Question

why plasma membrane called selectively permeable membrane

Answer 1

It is selectively permeable because the membrane can choose what it allows to enter and exit the cell. For instance, it may have a sodium channel that allows for sodium to flow into the cell, but won't allow any other chemicals into the cell. This is to prevent unwanted chemicals to enter.

Answer 2

Plasma membrane is called selectively permeable membrane because it does not allow all substances to pass through it , instead it allows only some selective materials to cross the phospholipid wall based in the nature and use of the particular material.
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