This definition enjoyed widespread currency for decades.
This chapter focuses on transition metals. All transition metal cations are toxic—those that are essential for Escherichia coli and belong to the first transition period of the periodic system of the element and also the "toxic-only" metals with higher atomic numbers.
Common themes are visible in the metabolism of these ions. First, there is transport. High-rate but low-affinity uptake systems provide a variety of cations and anions to the cells. Control of the respective systems seems to be mainly through regulation of transport activity flux controlwith control of gene expression playing only a minor role.
If these systems do not provide sufficient amounts of a needed ion to the cell, genes for ATP-hydrolyzing high-affinity but low-rate uptake systems are induced, e.
On the other hand, if the amount of an ion is in surplus, genes for efflux systems are induced. By combining different kinds of uptake and efflux systems with regulation at the levels of gene expression and transport activity, the concentration of a single ion in the cytoplasm and the composition of the cellular ion "bouquet" can be rapidly adjusted and carefully controlled.
The toxicity threshold of an ion is defined by its ability to produce radicals copper, iron, chromateto bind to sulfide and thiol groups copper, zinc, all cations of the second and third transition periodor to interfere with the metabolism of other ions. Iron poses an exceptional metabolic problem due its metabolic importance and the low solubility of Fe III compounds, combined with the ability to cause dangerous Fenton reactions.
This dilemma for the cells led to the evolution of sophisticated multi-channel iron uptake and storage pathways to prevent the occurrence of unbound iron in the cytoplasm.
In the unique case of mercury, the cation can be reduced to the volatile metallic form. Interference of nickel and cobalt with iron is prevented by the low abundance of these metals in the cytoplasm and their sequestration by metal chaperones, in the case of nickel, or by B12 and its derivatives, in the case of cobalt.
The most dangerous metal, copper, catalyzes Fenton-like reactions, binds to thiol groups, and interferes with iron metabolism. Pathways to Central Metabolism:Search the world's information, including webpages, images, videos and more.
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A different strain of experimentalism, with more connection to physiology, emerged in South America, under the leadership of Horacio G. Piñero at the University of Buenos Aires.
Russia, too, placed greater emphasis on the biological basis for psychology, beginning with Ivan Sechenov's essay, "Who Is to Develop Psychology and .