Question: 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?