obligate
     adj : restricted to a particular condition of life; "an obligate
           anaerobe can survive only in the absence of OXYGen"
           [ant: {facultative}]
     v 1: force or compel somebody to do something; "We compel all
          students to fill out this form" [syn: {compel}, {oblige}]
     2: commit in order to fulfill an obligation; "obligate money"
     3: bind by an obligation; cause to be indebted; "He's held by a
        contract"; "I'll hold you by your promise" [syn: {oblige},
         {bind}, {hold}]
Source: WordNet® 2.0