February 18, 2014

10th Biology Photosysthesis and Light - New Syllabus

Light and Photosynthesis

In Priestley’s time, scientists didn’t quite understand about energy, but later on much was discovered about it. If combining oxygen with carbon and hydrogen atoms to form carbon dioxide and water produced energy , 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 . This meant that if plants formed oxygen they had to get energy to make that possible. Where did the energy come from?

A Dutch scientist, Jan Ingenhousz ( 1730-1799), found the answer. He kept studying the way in which plants formed oxygen and, in 1779, noticed that this only happened in the light. In an experiment with the aquatic plant, Hydrilla, he observed that in bright sunlight, small bubbles formed around the green parts while in the dark they did not form. He also found that the gas present in the bubbles was oxygen.

It was further confirmed when Engelman in the early 20th century ingeniously detected the point of maximum photosynthesis. He used a strand of algae and exposed it to different colours of light (the colours that we see in a rainbow) He then used oxygen sensitive bacteria and found them to crowd areas illuminated with red and blue rays of light. This lead to more studies on light and photosynthesis and the role of different coloured compounds called pigments in plants and the utilization of light energy.

Activity-3

Oxygen is produced during Photosynthesis in the presence of light
• Arrange the apparatus as shown in the figure.
• Place some water plant (Elodea or Hydrilla) in a beaker containing pond water and cover these by a shortstemmed
funnel.
• Invert a test-tube full of water over the stem of the funnel. Ensure that the level of water in the beaker is
above the level of stem of the inverted funnel.

Place the apparatus in the sun for at least 3 days (when you leave school keep it back in your classroom). You would see that in place of water there is air that fills in. It is actually a gas that will collect in the test-tube. Test the gas in the test-tube by inserting a glowing incense stick which would burst into flames. This shows the presence of oxygen.

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