Relative Quantum Efficiency of Photosynthetic Oxygen Production in Chloroplasts of Spinach

Abstract

ISOLATED chloroplasts retain most of their structural and physico-chemical properties. Unpublished experiments carried out in this laboratory have shown that the pigments chlorophyll a and β-carotene are held very close together; both are partly removed from the chloroplast by mixtures of ethyl alcohol and water. They are removed in the same proportions and the intensity of photosynthesis is determined from the quantities remaining. Chlorophyll b is more soluble than chlorophyll a and the quantity left after the treatment is not directly related to the intensity of photosynthesis. Reducing system I (β-carotene–ferredoxin–TPN–TPNH)^1–3 is also affected by the surrounding solution. Specific quantities of ferredoxin and TPNH are removed by water. After careful preparation in small volumes of solution, however, so much ferredoxin and TPNH is usually left that optimal photosynthesis may occur in red light for 5–10 min. An oxidizing system¹ consisting of the cytochromes f, b₃ and b₆ is also well constructed, but its connexion with chlorophyll (the boundary between cytochrome f and chlorophyll) is more sensitive, and portions of the cytochrome system are frequently removed from the rest of the chloroplast during grinding or sonication. The intensity of photosynthesis then decreases by a proportional extent. Careful isolation in small quantities of liquid leaves most of the chloroplasts untouched and they can then be used for the experiments. The most obvious difference between chloroplasts in whole cells and isolated chloroplasts is that the isolated chloroplasts produced strongly reduced oxygen. Isolated chloroplasts with no additions only occasionally produce limited amounts of oxygen. In most cases, however, some oxygen is absorbed. It is known that isolated chloroplasts are cut off from the metabolic system of the cytoplasm and that they do not respire in the dark. A fundamental difference between chloroplasts in whole cells and isolated chloroplasts is therefore the inability or very small capacity of the latter to produce oxygen. According to the Hill reaction for intense oxygen production an oxidant has to be added; for example, quinone or potassium ferricyanide, both substances with a positive oxidation-reduction potential.