Why is the cell membrane called semi-permeable?

Answer 1

The cell membrane is hydrophilic outside and hydrophobic from inside thanks to the phospholipid. The membrane also contains protein gated channels which allow some molecules to pass through and ion channels. The transport in an out of cells is also controlled by osmotic pressure, the electric charge etc.

Answer 2

It is called partially permeable because only molecules of a certain size can pass through it. An example of this is the way water molecules can be filtered into the kidneys to make urine, whereas larger protein molecules in the bloodstream cannot be filtered through, and so are not present in urine.

Think of it as a bit like a sieve.

Answer 3

because only some things can pass through it