Re: what is the molecular structure of a serum phosphate molecule?

Good question:

In the body free phosphate exists as a mixture of two deprotonated forms of phosphate, depending on the pH. At the pH of blood, [pH 7.4 (Slightly basic)] the phosphate exists almost predominately as H₂PO₄^- and HPO₄^-2.

PO₄^-3 is not present in statistically meaningful amounts at physiological pH's.

The PubChem site (below) lists chemical information and structures for these two forms:

HPO₄^2- : hydrogen phosphate http://pubchem.ncbi.nlm.nih.gov/summary/summary.cgi?cid=3681305&loc=ec_rcs

H₂PO₄^- : The dihydrogen phosphate link is here: http://pubchem.ncbi.nlm.nih.gov/summary/summary.cgi?cid=1003&loc=ec_rcs

They are present at about a 60/40 (%) ratio at pH 7.4, as determined using mixed equilibria or the Henderson-Hasselback equation.

The size of phosphate is not substantially different than most zwitterionic compounds removed from dialysis, e.g bicarbonate, and its permability isn't substantially different from those either, but it is less so than single mono or divalent cations, e.g. Na and Ca. In most of the published information on the efficiency of various membrane configurations, PO₄ diffuses at about the same rate as creatnine, which is similarly sized, but slower than urea (uric acid) which is smaller. It certainly dialyzes faster than inulin, and Beta-2 macroglobulin, which are commonly used in dialysis monitoring.

Hope this helps. Thanks for writing in.