Question

Which of these conclusions drawn by Cornelius van Neil in his experiment on purple-sulphur bacteria is correct?

• A. He inferred that the oxygen in plants comes from water, which was later proved using radioisotope techniques.
• B. He demonstrated that photosynthesis is essentially a light-independent reaction.
• C. In purple sulphur bacteria, the oxidation products are oxygen and sulphates.
• D. Water is the hydrogen donor in purple sulphur bacteria.

Answer 1

Answer: (A)

Cornelius van Niel (1897-1985), based on his studies of purple and green bacteria, 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. Some organisms do not release O₂ during photosynthesis. When H₂S, instead is the hydrogen donor for purple and green sulphur bacteria, the ‘oxidation’ product is sulphur or sulphate depending on the organism and not O₂. Hence, he inferred that the O₂ evolved by the green plant comes from H2O, not from carbon dioxide. This was later proved by using radioisotopic techniques.