The term, selectively permeable, refers to the fact that the membrane allows some substances or molecules to enter the cells, and others to be prevented from entering. It also allows substances to leave the cell. All of this is dependent on the size of the molecule and the concentrations on either side of the membrane.
Selectively permeable membranes are important in maintaining homeostasis, fluid and electrolyte balance, and cellular health. Cells need to receive glucose and other nutrients in order to carry on normal cellular processes, and eliminate waste products of metabolism to avoid the accumulation of cellular toxins which can disrupt cellular processes and lead to cellular death.
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