Anion transport processes phosphates

Fig. 10.8. Simple biogeochemical model for metal mineral transformations in the mycorhizosphere (the roles of the plant and other microorganisms contributing to the overall process are not shown). (1) Proton-promoted (proton pump, cation-anion antiport, organic anion efflux, dissociation of organic acids) and ligand-promoted (e.g. organic adds) dissolution of metal minerals. (2) Release of anionic (e.g. phosphate) nutrients and metal cations. (3) Nutrient uptake. (4) Intra- and extracellular sequestration of toxic metals biosorption, transport, compartmentation, predpitation etc. (5) Immobilization of metals as oxalates. (6) Binding of soluble metal species to soil constituents, e.g. clay minerals, metal oxides, humic substances.

AletabolicFunctions. The chlorides are essential in the homeostatic processes maintaining fluid volume, osmotic pressure, and acid—base equihbria (11). Most chloride is present in body fluids a Htde is in bone salts. Chloride is the principal anion accompanying Na" in the extracellular fluid. Less than 15 wt % of the CF is associated with K" in the intracellular fluid. Chloride passively and freely diffuses between intra- and extracellular fluids through the cell membrane. If chloride diffuses freely, but most CF remains in the extracellular fluid, it follows that there is some restriction on the diffusion of phosphate. As of this writing (ca 1994), the nature of this restriction has not been conclusively estabUshed. There may be a transport device (60), or cell membranes may not be very permeable to phosphate ions minimising the loss of HPO from intracellular fluid (61). 

Neither inhibitors of glycolysis, nor uncouplers of cellular energy-releasing processes had any significant effect on the rate of vanadate influx. Phosphate, which is readily taken up by the cells inhibited vanadate influx. Sulfate is not accumulated by the cells, and neither sulfate nor chromate appreciably inhibited vanadate influx at concentrations up to 10.9 mM and 200, uM, respectively. Most significantly, inhibitors of anion exchange across the human red cell membrane such as DIDS (4,4 - diisothiocyanostilbene - 2,2 -disulfonic acid) were also found to block vanadate transport into vanadocytes. 

The carrier protein facilitating Pj and phosphate ester transport is of particular interest in leaves in connection with carbon processing - i.e., the synthesis, transport and degradation of carbohydrate, all of which occur in the cytosol [51]. This metabolite carrier, called the phosphate translocator, is a polypeptide with a molecular mass of 29 kDa and is a major component of the inner envelope membrane [52,53]. The phosphate translocator mediates the counter-transport of 3-PGA, DHAP and Pj. The rate of Pj transport alone is three orders of magnitude lower than with simultaneous DHAP or 3-PGA counter-transport [54]. Consequently operation of the phosphate translocator keeps the total amount of esterified phosphate and Pj constant inside the chloroplast. Significantly, the carrier is specific for the divalent anion of phosphate. 

The pyruvate, glutamate and phosphate transporters catalyze net uptake and release of their substrates with stoicheiometric amounts of protons [6]. Early evidence for the electroneutrality of the process was the good inverse correlation between the H gradient across the mitochondrial membrane and the gradients of these permeant anions, especially at equilibrium and at low metabolite concentrations [96,97]. At equilibrium the rate of inward transport should equal the rate of efflux and the distribution of permeant anion should be proportional to the A pH since ... 

The phase-transfer catalyst is soluble in both water and the organic solvent. It is water soluble because it is an ion, and it is hexane soluble because of the three long-chain alkyl groups. Thus, the phase-transfer catalyst distributes itself in both phases, and freely shuttles back and forth through the phase boundary between solvent layers. In aqueous NaOH, the chloride anion exchanges with hydroxide anion, as the counterion to the ammonium cation. When it does this, the catalyst carries the hydroxide ion from the aqueous phase, as an ion-pair, across the phase boundary into the organic phase, where the base then reacts with the diethyl benzylphosphonate. The Homer-Wadsworth-Emmons reaction then occurs, producing the alkene and diethyl phosphate anion. This anion becomes associated with the ammonium cation of the phase-transfer catalyst and is transported to the aqueous layer, where the catalyst picks up another hydroxide ion and repeats the entire process. 

Anions are involved in many fundamental processes in all living things. Recognition, transport, and concentration control of anions such as chloride, phosphate, and sulfate is carried out by biological systems on a continual cycle. Fluoride, nitrate, and pertechnetate are potentially dangerous contaminants that can gain access to our water systems by various means. 

On the other hand, acidification of phosphate buffer, assuming absence of loss of alkaline or acid salt by a specific transport mechanism, can be shown to evaluate H ion secretion. Actually, since there is evidence that there may be greater permeability to the acid salt which includes a monovalent anion, while the alkaline salt includes phosphate in the form of a divalent anion, such rates may be underestimates of H ion secretion (1). In Fig. 4, a comparison between bicarbonate and phosphate acidification rate coefficients is made based on this view. This comparison must be made based on the respective rate coefficients, since acidification rates depend on the product of such coefficients with the driving force for acidification, that is, the difference between initial and final buffer base concentration in the tubular lumen. For a valid comparison, these driving forces have to be equal. It can be noted that both in control and in acetazolamide infused rats the sum of the phosphate and the dilution coefficient approximates that related to overall bicarbonate reabsorption. Thus, bicarbonate reabsorption could be broken down into two main components, one of them being H ion secretion, and the other related to processes like dilution and transport of bicarbonate out of the lumen, as evaluated by chloride inflow into non-buffer anion solutions like sulfate. According to this estimate, at least 55% of the bicarbonate reabsorbed under the described experimental conditions is transferred via the H ion secretory system, while the remainder may be attributed to the above described processes measured by means of estimation of... 

At least seven carriers control entry through the mitochondrial membrane. One carrier facilitates entry of succinate, D- and L-malate, malonate, and w 5o-tartrate anions, but not tartrate, maleate, or fumarate. Another mediates the entry of citrate, c/s-aconitate, wocitrate, and d- or L-tartrate, but not furmarate or maleate. A third carrier transports adenosine nucleotides. Also phosphate anions can enter mitochondria whereas other inorganic anions cannot (Chappell, 1966). Apart from these processes, many substances (e.g. ammonia) enter by simple diffusion as non-ionized substances, and many anions flow in and out of mitochondria if cations are present (Pressman, 1970). 

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