Cell membrane is considered because it regulates what goes in and out of the cell

The Plasma Membrane controls what goes into & comes out of a cell, the plasma membrane is a selectivity permeable; it regulates what passes from one side to another. Water moves easily into and out of cells by Osmosis. Small molecules without charges such as oxygen and carbon dioxide flow through a plasma membrane without assistance, other substances such as proteins, glucose and charged particles called ions cannot pass through the selectively permeable membrane.

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