title: Freeze-fracture analysis of phloem structure in plant tissue cultures. II. The sieve element plasma membrane.
abstract: Sieve element plasma membranes reveal a unique distribution of intramembrane particles (IMPs) in tissue cultures fixed and cyroprotected prior to freeze-fracturing. Sieve element IMPs are smaller than those found in the plasma membranes of callus parenchyma cells from these same cultures. The PF/EF ratio of plasma membrane IMPs is 9.6 for parenchyma cells and 1.21 for sieve elements. The increased binding of IMPs to the sieve element E face may be related to the role of membrane proteins in the loading of sucrose and other molecules by these cells. The enlargement of the cell wall at the site of sieve area pores creates complementary ridges and depressions in the E and P fracture faces of sieve element plasma membranes. No alteration of IMP density is seen at the sieve area pore site.
mesh_terms: D002462:Cell Membrane / Q000648:ultrastructure; D046508:Culture Techniques; D004721:Endoplasmic Reticulum / Q000648:ultrastructure; D005614:Freeze Fracturing; D008854:Microscopy, Electron; D008954:Models, Biological; D010944:Plants / Q000648:ultrastructure*