Class 11 Biology Photosynthesis In Higher Plants Early experiments on photosynthesis

Early experiments on photosynthesis

  • Joseph Priestley (1733-1804) in 1770 performed a series of experiments that revealed the essential role of air in the growth of green plants.
  • Priestley observed that a candle burning in a bell jar, soon gets extinguished and a mouse would soon suffocate in the bell jar because a burning candle or an animal that breathe the air, both somehow, damage the air.
  • When he placed a mint plant in the same bell jar, he found that the mouse stayed alive and the candle continued to burn.
  • Jan Ingenhousz, using a similar setup as the one used by Priestley, but by placing it once in the dark and once in the sunlight, showed that sunlight is essential to the plant process that somehow purifies the air fouled by burning candles or breathing animals.
  • Ingenhousz with an aquatic plant showed that in bright sunlight, small bubbles were formed around the green parts while in the dark they did not, and later he discovered that these bubbles to be of oxygen.
  • Small bubbeles which formed were to be of oxygen; hence, he showed that it is only the green part of the plants that could release oxygen.
  • Julius von Sachs provided evidence for production of glucose in chlorophyll located in chloroplasts within plant cells and glucose is usually stored as starch.
  • W Engelmann used a prism splitted light into its spectral components and then illuminated a green alga, Cladophora, placed in a suspension of aerobic bacteria, which were used to detect the sites of O2 evolution.
  • Engelmann observed that the bacteria accumulated mainly in the regions of blue and red light of the split spectrum, which resembles roughly the absorption spectra of chlorophyll a and b.
  • By the middle of the nineteenth century, the key feature of photosynthesis, i.e., plants could use light energy to make carbohydrates from CO2 and water, was known.
  • Cornelius van Niel demonstrated that photosynthesis is essentially a light-dependent reaction in which hydrogen from a suitable oxidisable compound reduces carbon dioxide to carbohydrates.
  • In green plants, H2O is the hydrogen donor and is oxidised to O2, when H2S, instead is the hydrogen donor for purple and green sulphur bacteria, the ‘oxidation’ product is sulphur or sulphate.
  • Hence, it was inferred that the O2 evolved by the green plant comes from H2O, not from carbon dioxide, and the equation of photosynthesis is

6CO2 + 12H20 à C6H1206


Fig. Priestley’s experiment

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