Photosynthesis Problem Set 2

Problem 4 Tutorial: Calvin cycle experiment.

Calvin and colleagues determined the pathway of carbohydrate synthesis in plants by studying the incorporation of radioactive carbon dioxide into biological compounds. Suppose that photosynthesis is proceeding at a steady pace in a typical experiment with the lights on, and carbon dioxide is being combined with ribulose-bisphosphate (RuBP) to produce 3-phosphoglycerate (3PG). Then suddenly the source of carbon dioxide is eliminated. What changes in the concentrations of 3PG and RuBP would occur?

Reactions of the Calvin-Benson cycle

The key steps of the reactions of the Calvin-Benson cycle, also known as the dark reactions of photosynthesis, are shown in the diagram.

Reaction catalyzed by RUBISCO

CO2 enters the cycle as a substrate for RUBISCO, the CO2-fixing enzyme of photosynthesis. The reaction catalyzed by RUBISCO is as follows:

This problem describes an actual experiment done by Calvin and Benson to help determine the cyclic nature of the pathway. There is a steady supply of NADPH and ATP because the lights are on. When the source of CO₂ is eliminated, the RUBISCO reaction is blocked. The production of 3PG and the consumption of RuBP both stop. The other reactions of the cycle can continue to synthesize RuBP until their substrates are used up. Therefore 3PG levels will fall, and RuBP levels will rise.

The Biology Project
University of Arizona
Thursday, October 3, 1996
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