CHAPTER 7 - Absorption, Secretion and Translocation

Abstract

This chapter discusses that across the boundary between a living cell and its environment, solutes continuously pass in both directions. When, during a given time interval, there is a net movement of a given solute into the cell, it is known as absorption or influx of the solute and if this absorption leads to the establishment of a higher concentration inside than outside the cell, it is known as solute accumulation. When the net movement is in the reverse direction, it is known as solute release or efflux and the resultant depletion. In some cases and at some times, such movements across cell boundaries appear to be controlled in direction and rate by physical forces such as diffusion, particularly diffusion across cell membranes allowing free movement of some molecules or ions and preventing or restricting the movement of others. Such movements are referred to as passive, in that they tend to decrease the energy potential of the system. In other cases the solute movement is opposite in direction and/or different in rate from that expected from physical considerations. This movement involves consumption of cellular energy and is said to be active, and is often referred to as secretion.