Some of the factors affecting penetration in living cells may be advantageously studied in models in which the organic salts KG and NaG diffuse from an aqueous solution A, through a non-aqueous layer B (representing the protoplasmic surface) into an aqueous solution C (representing the sap and hence called artificial sap) where they react with CO2 to form KHCO3 and NaHCO3. Their relative proportions in C depend chiefly on the partition coefficients and on the diffusion constants in the non-aqueous layer. But the ratio is also affected by other variables, among which are the following:
1. Temperature, affecting diffusion constants and partition coefficients and altering the thickness of the unstirred layers by changing viscosity.
2. Viscosity (especially in the non-aqueous layers) which depends on temperature and the presence of solutes.
3. Rate of stirring, which affects the thickness of the unstirred layers and the transport of electrolyte in those that are stirred.
4. Shape and surface area of the non-aqueous layer.
5. Surface forces.
6. Reactions occurring at the outer surface such as loss of water by the electrolyte or its molecular association in the non-aqueous phase. The reverse processes will occur at the inner surface and here also combinations with acids or other substances in the "artificial sap" may occur.
7. Outward diffusion from the artificial sap. The outward movement of KHCO3 and NaHCO3 is small compared with the inward movement of KG and NaG when the concentrations are equal. This is because the partition coefficients3 of the bicarbonates are very low as compared with those of NaG and KG.
Since CO2 and HCO3- diffuse into A and combine with KG and NaG the inward movement of potassium and sodium falls off in proportion as the concentration of KG and NaG is lessened.
8. Movement of water into the non-aqueous phase and into the artificial sap. This may have a higher temperature coefficient than the penetration of electrolytes.
9. Variation of the partition coefficients with concentration and pH.
Many of these variables may occur in living cells. (It happens that the range of variation in the ratio of potassium to sodium in the models resembles that found in Valonia.)