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The aim of this symposium was to provide a framework for fruitful discussion on intestinal transport, not only for advanced scientists but also for younger people starting in this field of research. Invited lectures, communications and poster presentations were focused on four central themes, all treating the prop erties of the sole intestinal epithelium, deliberately leaving aside problems dealing with more integrative functions of the whole intestine. The importance of motility or blood circulation, for instance, is certainly capital in the overall intestinal function, but these aspects by themselves deserve another meeting. This volume has compiled the manuscripts of the invited lectures which sub stantially comprised the four sessions of the Symposium. Part 1 is designed to emphasize actual knowledge of the transport of water, inorganic as well as organic ions and molecules across the isolated intestinal epithelium. An enormous wave of investigations has emerged from studies per formed with "Ussing chambers", which roused interest in studies on absorption mechanisms and subsequently on secretory processes. This has triggered off a trend to research on isolated cells as absorption and secretion are the main func tion of the different cell types constituting the intestinal epithelium. In this first session not only the importance of the parallel arrangement of these different cellular entities is stressed, but also the role played by the paracellular route.
This Volume forms the cornerstone of this series of four books on Membrane Transport in Biology. It includes chapters that address i) the theoretical basis of investigations of transport processes across biological membranes, ii) some of the experimental operations often used by scientists in this field, iii) chemical and biological properties common to most biological membranes, and iv) planar thin lipid bilayers as models for biological membranes. The themes developed in these chapters recur frequently throughout the entire series. Transport of molecules across biological membranes is a special case of diffu sion and convection in liquids. The conceptual frame of reference used by investigators in this field derives, in large part, from theories of such processes in homogeneous phases. Examples of the application of such theories to transport across biological membranes are found in Chapters 2 and 4 of this Volume. In Chapter 2, Sten-Knudsen emphasizes a statistical and molecular approach while, in Chapter 4 Sauer makes heavy use of the thermodynamics of irreversi ble processes. Taken together, these contributions introduce the reader to the two sets of ideas which have dominated the thinking of scientists working in this field. Theoretical consideration of a more special character are also included in several other Chapters in Volume I. For example, Ussing (Chapter 3) re-works the flux ratio equation which he introduced into the field of transport across biological membranes in 1949.
The above consideration indicates that at present many of the experi mental facts on PS in animals can be quantitatively explained within the limits of the "universal" photoreceptor membrane concept. Of course, existence of preferential orientation of the absorbing dipoles in the tubuli of the rhabdomeres can not be totally rejected. We hope that the concept of the "universal" photoreceptor membrane may serve as the useful instrument when dealing with newly discovered properties of visual cells so that true mechanisms of electrical and optical coupling will be searched for instead of assumptions being made on additional properties of the photoreceptor membrane in every new animal under study. 5. Absorption Spectrum of the Universal Photoreceptor Membrane and Spectral Sensitivity of the Photoreceptor 5. 1 Preliminary Notes It seems nearly self-evident that the absorption spectrum of the pho toreceptor membrane coincides exactly with that of the visual pigment it contains. Hence, the membrane must exhibit three bands of absorp tion - the principal band with its peak within the limits of visible spectrum (or a-peak); the secondary band between 340 and 380 nm (S peak); and the third, protein band, in the ultraviolet (UV) at 280 nm (COLLINS et al. , 1952). The main peak of absorption is located within the range 433-575 nm for retinol-based pigments and between 438 and 620 nm for 3-dehydroretinol-based pigments, the position of Amax de pending on many ecological factors.