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karthikmns
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eval_testing_mns

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Massive amounts of gaseous exchange occur through the stomata
(usually present in leaves) as long as they are open .While plants also
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carry on gaseous exchange through loose tissues in the stems, roots etc.
It is actually at the level of the organelles involved in the process of
photosynthesis and respiration that the choice of the gas required is made.
Activity-2
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Carbondioxide is necessary for Photosynthesis
We need a destarched plant to start with. We need to keep the plant in
the dark for nearly three days to remove the starch (destarch). By placing
the plant in dark the starch is removed from the leaves.
Arrange the apparatus as shown in the figure 4.
Take a wide mouthed transparent bottle.
Free distribution by T.S. Government 2019-20 5
blue black
(starch formed)
split cork
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potted plant
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wide mouthed
bottle
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KOH
colourless (no starch)
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fig-4: Mohl’s half leaf experiment
Put potassium hydroxide pellets or potassium hydroxide solution in
the bottle. Potassium hydroxide absorbs carbon dioxide.
Insert splitted cork in the mouth of the bottle.
Insert one of the leaf of destarched plant (through a split cork) into
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transparent bottle containing potassium hydroxide pellets/potassium
hydroxide solution.
Leave the plant in sunlight.
After a few hours, test this leaf and any other leaf of this plant for
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presence of starch, as mentioned in activity-1.
The leaf part which was exposed to the atmospheric air and light becomes
bluish-black, and the one inside the bottle containing potassium
hydroxide which absorbs carbon dioxide in the bottle remains
colourless. This proves that carbon dioxide is necessary for
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photosynthesis.
Why was the plant kept in dark and then in sun light ?
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Why did we test two leaves in this experiment?
We have so far discussed the role of water and gases in the process of
photosynthesis. Scientist who had been working on these lines had observed
some other factors that affect the process of photosynthesis.
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Light and Photosynthesis
In Priestley’s time, scientists didn’t understand about energy, but later
on much was discovered about it. If energy is released when carbon dioxide
and water is formed by combining oxygen with carbon and hydrogen, then
what about the reverse?. What about forming oxygen again and putting it
back in the air. Eventually, scientists learned that the energy situation would
also reverse. Oxygen formation would use up energy. That means if plants
6 X Class Nutrition - Food supplying system
form oxygen they have to get energy to make it possible. Where did the
energy come from?
A Dutch scientist, Jan Ingenhousz (1730-1799), found the answer. He
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