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maybeareleaseofotherelementsintotheenvironmentthatcompetewiththeexi-
stingspeciationpathways.
Intheprocessesofaccumulationandexcretionoftoxicelementsbylivingorga-
nisms,thepossibilityoftheirappearingintheformofbiologicalmineralsisofsigni-
ficantimportance.Biologicalmineralsarealmostalwaysformedfromtheaqueous
phase.Ionsarepackedbestintothestructureofacrystaliftheyareofasimilarsize
andtheextentofhydration(namely,whentheirspeciationisnotgreat).
Manyinteractionsoccurduringthecrystalgrowthprocess.Ionslosesomeof
theirhydrationshellsinordertofitintothecrystalstructure.Thecrystalsforma-
tionisfacilitatedifanucleationstructureexists,andtheirgrowthisfasterifthere
arestepsontheirsurface.Thesestepsappearfrequentlyastheresultofspeciation
offoreignionsthataretoolargetofitintotheregularcrystallattice.Theprocesses
whichhaveaneffectonmineralformationandwhichareinconnectionwithions
speciationphenomenonplayasignificantroleinbiology.Manybodyfluidsaresu-
persaturated.Amorphousformsofcrystalsaremostoftenthefirsttypewhichde-
velopsinsupersaturatedliquids.Ionsinamorphoussolidsarearrangedirregularly,
astheycanexistintheformswithvariousdegreeofhydration.Theirtransitioninto
morestableandlesssolublecrystalsinvolvesthegraduallossofwaterandinterfe-
ringions.
Oneofthemostinterestingexampleswhichillustratehowalivingorganism
usesthemineralphaseisthetraceelementsexcretionthroughtheirembedding
intothestructureoftheamorphousminerals.Practicallyallinvertebratespossess
cellswhichhavethecapabilityofstoringamorphousformsofcalciumphosphates
(Simkiss,1993).Theelementswithvariousdegreeofspeciation,incorporatedin
thesestructures,canbenextexpelledfromtheorganismwithfaecesasmicroscopic
spheres.
Speciationonthecellmembranesurfacehasanimpactoncellmetabolismand
alsoonionstransportacrossthemembrane.Freeionscanbeboundbyspecificpro-
teinsonthesurfaceofthecellmembraneand,amongothers,inthiswaytheyplay
apartintissuesintegritypreservation(Hynes,1999).Next,bindingionsintothe
membranereceptorcanconstituteasignalfortheinternalcellenvironment,which
carriesseriousconsequences,expressedintheirmetabolismchanges.Thecellcan
takeupelementsbythewayofendocytosis,thatisbyformationofaninvaginated
membrane-boundvesicle.Inthisway,elementsaretakenupintheformofmineral
compounds.Thespeciationfacilitatesandinducesendocytosis,e.g.,throughbinding
particlestoopsoninsorothersubstances.
Theprocesseswhichaccompanythetransportofionsacrossthecellmembrane
areverysophisticatedandthespeedofionmovementandtheiramountaredepen-
dentontheirspeciation.Mostofthecationicchannelshaveahighlyspecificstruc-
ture,thus,specificfreemetalionscangetinthecellthroughthem.Thetransport
regulationthroughtheionchannelsisincloserelationtothehydrationstateofthe
metalion,itssizeandbindingtothechannelwalls(Wallace,2000).
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