Why are transport proteins needed to move certain substances across a cell membrane?

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Cellular membranes can be thought of as the best of both worlds.  They have a highly polar polyphosphate backbone on the outer surfaces and a highly nonpolar interior made of lipid hydrocarbon chains.  As such, there are few compounds that could naturally make their way through both of these polar opposite environments.  As such, there are proteins called transport proteins that are actually embedded in the cell membrane and span it, thus connecting the interior of the cell with the exterior.  The transport proteins are designed to allow specific substances to traverse the membrane while keeping everything else out.

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