Translocation determining factors

Elongation is a cycUc process on the ribosome in which one amino acid at a time is added to the nascent peptide chain. The peptide sequence is determined by the order of the codons in the mRNA. Elongation involves several steps catalyzed by proteins called elongation factors (EFs). These steps are (1) binding of aminoacyl-tRNA to the A site, (2) peptide bond formation, and (3) translocation. 

The translocation properties of CNTs through cellular membranes may be a key factor in determining the biodistribution and cytotoxicity of CNTs. 

From this outline of the factors that determine availability, we see that the soil controls elemental availability to the extent that it limits mobility (steps 1-3). The soil may also influence absorption by roots (step 4) because it has some control over the chemical forms (speciation) that elements take in solution. The important effect of speciation is discussed in more detail in the next section. The soil may even affect translocation of elements within the plant, since there is evidence that mobility in plants is sensitive to the specific chemical form absorbed from soil solution. For example, it appears that iron absorbed from soil solution as a bicarbonate salt is immobile within the plant root and not translocated to the top. Another example is the immobility of zinc and lead in plant roots that are well supplied with phosphate. It may be that chemical precipitation reactions within (or possibly on) roots are limiting translocation. The questions raised here about the chemical forms of elements within plants fall into the realm of plant physiology and will not be pursued further. 

Fig. 1.27 Mechanisms for the control of the activity of transcription factors. Regulatory DNA binding proteins can occur in active and inactive forms. The transition between the two forms is primarily controlled by the mechanisms indicated. Activation or inactivation of transcription factors is determined by signals that become effective either in the cytoplasm or in the nucleus. Signal-directed translocation of transcription factors into the nucleus is a major mechanism for transcriptional regulation. The amount of available transcription factor can also be regulated via its degradation rate or rate of expression. Furthermore, the interaction between DNA-bound activators and the transcription complex can be regulated by various signals.

Derivatives with good water-solubility can also, in the case of lower soil moisture, reach the root zone of the plants and, being absorbed there, exert their action. However, several other factors contribute to the exertion of a selective action. Primarily responsible are the individual resistances of the plants, for which the factors determining uptake and translocation are still unknown today. The better-known biochemical factors of resistance also contribute. 

In addition to the detoxification processes described, other factors play important parts in determining the physiological selectivity. For example, the translocation rate of the. r-triazine taken up by a plant varies considerably, depending on the plant species. When translocation is rapid, the quantity of triazine ne ed to cause wilting more easily reaches the chloroplasts. 

---