One feature characterizing the transition from prokaryote to eukaryote is the ‘sudden’ appearance of centrioles and their highly structured products, the typical eukaryotic flagella and cilia. These mechanochemical systems appear as fully developed machines, containing some 200 diffierent proteins (Luck et al. 1978) arranged in a remarkably complex organization which has undergone little modification since the advent of the first eukaryotic cells. It is now well established (see, for example, Satir, 1974) that ciliary and flagellar motility is based on a sliding filament mechanism that superficially resembles the far more extensively studied sliding filament system of striated skeletal muscle.The flagellar system, however, appears to be much more complex than the muscle system, because it does not ‘merely’ shorten and generate force, but develops propagating waves and exerts its effects via hydrodynamic interactions with a viscous medium.

Calcium ion plays an essential role in many biological processes. The environment about Ca2+ may be probed by substitution of tripositive lanthanide ions, Ln3+. Ca2+ proteins fall into two broad classes: those that are inhibited by Ln3+ substitution and those that are not. It is suggested that although Ca2+ undertakes a primarily structural role in the Ln3+ non-inhibited proteins, Ca2+ may be near the active site or participate in the mechanism of action of Ln3+ inhibited proteins. Ca2+ and Ln3+ radii are similar; most Ln3+ are slightly larger than Ca2+ in complexes of the same coordination number, and substitution of Ln3+ for Ca2+ is accommodated by a slight decrease in bond distance or by an increase in coordination number. Luminescence from Tb3+ has been demonstrated to be a sensitive environmental probe of Ca2+ binding sites in proteins.