The photochemical reaction in photosynthesis

Abstract

From the experiments of Warburg and Negelein (1923), we know that the green alga Chlorella pyrenoidosa can reduce one molecule of carbon dioxide for each four quanta of light absorbed, when conditions permit maximum efficiency. Chlorophyll is clearly the substance absorbing the light quanta, so we may inquire how much chlorophyll must be present for the reduction of one molecule of carbon dioxide. In a preceding paper (1932) we have presented evidence that the mechanism involved in the photochemical reaction must undergo a slower reaction, the so called Blackman reaction, before it can again take part in the photochemical reaction. Let us consider a cell in flashing light when the dark periods between flashes are so long that each unit activated in a given flash has time to complete the Blackman reaction before the next flash. Increasing the intensity of the flashes should increase the carbon dioxide reduction per flash until each unit capable of undergoing the photochemical reaction does so once in each flash. We say then that the photochemical reaction is saturated with light. The possibility that any unit will undergo the light reaction more than once in a single flash may be neglected, because the time required for the completion of the dark reaction is about 0.02 sec. at 25°C., while the duration of a light flash is 10^-5 sec. We define one unit arbitrarily as the mechanism which must undergo the photochemical reaction to reduce one molecule of carbon dioxide. If we can obtain light flashes of sufficient intensity to saturate the photochemical reaction, then the number of units in a sample of cells will equal the number of carbon dioxide molecules reduced per flash. The total chlorophyll content of the sample divided by the number of carbon dioxide molecules reduced per flash will give the number of chlorophyll molecules per unit, or per molecule of carbon dioxide. The measurement of this ratio was the objective of the work described in this paper.