To which materials, such as molecules or charged particles, is the cell membrane permeable; to which is it non-permeable?
Besides giving a cell shape and strength, the cell membrane is differentially permeable, allowing some materials to pass through while preventing others from doing so. With regard to molecules, the cell membrane is very permeable to water, certain lipid-soluble substances, amino acids, and sugars but relatively impermeable to large molecules such as proteins and polysaccharides. Although more permeable to uncharged than to charged particles, with regard to ions, the cell membrane is more permeable to negative ions than positive ions.
