What makes a cell membrane selectively permeable?

A cell membrane is selectively permeable because it only allows particles to move across them due to different concentrations. Molecules move from areas of low concentration to high concentration.The cell needs to control what enters and leaves. It needs to keep somethings in and other things out. However, only the Selectively cells can enter and go out of the cell! This is why the selectively permeable membrane is permeable!

A cell membrane (the outer casing or inner casing in a plant cell) is selectively permeable because some materials can pass freely through the cell membrane while others cannot.

The Lipid bi-layer of the cellular membrane allows simple molecules to pass through. However, for certain proteins, a special transportation mode is needed.

"1. it should be, it has been for about 20 years, and it's really not much different in the end than breeding animals to have particular traits. there are always concerns about genetically engineered animals/plants affecting the environment in unexpected negative ways, but i can't think of an example where the fears didn't turn out to be baseless.  

first, as you point out, it would be a great advance in pharmacology, to be able to tailor drugs and their doses to me. the pros involve better medical treatment. the cons involve privacy issues like insurance companies denying coverage to people with genetic risk factors. looking at dna markers wouldn't affect the frequency of genetic diseases, but it would allow detection of them earlier. and ok, actually i suppose that if people used this information to avoid having children with genetic diseases, some might decrease in frequency."

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