Tetrahymena has been used as a model cell system in many studies of morphogenesis, conjugation, gene mapping, cell division and growth kinetics. In this article, we consider some advances which have resulted from the successful development of a chemically defined medium (CDM), and how subsequent work has extended the contribution that this organism has made to our understanding of different aspects of growth, nutrition, cell cycle control, cytokinesis and intercellular signalling. Finally, we discuss the considerable potential that has arisen for (...) the biotechnological exploitation of this big and rapidly growing eukaryotic cell. (shrink)

Diffusion theory explains in physical terms how materials move through a medium, e.g. water or a biological fluid. There are strong and widely acknowledged grounds for doubting the applicability of this theory in biology, although it continues to be accepted almost uncritically and taught as a basis of both biology and medicine. Our principal aim is to explore how this situation arose and has been allowed to continue seemingly unchallenged for more than 150 years. The main shortcomings of diffusion theory (...) will be briefly reviewed to show that the entrenchment of this theory in the corpus of biological knowledge needs to be explained, especially as there are equally valid historical grounds for presuming that bulk fluid movement powered by the energy of cell metabolism plays a prominent note in the transport of molecules in the living body. First, the theory's evolution, notably from its origins in connection with the mechanistic materialist philosophy of mid nineteenth century physiology, is discussed. Following this, the entrenchment of the theory in twentieth century biology is analyzed in relation to three situations: the mechanism of oxygen transport between air and mammalian tissues; the structure and function of cell membranes; and the nature of the intermediary metabolism, with its implicit presumptions about the intracellular organization and the movement of molecules within it. In our final section, we consider several historically based alternatives to diffusion theory, all of which have their precursors in nineteenth and twentieth century philosophy of science. (shrink)

For many years, biology was largely descriptive (natural history), but with its emergence as a scientific discipline in its own right, a reductionist approach began, which has failed to be matched by adequate understanding of function of cells, organisms and species as whole entities. Every effort was made to explain biological phenomena in physico-chemical terms.It is argued that there is and always has been a clear distinction between life sciences and physical sciences, explicit in the use of the word biology. (...) If this distinction is real, it implies that biological phenomena can never be entirely satisfactorily explained in terms of extant physicochemical laws. One notable manifestation of this is that living organisms appear to -- actually do -- behave in purposeful ways, and the inanimate universe does not. While this fundamental difference continues to be suppressed, the purposiveness (or teleology) which pervades biology remains anathema to almost all scientists (including most biologists) even to the present day. We argue here that it can, however, become a perfectly tenable position when the Theory of Natural Selection is accepted as the main foundation, the essential tenet, of biology that distinguishes it from the realm of physical sciences. In accepting this position, it remains quite legitimate to expect that in many but not all circumstances, extant physical laws (and presumably others still to be discovered) are in no way breached by biological systems, which cannot be otherwise since all organisms are composed of physical material. (shrink)