Intermediate diffusion hypothesis

Weak Acid" and "Intermediate-Diffusion" Hypotheses. Two hypotheses (which are not necessarily mutually exclusive) have been proposed for the entry and systemic mobility of chemicals in the phloem the "weak-acid" hypothesis proposed by Crisp and colleagues (5), and the "intermediate-diffusion" hypothesis proposed by Edgington and Peterson (4). These are illustrated in Figure 5. 

The "intermediate diffusion" hypothesis proposes that the critical determinant of phloem mobility is the optimum permeability coefficient of a given molecule, P. P is calculated as ... 

Although the pathway of Eq. (1) is now based on much evidence (Section 111) and is unambiguous in the case of at least one bacterium [Pseudomonas stutzeri strain Zobell (f. sp. P. perfectomarina)], there have been alternative hypothesis. One hypothesis, advanced by the Hollocher group (Garber and Hollocher, 1981 St. John and Hollocher, 1977), considered NO as a likely intermediate, but one that remained at least partly enzyme-bound and was not entirely free to diffuse. This view was based on the outcome of certain kinetic and isotope experiments which can be summarized as follows. When denitrifying bacteria were challenged simultaneously with [ N]nitrite and ordinary NO, the cells reduced both compounds concomitantly to N2 (or to N2O in the presence of acetylene which is a specific inhibitor (Balderston et al., 1976 Yoshinari and Knowles, 1976) of nitrous oxide reductase). In the process, little NO was generally detected in the gas phase pool of NO and there was relatively little isotopically mixed N2O formed. That is, most of the N and N reduced to NjO appeared as N2O... 

The main postulate of this delayed diffusion model is that phenyl anion II (designated as S), formed as the solid intermediate, reacts with the generated gas R after a certain minimum pressure is reached to give the final product Pj. Based on this hypothesis, plots have been prepared for the rate of outward advance (as against inward advance in the normal shrinking core model) of the second solid phase as a function of position within... 

Thus, in O.O5MH2SO4, pH 1.6, an anomalous dissolution rate of iron was found experimentally, independent of the diffusion of hydrogen from inside the metal, of the cathodic potential, and of hydrodynamics. The dissolution rate slightly increased with hydronium concentration with a reaction order close to 0.3 and with iron impurity content. In an attempt to explain this phenomenon, Kolotyrkin and Florianovich advanced the hypothesis of a chemical dissolution via the intermediate formation of hydroxo ferrous ions ... 

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