A chemical substance which is the principal constituent of silk, cobwebs, and the horny skeleton of sponges.

Biology and Biochemistry. Originally (following the terminology of C. Nägeli): a submicroscopic particle consisting of a number of molecules…

Biology. A substance obtained by digestion with heat, etc.

any radioactive isotope (typically of an alkaline earth metal) that accumulates mainly in bone after entering the body.

any tissue which responds to a particular hormone or hormones within an organism (cf. target organ n. above).

An inorganic mineral substance produced by a living organism, typically forming hard supporting structures such as bone, shell, and coral; a…