Page 87 - NCERT Science Class 10 English Medium

Let us now see what actually happens during the process of
photosynthesis. The following events occur during this process –

(i) Absorption of light energy by
chlorophyll.
(ii) Conversion of light energy to chemical
energy and splitting of water molecules
into hydrogen and oxygen.
(iii) Reduction of carbon dioxide to
carbohydrates.
These steps need not take place one after
the other immediately. For example, desert
plants take up carbon dioxide at night and
prepare an intermediate which is acted upon
by the energy absorbed by the chlorophyll
during the day.
Let us see how each of the components of
the above reaction are necessary for
photosynthesis.
If you carefully observe a cross-section of a
leaf under the microscope (shown in Fig. 5.1),
you will notice that some cells contain green
dots. These green dots are cell organelles called
chloroplasts which contain chlorophyll. Let us
Figure 5.1
Figure 5.1 do an activity which demonstrates that
Figure
Figure 5.15.1
Figure 5.1
Cross-section of a leaf
chlorophyll is essential for photosynthesis.
Activity 5.1
Activity
Activity 5.1
Activity 5.15.1
Activity 5.1
n Take a potted plant with variegated leaves – for example, money plant
or crotons.
n Keep the plant in a dark room for three days so that all the starch
gets used up.
n Now keep the plant in sunlight for about six hours.
n Pluck a leaf from the plant. Mark the green areas in it and trace them
on a sheet of paper.
n Dip the leaf in boiling water for a few minutes.
n After this, immerse it in a beaker containing alcohol.
n Carefully place the above beaker in a water-bath and heat till the
alcohol begins to boil.
n What happens to the colour of the leaf? What is the colour of the
solution?
n Now dip the leaf in a dilute solution of iodine for a few minutes.
n Take out the leaf and rinse off the iodine solution.
n Observe the colour of the leaf and compare this with the tracing of
Figure 5.2
Figure 5.25.2
Figure
Figure 5.2 the leaf done in the beginning (Fig. 5.2).
Figure 5.2
Variegated leaf (a) before n What can you conclude about the presence of starch in various areas
and (b) after starch test of the leaf?
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