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karthikmns
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eval_testing_mns

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What do you see/notice?
The presence of starch will be indicated by a blue-black colour.
Do you think solar energy transforms into chemical energy by the
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process of photosynthesis?
Factors (Materials) essential for the process of Photosynthesis
What are the materials that you think would be essential for the
synthesis of carbohydrates in the process of photosynthesis? (Hint: see
the equation proposed by Van Neil)
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Do you think the equation tells us about all the materials involved?
It took scientists over 300 years to find out about them. They opined
that we don’t know about several materials involved in the process still.
Let us study how scientists experimented to find out about some of
the materials required for the process of photosynthesis.
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Water and Photosynthesis
In class VII we already studied how Von Helmont found that the water
is essential for the increase of plant mass.
He did not know about photosynthesis then. Later, it was found that
increase in plant body mass or material occurred due to the process of
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photosynthesis. We shall study more about it in the following sections.
Read the chapter on ‘Nutrition in Plants’ in class VII. Discuss with
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your friends and write a note on Von Helmont’s experiment focussing on
how he concluded that water was important for plant growth and increase
in body mass.
Air and Photosynthesis
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Let us discuss an experiment on photosynthesis. This one helps us to
find out about the role of air in the process of photosynthesis. It is
interesting to learn about the experiment which is one of the several
milestones in the gradual development of our understanding of
Photosynthesis.
Joseph Priestly (1733-1804) in 1770 performed a series of experiments
that revealed the essential role of air on the growth of green plants
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4 X Class Nutrition - Food supplying system
(photosynthesis was still not known to scientists at
that time). You may recall, Oxygen was discovered
by Priestly in 1774. The name oxygen was coined
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later by Lavoisier in the year 1775. Priestly observed
that a candle burning in a closed bell jar, soon gets
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extinguished. Similarly, a mouse would soon
suffocate in a closed space of the bell jar. He
concluded that a burning candle or an animal, both
somehow, damage air. When he placed a mint plant
in the same bell jars, he found that the mouse stayed
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alive and the candle when lighted from outside
continued burning in the presence of the mint plant.
Priestly hypothesized as follows: ‘Plants restore the
air what breathing animals and burning candles
remove’. fig-3: Priestly experiment
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Do you find any relationship between candle, rat, mint plant ? Discuss.
Do similar experiment in your school. Use a candle, plant in a small
pot and a bell jar. Note down your observations and explain.
Priestly’s experiment confirmed that gaseous exchange was going on
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and plants were giving out a gas that supported burning and was essential
for the survival of animals.
But how do plants take in air and utilize carbon dioxide for
photosynthesis and oxygen for respiration?
How do they make the choice?
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