Soluble proteins contained with organelles of the endomembrane fraction (ER, Gol

Question

Soluble proteins contained with organelles of the endomembrane fraction (ER, Golgi, lysosomes, vacuoles, plasma membrane) are found to be confined to those compartments. Microscopic staining techniques will detect a protein in these organelles, but there will be no evidence that these proteins cross membranes to enter the cytosolic compartment. How can the barrier to trans-membrane diffusion be explained for water-soluble proteins? Detergents break the bonds between the hydrophobic tail and hydroghilic head of membrane lipids. Disruption of membrane lipids releases the integral proteins.

Explanation / Answer

Substances which are water soluble typically do not cross lipid membranes easily there is some transport mechanisms are. So we can assume that water would also not cross cell membranes easily. Indeed, in artificial lipid bilayers, water does not cross easily and this is consistent with our expectation but water crosses nearly all the membranes in the body easily.It can be explained in terms of 2 pathways which can be called as lipid pathway & the water channel pathway.

lipid pathway refers to water crossing the lipid bilayer of the cell membrane by diffusion.
In some membranes the water flux is very high and cannot be accounted for by water diffusion across lipid barriers. A consideration of this fact lead to the hypothesis that membranes must contain protein which provide an aqueous channel through which water can pass. The water channels have now been found and are discussed below. Flow of water through these channels can occur as a result of diffusion or by filtration.

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