Urea and membrane permeability of RBC

[micco13]

Hi there,

I'm a little confused with the ability of urea diffusing across membranes of red blood cells ... I was told that urea can easily penetrate the membrane as they are not present in high concentration inside the cell. If I am mistaken, isn't urea a large polar molecule? I thought they would be impermeable to membranes (unless facilitated by channels).

We had a lab experiment on this, recording the haemolysis time of different solutions ...
I used two different concentrations of urea - 0.28M and 1.0M. It was found that 0.28M of urea caused faster cell lysis than 1.0M urea. Why does a less conc. urea haemolysis faster than a more conc. urea?

Any help will be much appreciated

[blcr11]

Urea is polar, but not very large as organic molecules go—MW of only 60. Provided that the nitrogens are not protonated, which at physiologic pH they are not, urea can cross the membrane by passive diffusion; not so quickly as water, but the transfer rate is much faster than, say, glucose (MW of 180) which requires some form of facilitative transport to get across a cell membrane. 1.8% urea (ca. 0.3M urea) is isosmotic with plasma (as is 0.9% NaCl, or 5% glucose). A 0.28M solution of urea must be just hypotonic enough for water to flow into and lyse the RBCs. 1M urea should be hypertonic. The RBCs should shrivel up as water is driven out of the cells and they may not lyse at all.

The issue here is whether the concentration of urea is less (=hypotonic) or greater (=hypertonic) than the isotonic or isosmotic concentration of urea.