Thank you for the opportunity to help you with your question!
in the first place it is good to understand that the plasma membrane
is specifically designed in a way that allows materials to pass through
it. this is so because it is simply like a fluid in structure and
therefore not hardened or rigid. its contents or the materials that form this membrane allows this movement of materials. the integral proteins forms this structure and also allow the in and outside movement of materials. there is also the number of receptors that are able to recognize the materials as well as their sizes and whether should be allowed in and out through the membrane.
on the other hand, a cell is able to maintain homeostasis through a technique that controls the substances or materials that go through the cell. the phospholipid bilayer which is actually the cell membrane is the one that controls these transition. the transition involves both water and ions as well.
Please let me know if you need any clarification. I'm always happy to answer your questions.