Relative Quantum Yields for Anaerobic Photoassimilation of Glucose

Abstract

TWO sets of photochemical reactions are assumed to originate in the two chlorophyll systems II and I which together promote photosynthesis. The main point in recent discussions of the recognized partial reactions is the attribution to either of these sets or to both. Evolution of oxygen occurs only when light is absorbed by system II, and the co-operation of system I is indispensable whenever the evolution of oxygen is coupled to the reduction of carbon dioxide. To find whether excitation of system I alone can produce useful metabolites in the intact cell, experiments are required in which long wave red light, λ > 690 mµ, elicits measurable responses. In this dark red region the chlorophyll I complex absorbs far more strongly than the chlorophyll II complex. Two metabolic processes of green algae have been found which respond particularly well to selective illumination in the region between λ690 and 730 mµ; the anaerobic reactions of algae adaptable to hydrogen and the aerobic assimilation of acetate in Chlamydobotrys^2,3. A third reaction of this kind is the incorporation of glucose by Chlorella induced by light under anaerobic conditions⁴. This reaction proceeds better in the near infra-red than in orange light at λ620 mµ. The common feature of these three metabolic reactions is that theoretically they can proceed with an adequate supply of ATP. This agrees with Tagawa's observation^5,6 that chloroplast preparations in dark red light carry out cyclic photophosphorylation faster than a Hill reaction with evolution of oxygen.