.

    Set-up of apparatus to prove CO2 is essential for photosynthesis (A) with potassium hydroxide (B) without potassium hydroxide

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