Search

Chloroplasts, both in situ and isolated, were dehydrated at low temperatures to avoid chlorophyll extraction. These chloroplasts were fixed with glutaraldehyde only or with glutaraldehyde followed by osmium. Chloroplasts dehydrated at low temperatures have narrower fret membranes, but the same dimensions of the partitions as chloroplasts dehydrated by standard techniques. However, the two membranes forming the partition are separated by an electron transparent region or A-component in low temperature dehydrated chloroplasts post-fixed with osmium. With glutaraldehyde only, the A-component appears as a thin discontinuous dark line. The results indicate that the A-component is a structural component of the partition and supports the hypothesis that chlorophyll and the hydrophibic lipids are localized there. [ABSTRACT FROM AUTHOR]

The ultrastructure of isolated mesophyll chloroplasts of Zea mays L. was studied using the shadow casting technique. Grana and interconnecting fret membranes were observed in the same basic arrangement as they appear in thin sections. Quantasome units, as previously described by other authors for other species, were detected. Portions of the peripheral reticulum and envelope were also observed. Swollen membranes, comparable to those observed when isolated plastids are placed in water, were frequent in the preparations. It can be assumed, then, that in any preparation of this nature the hydrophillic spaces will absorb a large amount of water, swelling the intramembranous spaces and giving the membranes the appearance of large vesicles which will appear as flattened sacs when dried on the electron microscope grids. As there is no indication of any quantasome pattern in these membranes it is assumed that the particle arrangement of these membranes was altered during the procedure. It was not possible to determine whether the membranes observed in the swollen vesicles belonged to the grana or frets. A change in the morphology of the membranes during the processing may be observed with the light microscope. Recognition of this change is frequently difficult in electron micrographs, consequently, membranes with similar appearance under the electron microscope may actually have arisen from different portions of the plastid membrane systems. [ABSTRACT FROM AUTHOR]

For the cell physiologist interested in the structure of the mesophyll cell and the functions of its various parts the immediate problems may be listed as follows: First, pure suspensions of chloroplasts, nuclei, mitochondria and hyaloplasm must be obtained. Prerequisite to the obtaining of such pure suspensions are adequate means of identifying structures occurring in both leaf cell and in leaf homogenate. Such a study may well lead a revolution of the cytologists conception of the mesophyll cell. Secondly, it is not sufficient to obtain pure fractions of chloroplasts, nuclei, mitochondria and hyaloplasm. These materials must be obtained with as little change from their living state as possible and such changes as occur must be carefully evaluated and interpolated in the final conclusions. [ABSTRACT FROM AUTHOR]