Biology Class 11th Photosynthesis

Photosynthesis (Introduction)

In this article we will discuss Photosynthesis (Introduction)

Photosynthesis is an enzyme regulated anabolic process of manufacture of organic
compounds inside the chlorophyll containing cells from carbon dioxide and water with the
help of sunlight as source of energy.

6CO2 + 6H2O → C6H12O6 + 6O2

Photosynthesis is the basis of life on earth because it the primary source of all food on earth and it is responsible for release of O2 in the atmosphere.

Chlorophyll, light and CO2 is required for photosynthesis. It occurs only in green part of
leaves in presence of light.

Early Experiments
Joseph Priestley in 1770, on the basis of his experiments shows the essential role of air in growth of green plants. A mouse kept in
closed space could get suffocated and die but if a mint plant is kept in bell jar neither candle will extinguish nor will the mouse die. He
concluded that foul air produced by animal is converted into pure air by plants. Priestley discovered Oxygen gas in 1774.

Julius Van Sachs in 1854 shows that green plant in plants produces glucose which is stored as starch. Starch is the first visible product of photosynthesis.

T.W.Engelmann (1843-19090 splits light into components by prism and then illuminated
Cladophora (an algae) placed in a suspension of aerobic bacteria. He found that bacteria
illuminated in blue and red light of the split spectrum. He thus discovered the effect of
different wavelength of light on photosynthesis (action spectrum).

Cornelius Van Neil (1897-1985) on the basis of studies with purple and green sulphur
bacteria shows that photosynthesis is a light dependent reaction in which hydrogen from an oxidisable compound reduces CO2 to form sugar.

2 H2A + CO2 → 2A + CH2O + H2O

In green sulphur bacteria, when H2S, instead of H2O was used as hydrogen donor, no O2 was evolved. He inferred that O2 evolved by green plants comes from H2O but not from CO2 as thought earlier.


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