The growth and division of the single mitochondrion and other organelles during the cell cycle of Chlorella, studied by quantitative stereology and three dimensional reconstruction

During the cell cycle of Chlorella fusca var. vacuolata 211-8p, the growth of individual organelles is not restricted and holds their average percentage contribution to total cell volume at levels close to 40% for the chloroplast, 3% for the mitochondrion and 10% for the vacuoles and nucleus throughout the cycle. In contrast pyrenoid growth is restricted to the beginning of the cycle and its volume declines after 10 hours, while starch reaches a maximum at 16 hours and disappears during the ensuing dark period. Consumption of pyrenoid starch begins first as cells enter the S phase. Total tonoplast area is equal to at least half of plasmalemma area and an increase in its area is maintained throughout the cycle, even during the period of cytokinesis because although total vacuole volume falls then, there is at this time an increase in the number of vacuoles. The vacuoles contain polyphosphate bodies, which were examined by energy-dispersive x-ray microanalysis. Three dimensional analysis by model building shows that the Chlorella cell possesses a single mitochondrion and microbody. The microbody is consistently located close to the pyrenoid. The mitochondrion forms an extensive reticulum, in places passing through the chloroplast. The total average length of the reticulum is 22.5 μm in new daughter cells and reaches a maximum of 147 μm at 10 hours, when its diameter is at a minimum of 0.17 μm. None of the above organelles has been observed to divide autonomously prior to cytokinesis. The mitochondrion and chloroplast are partitioned among the daughter cells by the outward progress of septa in the plane of phycoplast microtubules. Microtubules can play a special role in chloroplast division by directing the septum through pores, which therefore form partial cleavage planes in this organelle. No division stages of either microbody or pyrenoid have been observed and it is suggested that these may be formed de novo in daughter cells.