FACTORS AFFECTING PERMEABILITY 123 



As the preceding discussion has indicated environmental factors influence the 

 permeability of the cytoplasmic membranes to polar compounds more mark- 

 edly than to non-polar compounds. 



I. Tefnperature.— The apparent permeability of membranes to many sub- 

 stances increases with temperature. In part this effect may be due to increased 

 kinetic activity of the molecules which pass through the membrane rather 

 than to any effect of temperature on the membrane itself. A given substance 

 will penetrate through a membrane more and more rapidly as the temperature 

 is raised, until the membrane itself is destroyed, or profoundly modified by 

 the temperature attained. In living membranes the upper limit of temperature 

 which results in the destruction of the membrane is the thermal death point 

 of the protoplasm which in most active cells usually lies between 50 and 60° C. 

 At this point a sudden, irreversible increase in permeability occurs. This is 

 due to death of the protoplasm and consequent destruction of the property 

 of differential permeability in the cytoplasmic membranes. This phenomenon 

 may be observed readily by heating a small piece of red beet tissue in a test 

 tube of water. As soon as the thermal death point of the protoplasm is ex- 

 ceeded, the red pigment dissolved in the cell sap (an anthocyanin), to which 

 the living cytoplasmic membranes are impermeable, rapidly diffuses out into 



the water. 



Temperatures low enough to induce ice formation in plant tissues also 

 result in an increase in the permeability of the cells of many (but by no means 

 all) tissues, due to the resultant destruction of the protoplasm (Chap. 

 XXXIII).' This effect can be conveniently illustrated by freezing a small 

 block of red beet tissue and immersing it in water as soon as it has thawed. 

 The almost immediate outward diffusion of anthocyanins which ensues is 

 evidence of the destruction of the cytoplasmic membranes. 



2. Electrolytes.— According to Osterhout (1922) investigations made on 

 the tissue of Laminaria (a kelp) by the conductivity method^ indicate that 

 the cations Li+, Na+, K+, NH4 + , Cs+, and Rb+ all increase permeability. 

 This seems to be true regardless of the anion with which the cations are 

 paired. The cations are arranged from left to right in their probable order 

 of effectiveness in increasing permeability. This, it will be noted, is approxi- 

 mately the order of the lyotropic series. 



The initial effect of bivalent and trivalent cations such as Ca++, Ba+ + , 

 Sr+ + , Mg+ + , Fe++, La+ + +, Fe+ + + and A1+ + + as found by this method 



1 In this method permeability is determined by measuring the electrical con- 

 ductivity of cylinders or stacks o'f disks of plant tissue. The greater the electrical 

 conductivity, the greater the permeability. Since the conductivity of the tissue is 

 due to the movement of ions through it, this method measures permeability to 

 electrolytes only. 



