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Ultrastructure and physical characteristics of proplastids and chloroplasts isolated by zonal centrifugation from Euglena gracilis var. bacillaris.

Proplastids at various stages of development and mature chloroplasts were isolated by zonal centrifugation in a double-sigmoid, sucrose-sorbitol gradient from Euglena gracilis var. bacillaris grown in darkness and subjected to various periods of illumination with white light. Computer programs were used in characterizing physical properties of the isolated organelles and in predicting the location of plastids of different developmental stages in the gradient. The technique permits computation of these parameters in any gradient material. The convoluted nature of the early developing plastids, revealed by electron microscopy, suggests that the frictional coefficients should be taken into account in describing the physical constants of the organelles, since their sedimentation coefficients are lower than would be expected for spherical particles of the same size and density.