Freeze-fracture replicas of the plasma membrane of unfixed, uncryoprotected Paramecium tetraurelia bear large rectilinear arrays of 11 nm particles arranged in 7–11 parallel rows. The arrays are of sufficient size to leave impressions in replicas of the underlying outer alveolar membrane, and are apparent as parallel ridges in replicas of the surface coat of deep-etched cells. By noting the location of arrays in replicas of identified portions of the cortex of P. tetraurelia, it has been possible to map the distribution of arrays over the cell surface. The arrays are found primarily over the anterior surfaces of the cell, covering an area that extends from the preoral suture over the left adoral field and a large portion of the anterior dorsal surface. Freeze-fracture analyses of cells taken from a number of different stages of a culture cycle suggest that the particle arrays are not replicated as an integral part of the cortex during cell division, but are assembled and oriented in the membrane as the cells mature. The appearance of small intramembranous particle complexes in the plasma membrane of cells in logarithmic growth phase supports this hypothesis, possibly representing an assembly stage in the formation of the larger particle arrays. The facts that the particle arrays are apparent in replicas of the surface coat of cells, are found primarily at the anterior of the cell body, and have a highly specific orientation with respect to the cell surface, strongly suggest that they function as chemoreceptors in P. tetraurelia.

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