Summary
Preparations of chloroplasts fromAcetabularia carry out photosynthetic activityin vitro that is indistinguishable in rate and products from their activity in intact cells. The activity of the preparation is stable for hours. In the isolated chloroplasts, the bulk of carbon fixation occurs through the Calvin cycle; however some β carboxylation occurs. Glycolate and glycerate are rapidly turned over suggesting that they are participating in synthetic pathways. The most obvious end product of carbon fixation is sucrose, but some starch is formed in the chloroplasts. In intact cells the carbon flows into fructans which are formed in the cytoplasm. The evidence available suggests that contamination plays at most a very small role in the activities of the isolate.
In the isolate, there is a rapid accumulation of insoluble carbon, and much of it is protein. Hydrolysis of the protein reveals that all the amino acids are derived from photosynthetic products. Work in progress has also demonstrated the biosynthesis of the plastid pigments, lipids and nucleic acids from14CO2. Thus the chloroplasts are active in biosynthesis.
The properties of the isolate suggest that the chloroplast is a tight cytoplasmic compartment. Only a limited number of compounds are exchanged with cytoplasmic pools, and control mechanisms may involve the transport of photosynthetic products.
A preliminary study of chloroplasts isolated from enucleate cells has revealed a diminished rate of carbon fixation. A tentative conclusion is reached that the deficit is perhaps in the transport of HCO3-across the chloroplast membrane.
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This paper was presented at the Second International Symposium onAcetabularia in Wilhelmshaven, Germany, in July, 1972. The authors wish to thank the National Science Foundation U.S.A. for research grants supporting D. C. S. and the National Research Council of Canada and the Canada Council for financial support to R. G. S. B.
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Shephard, D.C., Bidwell, R.G.S. Photosynthesis and carbon metabolism in a chloroplast preparation fromAcetabularia . Protoplasma 76, 289–307 (1973). https://doi.org/10.1007/BF01279127
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DOI: https://doi.org/10.1007/BF01279127