Cell membranes (1) regulate material moving into and out of the
cell, and from one part of the cell to another; (2) separate the inside
of the cell from the outside; (3) separate various organelles
within the cell; (4) provide a large surface area on which specific
chemical reactions can occur; (5) separate cells from one another;
and (6) are a site for receptors containing specific cell identification
markers that differentiate one cell type from another.
The ability of the plasma membrane to let some substances
in and keep others out is called selective permeability (L. permeare
or per, through meare, pass) and is essential for maintaining
cellular homeostasis. Homeostasis (Gr. homeo, always
the same stasis, standing) is the maintenance of a relatively
constant internal environment despite fluctuations in the external
environment. However, before you can fully understand how
substances pass into and out of cells and organelles, you must
know how the molecules of those substances move from one
place to another.
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