March 4, 2014

10th Biology - Photosynthesis and Sun Light - New Syllabus

Sunlight is necessary to form starch: Take a plant with destarched leaves. Remember the process of destraching leaves.
• Cover one of its leaves with black paper on which a design is cut. Fix
the paper on the leaf in such a manner that light does not enter the dark
part.
• Place this plant in the sun.
• After few hours of exposure to bright sunlight, test the leaf which is
covered by black paper for the presence of starch.
• It will be observed that only the parts of the leaf, which could get light
through the cut out design, turns blue-black showing the presence of starch.

Chlorophyll and Photosynthesis

Ingenhousz wanted to find out more about photosynthesis and carried out several other experiments. He proposed that only green plant parts could carry out the process of photosynthesis.

What about plants having colored leaves? How is it that new leaves which look dark red in colour in several plants turn green? Do plants having reddish or yellowish leaves also carry out photosynthesis? Questions like these remained challenges until
scientists could isolate the green coloured substance from plant parts and study its nature.

Establishment of Ingenhousz’s proposition came after several experiments till the mid 20th century when scientists could also locate the site of photosynthesis and even isolate it. Around four decades after Ingenhousz’s proposition scientists could only isolate the green substance to observe its nature and find out whether photosynthesis could be carried out with it. This had become possible in the year 1817 due to the work of two scientists Pelletier and Caventou who obtained an extract of the green
colored substance and named it as chlorophyll means green leaf.

It was also found that the other pigments would aid in the process of photosynthesis by passing on the energy of sunlight trapped by them to chlorophyll.

Where does Photosynthesis take place?

Try to name some parts where you think photosynthesis occurs. Do you think the new reddish leaves of plants also carry out photosynthesis? What could be the role of their colour?

The exact location of the photosynthetic part or a part containing chlorophyll was not known till another 60 decades after Pelletier and Caventou discovered chlorophyll. It was believed to be spread in the cells of green plant parts. In 1883, Julius von Sachs, observed that chlorophyll in plant cells is not spread through the entire cell. It is found in organelles within the cell. Such organelles were named as chloroplasts. These are present in large numbers in the cells (around 40 – 100) of parts like the stomatal guard cells and ground tissues of plants .

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