What is the flux of formamide through a membrane when thickness lipid layer is 8 nm thick, if the diffusion coefficient is 1.4 10-8 cm2 / s, K=1, the concentration on one side of the membrane is 2 10-4 mol / l, and on the other side - ten times less?
Answers
We have developed an experimental approach that allows us to quantify unstirred layers around cells suspended in stirred solutions. This technique is applicable to all types of transport measurements and was applied here to the 18O technique used to measure CO2 permeability of red cells An external file that holds a picture, illustration, etc.
Object name is tjp0587-1153-mu1.jpg. We measure An external file that holds a picture, illustration, etc.
Object name is tjp0587-1153-mu2.jpg in well-stirred red cell (RBC) suspensions of various viscosities adjusted by adding different amounts of 60 kDa dextran. Plotting An external file that holds a picture, illustration, etc.
Object name is tjp0587-1153-mu3.jpgvs. viscosity ν gives a linear relation, which can be extrapolated to ν= 0. Theoretical hydrodynamics predicts that extracellular unstirred layers vanish at zero viscosity when stirring is maintained, and thus this extrapolation gives us an estimate of the An external file that holds a picture, illustration, etc.
Object name is tjp0587-1153-mu4.jpg free from extracellular unstirred layer artifacts. The extrapolated value is found to be 0.16 cm s−1 instead of the experimental value in saline of 0.12 cm s−1 (+30%). This effect corresponds to an unstirred layer thickness of 0.5 μm. In addition, we present a theoretical approach modelling the actual geometrical and physico-chemical conditions of 18O exchange in our experiments. It confirms the role of an extracellular unstirred layer in the determination of An external file that holds a picture, illustration, etc.
Object name is tjp0587-1153-mu5.jpg. Also, it allows us to quantify the contribution of the so-called intracellular unstirred layer, which results from the fact that in these transport measurements – as in all such measurements in general – the intracellular space is not stirred. The apparent thickness of this intracellular unstirred layer is about 1/4–1/3 of the maximal intracellular diffusion distance, and correction for it results in a true An external file that holds a picture, illustration, etc.
Object name is tjp0587-1153-mu6.jpg of the RBC membrane of 0.20 cm s−1. Thus, the order of magnitude of this An external file that holds a picture, illustration, etc.
Object name is tjp0587-1153-mu7.jpg is unaltered compared to our previous reports. Discussion of the available evidence in the light of these results confirms that CO2 channels exist in red cell and other membranes, and that An external file that holds a picture, illustration, etc.
Object name is tjp0587-1153-mu8.jpg of red cell membranes in the absence of these channels is quite low.
It has long been recognized (Gutknecht et al. 1977; Holland et al. 1985; Wunder et al. 1997; Wunder & Gros, 1998; Endeward & Gros, 2005; Endeward et al. 2006b; Missner et al. 2008) that unstirred layers on cellular surfaces constitute a special problem when the transport of gases across cell membranes is measured. This is due to the fact that in many, although not all, cell membranes the permeability to gases is several orders of magnitude greater than that of ions. This holds even in the case of ions that are as permeable as bicarbonate in the red blood cell membrane. On the other hand, the diffusion coefficients of CO2 and other gases in water are not much greater than those of ions (Gros et al. 1976). While in the case of ions the diffusion resistance of an unstirred layer is often negligible in comparison to the diffusion resistance of the membrane itself, this implies that in the case of CO2 and other gases the diffusion resistance exhibited by an unstirred layer of solution around the cell can become much greater than that of the membrane itself. If the unstirred layer is of considerable thickness, this problem can render the membrane permeability for a gas undetectable.