Membranes root transport processes

In the beginning of the last century, it was observed that the permeability of a substance across a cell membrane is proportional to the relative partition coefficient of the permeating substance between phases of oil and water. In later years, it was found that the product of the permeability coefficient P and the square root of the molecular weight of a permeant shows a better correlation with the partition coefficient than does permeability alone. This correlation has lead to the idea that permeation is limited not only by the hpid solubility of the permeant but also by a screen-like property of the membrane because small molecules penetrate faster than would be predicted from their oil-water partition coefficients. For larger molecules, however, one would expect a product of permeability with the cube root, rather than the square root, of the molecular weight to be more closely correlated with the oil-water partition coefficient. Likewise, the apparent partition coefficient to the membrane plays a major role for the transport processes across the membrane. In strict physico-chemical terms, a membrane is not a phase, but it is most common to treat it as such anyway. 

Whatever the ulitmate site(s) of action in the plant, the metals have to cross the plasma membrane of the root cells before reaching the intracellular compartment. Therefore, this barrier (including its associated transport systems) can be considered as the first target for metal action. Only after passing the membrane, metals can interact with other cellular components and processes. 

Intracellular distribution of essential transition metals is mediated by specific metallochaperones and transporters localized in endomembranes. In other words, the major processes involved in hyperaccumulation of trace metals from the contaminated medium to the shoots by hyperaccumulators as proposed by Yang et al. (2005) include bioactivation of metals in the rhizosphere through root-microbial interaction enhanced uptake by metal transporters in the plasma membranes detoxification of metals by distributing metals to the apoplasts such as binding to cell walls and chelation of metals in the cytoplasm with various ligands (such as PCs, metallothioneins, metal-binding proteins) and sequestration of metals into the vacuole by tonoplast-located transporters. 

Many solute properties are intertwined with those of the ubiquitous solvent, water. For example, the osmotic pressure term in the chemical potential of water is due mainly to the decrease of the water activity caused by solutes (RT In aw = —V ri Eq. 2.7). The movement of water through the soil to a root and then to its xylem can influence the entry of dissolved nutrients, and the subsequent distribution of these nutrients throughout the plant depends on water movement in the xylem (and the phloem in some cases). In contrast to water, however, solute molecules can carry a net positive or negative electrical charge. For such charged particles, the electrical term must be included in their chemical potential. This leads to a consideration of electrical phenomena in general and an interpretation of the electrical potential differences across membranes in particular. Whether an observed ionic flux of some species into or out of a cell can be accounted for by the passive process of diffusion depends on the differences in both the concentration of that species and the electrical potential between the inside and the outside of the cell. Ions can also be actively transported across membranes, in which case metabolic energy is involved. 

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