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The Cell Membrane. The Structure of the Cell Membrane. The cell membrane (or plasma membrane) surrounds all living cells, and is the cell's most important organelle. It controls how substances can move in and out of the cell and is responsible for many other properties of the cell as well.
Both eukaryotic cells, including plant cells and animal cells, and prokaryotic cells, e.g. bacteria, are enclosed by a cell membrane. A cell membrane (see the simple diagram on the right) is a thin structure that is also known as the plasma membrane.The main functions of the cell membrane are: The cell membrane forms a barrier between the inside of the cell and the environment outside the cell-enclosing cytoplasm and any organelles within the cell, and enabling different chemical environments to exist on each side of the cell membrane.