ABSTRACT
Tracers can be tagged to specific molecules of biological interest and used to follow the fate of the latter within a cell. The use of the ultrastructural tracers in permeability studies assumes a thorough characterization of their molecular weight, size, shape, net electrical charge, and chemistry. The molecule's shape and flexibility or rigidity are also important parameters that determine the permeability behavior of the tracers. The procedures for protein cationization and anionization do not significantly change the molecular weight; however, some conformational modifications may occur. Based on the method of detection, the tracers used for macromolecular transport fall into four major classes: particulate tracers, peroxidatic tracers, radiolabeled tracers, and tracers tagged to biological molecules. Due to the variable degree of polycondensation, dextrans and glycogens have different molecular weights and sizes. Dextran and glycogen powders are soluble in 0.15 M NaCl and in various buffer solutions; the dissolution can be accelerated by sonication.
