Stimulated facilitated diffusion

Abstract

WHEN a substrate is transported through a membrane by means of diffusion and at the same time by means of a molecular carrier, which itself is transported through the membrane by diffusion, we speak of facilitated diffusion. This is a well known transport mechanism, which has been extensively studied (2, 6, 7, 9, 11, 12, 151. A classic example of facilitated diffusion is the transport of oxygen (0,) through membranes filled with hemoglobin (Hb) or myoglobin (Mb) [7]. H ere the presence of a carrier, Hb or Mb, may increase the flow of oxygen through the membrane considerably [7]. Hence the flux through the membrane may be quite sensitive to the concentration of carriers, and to substances which affect this concentration. Thus, it was observed that the presence of carbon dioxide (CO,) may affect the flux of O2 through a membrane containing Hb-carriers [8]. However, somewhat surprisingly it was observed that an appropriate concentration gradient of the CO* could actually enhance the flow of O2 through the membrane, to the point of “pushing” the O2 through the membrane against an adverse pressure gradient. In this paper we shall derive a few basic properties of the facilitated diffusion mechanism on the basis of the classical model for this mechanism [6] and we shall extend this model to incorporate the effect of COz, and explain the anomaly described above. In addition we shall derive the general shape of the concentration profiles for 02, Hb and HbOz, the carrier-oxygen complex, inside the membrane, both when CO2 is absent and when it is present. Typical curves are given in Fig. 1. In our model, the effect of the CO* is to cause an inhomogeneity inside the membrane, which in turn leads to the possibility of the membrane acting as an oxygen pump. Formally the model resembles that used by Thomas et al. [5, 111 to describe a membrane in which glucose can be transported against a concentration gradient.