Читать книгу Molecular Mechanisms of Photosynthesis - Robert E. Blankenship - Страница 51

In the 1940s and 1950s, a controversy raged in the field of photosynthesis over the minimum quantum requirement for the process (Nickelsen and Govindjee, 2011). The quantum requirement is the number of photons that need to be absorbed for a photochemical process to take place. It is the reciprocal of the quantum yield. Otto Warburg, the Nobel prize‐winning German biochemist who had developed the manometric techniques that were standard for measurement in these experiments, steadfastly maintained that the minimal quantum requirement for photosynthesis was three to four photons per O₂ evolved. Essentially everyone else obtained much higher values, in the range of 8–10 photons per O₂ produced. Foremost among these researchers was Warburg's former student, Emerson, who had earlier carried out the experiments with Arnold described above. The argument raged on for many years and was really only settled after Emerson's premature death in an airplane crash in 1959, followed by Warburg's death in 1970.

This disagreement may seem to be only an academic issue, but the outcome was essential to the development of a deeper understanding of the underlying chemical mechanism of photosynthesis. The discussion really boils down to energetics. The energy content of the three photons that Warburg thought were all that was needed is just barely enough to account for the free energy difference between the reactants and the products (see Chapter 13). Warburg was pleased with this result, which coincided with his nineteenth‐century romantic view of nature, summarized by the comment often attributed to him: “In a perfect world photosynthesis must be perfect.” Emerson's view was more practical, and thousands of subsequent measurements in many laboratories have supported his higher numbers for the quantum requirement for photosynthesis.

Exactly why Warburg obtained the results he did is still not entirely clear, but it is thought to have to do with interactions of photosynthesis and respiration, including transient “gushes” and “gulps.” The measurement shows only net oxygen production; to get the rate of photosynthesis, it is necessary to correct for the rate of respiration. If the rate of respiration is unchanged between light and dark, this correction will be accurate; but if photosynthesis inhibits respiration (as some modern evidence suggests), the correction will lead to erroneously low values for the quantum requirement. In retrospect, it is clear that Warburg, despite being a brilliant experimenter and very experienced professional scientist of the highest rank, fell into the very human trap of thinking that he knew what the answer should be and then not being sufficiently objective in evaluating his own experiments.

Unfortunately, the quantum requirement controversy took up the enormous time and effort of many of the foremost scientists of the day and didn't directly lead to a new understanding of the mechanism of photosynthesis. However, in the process of thoroughly examining the conditions required for the measurement of the quantum requirement for photosynthesis, some important new discoveries were made, which ultimately did lead to a much deeper understanding. Chief among these were the phenomena known as the “red drop” and “enhancement.”