The cell membranes of mammalian red blood cells are permeableto urea. If red blood cells are dropped into a solution of urea thatis identical in osmotic pressure (isosmotic) to the cytoplasm ofthe cells, although the cells do not swell and burst as quickly aswhen they are dropped simply into pure water, they eventuallyswell and burst. Explain. Also discuss how you would designa solution into which red cells could be placed without everswelling. (Hint: Think about whether urea will stay on theoutside of the cells and the implications for osmotic pressures.)

The cell membranes of mammalian red blood cells are permeable
to urea. If red blood cells are dropped into a solution of urea that
is identical in osmotic pressure (isosmotic) to the cytoplasm of
the cells, although the cells do not swell and burst as quickly as
when they are dropped simply into pure water, they eventually
swell and burst. Explain. Also discuss how you would design
a solution into which red cells could be placed without ever
swelling. (Hint: Think about whether urea will stay on the
outside of the cells and the implications for osmotic pressures.)