February 18, 2014

10th Biology - Water and Photosysthesis - New Syllabus

Water and Photosynthesis

In class VII we had already studied how Von Helmont found that water was essential for the increase of plant mass
He did not know about photosynthesis then. It was later found that increase in plant body mass or material occurred due to the process of photosynthesis. We shall study more about it in the following sections.

Once again read the chapter on nutrition in plants in class VII and write a note on Von Helmont’s experiment focusing on how he concluded that water was important for growth in plant body mass.

Air and Photosynthesis

Let us discuss a simple experiment about Photosynthesis. We have studied some others in our earlier classes. 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 was 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 in the growth of green plants (photosynthesis was still not known to scientists). Priestly, you may recall, discovered oxygen in 1774(the name oxygen was coined later by Lavoisier in the year 1775).

Priestly observed that a candle burning in a closed space, a bell jar, soon gets 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. But when he placed a mint plant in the same bell jars, he found that the mouse stayed alive and the candle when lighted from outside continued burning in the presence of the mint plant. Priestly hypothesized as follows: Plants restore to the air whatever breathing animals and burning candles remove.

What had Priestly done to introduce the mint plant without disturbing the experimental set up?
How did he light the candle from outside?
Priestey’s experiment confirms that gaseous exchange was going on and plants were giving out a gas that supported burning and was essential for the survival of animals.

But how do plants takein air and utilize carbon dioxide for photosynthesis and oxygen for respiration?
How do they make the choice?

Massive amounts of gaseous exchange occurs through the stomata present in leaves as long as they are open while plants also carry on gaseous exchange through loose tissues on 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.

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