Oxygen Concentration in Isolated Chloroplasts during Photosynthesis

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Abstract
The O2 concentration in intact and osmotically disrupted isolated spinach (Spinacia oleracea, L.) chloroplasts during photosynthesis was estimated. The chloroplasts were allowed to reduce 3-phosphoglycerate, CO2, or ferricyanide in light until the rate of O2 production was linear. When the light was turned off O2 evolution from the chloroplasts continued for a few seconds. This prolonged O2 evolution is due to an O2 surplus inside the chloroplasts which equilibrates with that in the medium. From this surplus the O2 concentration inside the chloroplasts at the moment when the light had been switched off was calculated. In all experiments the O2 concentration inside the photosynthesizing chloroplasts was higher than that outside, but was dependent upon the O2 concentration of the chloroplast medium. At low external O2 concentration (30 μm) the ratio of the internal to the external O2 concentration was about 5, whereas at concentrations corresponding to those in airsaturated water this ratio was close to 1. With osmotically broken chloroplasts this ratio was 1.2 at 30 μm O2 and almost 1 from 150 μm onward. When the O2 surplus found in broken chloroplasts during photosynthesis was related to the volume of the thylakoids, a ratio of about 2.3 was observed.