Structure
- The plasma membrane is the thin outer boundary of a cell; it separates the inside of the cell from the fluid and structures outside the cell.
- Plasmalemma is another name for the plasma membrane, so plasmalemma and cell membrane refer to the same cell boundary.
- The cytoplasm is the living material inside the cell membrane, excluding the nucleus.
- The extracellular environment is the space outside the cell, including surrounding fluid, neighboring cells, and extracellular matrix.
- The plasma membrane is about 7.5 nanometers (nm) thick, which means it is far too thin to see with an ordinary light microscope.
- A nanometer is one billionth of a meter, so membrane thickness is measured on a molecular scale.
- A lipid bilayer is a two-layer sheet of lipid molecules that forms the basic wall of the plasma membrane.
- A leaflet is one of the two lipid layers in a lipid bilayer.
- The inner leaflet is the membrane layer facing the cytoplasm.
- The outer leaflet is the membrane layer facing the extracellular environment.
- Integral proteins are membrane proteins embedded in the lipid bilayer or attached tightly enough to behave as part of the membrane.
- Peripheral proteins are membrane proteins attached to the membrane surface rather than deeply embedded in the lipid bilayer.
- The lipid bilayer provides the membrane's basic barrier, while integral and peripheral proteins provide many of the membrane's specific functions.
- Transmission electron microscopy is an imaging method that uses electrons rather than visible light to show structures as small as cell membranes.
- A trilaminar unit membrane has three visible layers under transmission electron microscopy: two darker outer lines and a lighter middle zone.
- The trilaminar appearance reflects the organization of membrane lipids and proteins rather than three separate membranes.
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