why is the the plasma membrane called a selectively permeable membrane?

It's simply because it does not allow each and everything to pass through it, so in other words it "chooses" what can enter a cell.

For e.g There are some people who allow others to enter their life (by becoming their friend) as they see profit in them. Plasma membrane is somewhat same.

Sorry if my explanation or answer is not upto the mark :P

 It is so because it allows or permits various material from going inside or outside the cell.....

Since it selects, what must enter or what must not, thats why it is selectively permeable membrane

Because it doesnt let any material enter the membrane .......therefore it selects what can enter and what cant enter

 its because it allows the entry of some materials and prevents the others from entering in the cell.therefore it is called selectively permeable membrane as it allows only selected materials to enter inside the cells!

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