452 CHEMICAL AGENTS 



posure to one material does not materially change the results with 

 another. Closely related chemicals appear, however, to compete for a 

 common receptor (246, 288). 



7. Absorbed toxicants of some types are held firmly in or on the 

 spore and can be released in quantity only by rather drastic treatment. 

 Silver, by contrast, is easily removed by exchange (292). 



The relation of uptake to effectiveness is shown in Figure 5 for a 

 series of related s-triazines. Fungitoxic action is seen to be correlated 

 with the capacity of the spore to remove the toxicant from solution. 



In general, two mechanisms suggest themselves as alternative ex- 

 planations of the phenomenon of rapid uptake against a concentra- 

 tion gradient. First, it is, of course, known that "active transport" 

 of nutrient materials into cells occurs commonly and results in accumu- 

 lation against a gradient, and Pramer (336) has shown that strepto- 

 mycin is so accumulated by Nitella clavata. However, to suggest active 

 transport as the mechanism of entrance of a variety of toxicants raises 

 the difficulty that active transport as usually understood necessarily 

 involves the expenditure of metabolic energy and is prevented by many 

 inhibitors. As discussed later in connection with the heavy metals, it 

 seems at present more promising to look toward a second mechanism, 

 the adsorption of the toxicant by a solid phase in or on the cell. For 

 both these alternative mechanisms, it should be noted, the same general 

 relation between uptake and external concentration holds; in both 

 there is some saturation value of the external concentration. Conse- 

 quently, whether or not the concentration-uptake relation is described 

 by the adsorption isotherm does not distinguish between active trans- 

 port and adsorption. More important results will come from studies 

 of the effect of metabolic state upon uptake. 



Drawing primarily from studies on the action of heavy metals 

 (see below), we may suggest that a toxic material which does not move 

 freely across the semi-permeable membrane undergoes a two-stage 

 process. In the first phase there is a rapid sorption of the toxicant 

 outside the permeability barrier, or perhaps both outside it and on it. 

 In the second phase, the material is released at a slower rate to the 

 interior of the cell from the saturated peripheral sites; only in this 

 phase, for most materials, does interference with growth occur. 



This hypothesis has many difficulties which are, however, better con- 

 sidered in connection with heavy metal toxicity. It ignores, for con- 

 venience, the possibility that the sorbed toxicant may exert specific 

 effects before it enters the cell at all; this possibility is treated as a sepa- 

 rate topic. 



