Chapter 4 - Mechanisms of Water Transport Across Cell Membranes and Epithelia

Abstract

In this chapter we discuss the pathways and mechanisms of water transport across cell membranes and epithelia. The concepts of diffusion and osmosis are presented at the biophysical level and applied to water transport phenomena across lipid bilayers and membrane-spanning pores. Water pores (“aquaporins”) are described from biophysical and molecular points of view. The mechanism of water transport across the cell membrane is osmosis and cell volume is determined by the intracellular solute content and the extracellular osmolality. Water permeates the cell membrane via aquaporins and/or the phospholipid bilayer; other pathways having minor importance. Water transport in the absence of a substantial osmolality difference between the adjacent solutions is most likely osmotically coupled to solute transport in the same direction (“near-isosmotic fluid transport”), without requirement for large standing osmotic gradients. Epithelia display a wide range of permeabilities to water, which has important physiological significance, in particular for kidney function.