Title: Optional

Text: Passive transport moves material down its concentration gradient. It requires no energy input and stops once equilibrium is reached. However, some cells require a concentration gradient to be created and maintained. Other cells must take in or expel materials too large to fit through a membrane protein. When passive transport will not get the job done, cells resort to active transport, which requires energy in the form of ATP (adenosine triphosphate). Each paragraph will be a test.

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