The cell membrane around a cell forms a barrier that protects and regulates the cell. Certain chemicals can pass through the plasma membrane because they have similar chemical properties as the membrane.

The plasma membrane is made largely of lipids, so substances that dissolve in lipids can pass through it more easily. Being soluble in the membrane’s lipid layer allows certain chemicals to cross; concentration, dilution, or heating alone does not explain the shared chemical property described. The correct choice is “They are soluble.”

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