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  <xsd:element id="plx_ChiesiAgreementsMember" name="ChiesiAgreementsMember" nillable="true" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" xbrli:periodType="duration" />
  <xsd:element id="plx_InterestReceivedNet" name="InterestReceivedNet" nillable="true" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" xbrli:balance="debit" />
  <xsd:element id="plx_SeniorVicePresidentAndChiefFinancialOfficerMember" name="SeniorVicePresidentAndChiefFinancialOfficerMember" nillable="true" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" xbrli:periodType="duration" />
  <xsd:element id="plx_SevenPointFivePercentageSeniorSecuredConvertibleNotesDue2024Member" name="SevenPointFivePercentageSeniorSecuredConvertibleNotesDue2024Member" nillable="true" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" abstract="true" xbrli:periodType="duration" />
  <xsd:element name="ConvertibleNotesDue2021And2024Member" id="plx_ConvertibleNotesDue2021And2024Member" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <xsd:element name="NumberOfGlobalLicensingAndSupplyAgreements" id="plx_NumberOfGlobalLicensingAndSupplyAgreements" type="xbrli:integerItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="instant" abstract="false" />
  <xsd:element name="PayableToCustomerCurrent" id="plx_PayableToCustomerCurrent" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="instant" abstract="false" xbrli:balance="credit" />
  <xsd:element name="AccountsPayableAndAccruedLiabilitiesOtherCurrent" id="plx_AccountsPayableAndAccruedLiabilitiesOtherCurrent" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="instant" abstract="false" xbrli:balance="credit" />
  <xsd:element name="StockIssuedDuringPeriodValueWarrantsAndStockOptionsExercised" id="plx_StockIssuedDuringPeriodValueWarrantsAndStockOptionsExercised" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="false" xbrli:balance="credit" />
  <xsd:element name="StockIssuedDuringPeriodSharesWarrantsAndStockOptionsExercised" id="plx_StockIssuedDuringPeriodSharesWarrantsAndStockOptionsExercised" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="false" />
  <xsd:element name="ProceedsFromWarrantsAndStockOptionExercises" id="plx_ProceedsFromWarrantsAndStockOptionExercises" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="false" xbrli:balance="debit" />
  <xsd:element name="SalesAgreementMember" id="plx_SalesAgreementMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <xsd:element name="CommonStockIncreaseInGrossSalesPriceOfSharesReservedForFutureIssuance" id="plx_CommonStockIncreaseInGrossSalesPriceOfSharesReservedForFutureIssuance" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="instant" abstract="false" xbrli:balance="credit" />
  <xsd:element name="CommonStockValueOfCapitalSharesReservedForFutureIssuance" id="plx_CommonStockValueOfCapitalSharesReservedForFutureIssuance" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="instant" abstract="false" xbrli:balance="credit" />
  <xsd:element name="GrossProceedsFromIssuanceOfCommonStock" id="plx_GrossProceedsFromIssuanceOfCommonStock" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="false" xbrli:balance="debit" />
  <xsd:element name="OutstandingStockOptionsUnvestedRestrictedStockAndWarrantsMember" id="plx_OutstandingStockOptionsUnvestedRestrictedStockAndWarrantsMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <xsd:element name="SingleReportableSegmentMember" id="plx_SingleReportableSegmentMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <xsd:element name="OutstandingStockOptionsUnvestedRestrictedStockWarrantsAndConvertibleNotesMember" id="plx_OutstandingStockOptionsUnvestedRestrictedStockWarrantsAndConvertibleNotesMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="true" />
  <xsd:element name="OtherNonoperatingExpenseNetOfgainLossForeignCurrencyTransactionBeforeTax" id="plx_OtherNonoperatingExpenseNetOfgainLossForeignCurrencyTransactionBeforeTax" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" nillable="true" xbrli:periodType="duration" abstract="false" xbrli:balance="debit" />
</xsd:schema>
