Photosynthesis of Lipids from 14CO2 in Spinacia oleracea

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Abstract
Young expanding spinach leaves exposed to 14CO2 under physiological conditions for up to 20 minutes assimilated CO2 into lipids at a mean rate of 7.6 micromoles per milligram chlorophyll per hour following a lag period of 5 minutes. Label entered into all parts of the lipid molecule and only 28% of the 14C fixed into lipids was found in the fatty acid moieties, i.e. fatty acids were synthesized from CO2in vivo at a mean rate of 2.1 micromoles per milligram chlorophyll per hour. Intact spinach chloroplasts isolated from these leaves incorporated H14CO3 into fatty acids at a maximal rate of 0.6 micromole per milligram chlorophyll per hour, but were unable to synthesize either the polar moieties of their lipids or polyunsaturated fatty acids. Since isolated chloroplasts will only synthesize fatty acids at rates similar to the one obtained with intact leaves in vivo if acetate is used as a precursor, it is suggested that acetate derived from leaf mitochondria is the physiological fatty acid precursor.