Читать книгу Molecular Mechanisms of Photosynthesis - Robert E. Blankenship - Страница 52

As part of his attempt to settle the controversy with Warburg, Emerson and coworkers made careful measurements in the 1940s of the quantum requirements for photosynthesis as a function of wavelength and obtained a most remarkable result. As the wavelength of light utilized for the experiment approached the red edge of the absorption of the chlorophyll, the quantum requirement went up dramatically. The action spectrum for photosynthesis is remarkably congruent with the absorption spectrum throughout much of the visible wavelength range, but drops off more quickly in this far‐red region (Fig. 3.4). An action spectrum is a plot of the effectiveness of light to cause a given effect, in this case oxygen evolution, versus the wavelength of light. This decrease in quantum yield (the reciprocal of the quantum requirement) at long wavelengths came to be known as the “red drop.” The interpretation of the red drop is that those chlorophyll molecules that absorb light at the extreme red edge of the absorption band do not do photosynthesis as efficiently as the chlorophylls that absorb light of shorter wavelengths. The long‐wavelength chlorophylls somehow behaved differently. Other measurements of photosynthesis and chlorophyll fluorescence in red algae, organisms that contain an antenna complex known as a phycobilisome, also suggested that the long‐wavelength chlorophylls were somehow inactive in photosynthesis. The red drop result was easily reproduced, but the significance of it was not understood until later.

However, the result of another experiment by Emerson and coworkers was even more bizarre (Emerson et al., 1957). He found that if the ineffective long‐wavelength light was supplemented with shorter‐wavelength light, it suddenly became capable of driving photosynthesis at good rates. A sample of algae was illuminated with red light, and the intensity adjusted to give a particular rate of O₂ production, measured as always using a manometer. This light was then turned off, and a second light source, this time the inefficient far‐red light, was directed on the sample. The intensity of this light was adjusted to give a rate of O₂ production comparable to that of the red light. This required that the intensity of the far‐red light be increased significantly, as expected from the earlier experiments that had shown its weak effect. The remarkable result was that, when both beams of light were directed on the sample at the same time, the rate of O₂ production was greatly increased and was much higher than the sum of the two individual rates! This result came to be known as the enhancement effect, because of the enhancing effect of the short‐wavelength light. Additional experiments by Jack Myers and Stacey French (1960) showed that enhancement worked even when the two beams of light were not present at exactly the same time. These results made no sense in the context of the 1950s understanding of the mechanism of photosynthesis. Several years went by before a reasonable explanation was proposed for these and other puzzling results.

Figure 3.4 Absorption spectrum of chloroplasts (dashed line) and action spectrum for photosynthesis (dotted line). The red drop in the quantum yield of photosynthesis (solid line).