The whole point of a selectively permeable membrane is to maintain a stable internal cellular environment. If anything could just get in and out, cells would just stop working completely.
A cell needs a selectively permeable membrane in order to maintain a balance of water in the cell and the ability to bring in glucose and other items it needs for both DNA and RNA processing along with waste removal.
Selectively permeable membranes are membranes that, in addition to allowing the passage of solvents, let specific solutes pass through while blocking others.
Cells allow some materials to enter or leave, but not others. Cells are said to be permeable to some materials and impermeable to others.
Permeable means “permitting passage,” and impermeable means “not permitting passage.” In general, small molecules pass through the cell membrane easily, medium-sized molecules pass through less easily, and large molecules cannot pass through without help from the cell.
Because the cell membrane allows certain substances to enter or leave, but not others, it is said to be selectively permeable.
Diffusion is one of the ways that substances move into and out of cells.
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