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Exercise 4.   Determination of the Presence of Water Channels in Mammalian Red Blood Cell Plasma Membranes


A. Introduction


Image Source: Wellcome Images

The lipid bilayer is moderately permeable to water, and for many membranes the water permeability of the lipid bilayer is sufficient to meet the water transport demands of the cell. However, for some biological membranes, the water permeability of the lipid bilayer is not high enough to meet water transport needs of the cell. This is particularly true of the plasma membranes of the epithelial cells that line many of the tubules of the kidney. Kidney epithelial cells are specialized for very rapid water transport, and their plasma membranes are ultra-permeable to water, far more permeable than the lipid bilayer. To achieve this high degree of water permeability, their plasma membranes are equipped with special proteins that form water channels, allowing the extremely rapid movement of water across the membrane.

We have known for a very long time that the mammalian red blood cell plasma membrane is very permeable to water. Even the introductory biology student understands that if you put mammalian red blood cells in water, they will rapidly swell and break open (hemolyze) due to the rapid osmosis of water into the cell. The question we are asking in today's lab experiment is "Is the high water permability of mammalian red blood cell plasma membrane due solely to the water permeability characteristics of the lipid bilayer, or are there also water channels present in the membrane?" Thus, the objective of today's laboratory experiment is to determine whether there are water channels in the mammalian red cell plasma membrane.