Question

Why is plasma membrame IA called selectively permeable membrane state it's function

Answer 1

your answer is here:
plasma membrane called  selectively permeable membrane because it allow selective material to pass through it that is required to the cell.
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Answer 2

the cell membrane allows certain thing to pass , and permeate the membrane, while blocking certain substances from entering and existing . so if something is said to be selectively permeable membrane, it has pores or openings that allow certain things to pass on