Biochemistry

Ghosties had a different blood biochemistry to humans. (AUDIO: Asylum)

According to the Seventh Doctor, the pH of the blood gave an idea of a species' basic biochemistry. (AUDIO: A Thousand Tiny Wings)

Elkinar was a doctor of biochemistry and applied xeno-agronomy. (AUDIO: The Seeds of War)

Hex noted that the Seventh Doctor had a different biochemistry to humans, and therefore using chloroform on him consistently was not having the "right" effect. (AUDIO: The Magic Mousetrap)