Question

why does a semi permeable allow only some substances to pass through it?

Answer 1

Dear student,
The pores of the semi permeable membrane are such that is allows only small uncharged molecules to pass through it easily. Permeability depends upon the size of the molecules and their solubility in the membrane. There are transmembrane proteins in the plasma membrane which acts as channel for transport of hormones and ions.
Hence, we can say that it is the property of the plasma membrane that only certain molecules can pass through it while remaining molecules are restricted.

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