Note that some molecular signals can be received by receptors inside the cell. How is it possible that some molecules ar

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Note that some molecular signals can be received by
receptors inside the cell. How is it possible
that some molecules are kept at the outside surface and others are
let into the cell? The cell membrane is selective.
If a substance can cross the cell membrane, the cell membrane
is_____to the substance. The substance itself is then said to be
permeant.