Membranes
Membranes are present throughout the cell and serve multiple functions. They form a unique membrane at the cell surface (plasma membrane), surround the nucleus, and form membranous intracellular organelles, such as endoplasmic reticulum, mitochondria and vesicles. All these membranes are unit membranes, consisting of a phospholipid bilayer and protein; integral membrane proteins span the bilayer; peripheral proteins associate with the inner surface of the membrane.
Plasma membrane >
The plasma membrane is the unique membrane at the cell surface, enclosing the cytoplasm of the cell.
Nuclear envelope >
Two membranes surround the nucleus, forming the nuclear envelope. Arrows indicate the outer nuclear membrane.
RER >
Rough endoplasmic reticulum (RER) is composed of membranes that form interconnected, flattened sacs with ribosomes on their cytoplasmic surfaces.
SER >
Smooth endoplasmic reticulum (SER) is composed of membranes that form sacs or tubules continuous with the membranes of the RER. SER lacks ribosomes.
Golgi apparatus >
The unit membranes that form the Golgi apparatus form stacks of independent flattened sacs, like a stack of dinner plates. They are not continuous with the membranes of the endoplasmic reticulum.
Mitochondria >
Mitochondria are composed of two unit membranes, inner and outer. The inner membrane extends into the interior of the mitochondria forming cristae.
Vesicles >
Vesicles contained within the cell, such as secretory vesicles, lysosomes and transport vesicles, are surrounded by single unit membranes.
