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Membranes
Membranes serve multiple functions. They form a unique membrane at the cell surface (plasma membrane), surround the nucleus, and form membranous intracellular organelles, such as rough endoplasmic reticulum (RER), mitochondria and vesicles. All membranes consist of a phospholipid bilayer composed of cholesterol and protein; integral membrane proteins span the bilayer; peripheral proteins associate with the inner surface of the membrane.
![Plasma membrane > <p>The plasma membrane is the membrane at the cell surface, enclosing the cytoplasm of the cell.</p>](https://digitalhistology.org/wp-content/uploads/2017/05/C.S.M-1-1-ping.png)
Plasma membrane >
The plasma membrane is the membrane at the cell surface, enclosing the cytoplasm of the cell.
![Nuclear envelope > <p>Two membranes surround each nucleus, forming the nuclear envelope. Arrows indicate the outer nuclear membrane. Intracellular membranes have a similar structure and appearance as the plasma membrane.</p>](https://digitalhistology.org/wp-content/uploads/2017/05/C.S.M-1-2-ping.png)
Nuclear envelope >
Two membranes surround each nucleus, forming the nuclear envelope. Arrows indicate the outer nuclear membrane. Intracellular membranes have a similar structure and appearance as the plasma membrane.
![RER > <p>Rough endoplasmic reticulum (RER) is also composed of membranes with a similar structure and appearance as the plasma membrane. RER is studded with ribosomes.</p>](https://digitalhistology.org/wp-content/uploads/2017/05/C.S.M-1-3-ping.png)
RER >
Rough endoplasmic reticulum (RER) is also composed of membranes with a similar structure and appearance as the plasma membrane. RER is studded with ribosomes.
![Mitochondria > <p>Intracellular membranes, e.g., those associated with mitochondria, have a similar structure and appearance as the plasma membrane. Mitochondria possess two membranes.</p>](https://digitalhistology.org/wp-content/uploads/2017/05/C.S.M-1-4-ping.png)
Mitochondria >
Intracellular membranes, e.g., those associated with mitochondria, have a similar structure and appearance as the plasma membrane. Mitochondria possess two membranes.
![Vesicles > <p>Vesicles contained within the cell, such as secretory vesicles, lysosomes and transport vesicles, are surrounded by membranes.</p>](https://digitalhistology.org/wp-content/uploads/2017/05/C.S.M-1-5-ping.png)
Vesicles >
Vesicles contained within the cell, such as secretory vesicles, lysosomes and transport vesicles, are surrounded by membranes.