Question

why plasma membrane is known as selectively permeable membrane

Answer 1

Because it allows the passage of certain molecules while resisting others. Mainly because of the sizes and polarity of molecules.

For example human cell membranes can allow the passage of fatty acids and water freely, while other biological molecules such as virus cannot pass through unless this virus has a specific glycoprotein on its surface that triggers an endocytosis. (HIV, can only enter cells with CD4 receptors but not any other cells)

This is what ‘selective’ means.

Answer 2

This is the answer
It allows entry and exit of some materials in and out of the cell.

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