Protoplasm was part of a cell. According to the Fifth Doctor, it was "the basic form of human life" from billions of years before Tegan Jovanka's time, from which humans evolved. (AUDIO: Interstitial) The natural forms of the Iyteans, a symbiont race, were little more than "three pounds of protoplasm". (GAME: The Iytean Menace [+]J. Andrew Keith, The Doctor Who Role Playing Game (FASA, 1985).)
The process used by the Chameleons to assume the identities of humans involved the use of armband-like devices placed on both subjects. If an armband was removed from the original, the Chameleon that assumed their identity would be killed, reduced to a small blob of protoplasm. (TV: The Faceless Ones)
The Saiph were able to turn into protoplasm. (AUDIO: The Stealers from Saiph)
Because computers were unable to withstand the reality-changing stress, TARDISes required morphologically unstable living organic matter, or protoplasm, for their Block Transfer Computations. (PROSE: Cat's Cradle: Witch Mark)