Boundary · lipid bilayer
Plasma membrane
The living skin of the cell
In view
The surface ripples and its proteins drift sideways, because the membrane is a two-dimensional fluid. Up close, lipids jostle and swap places, ions slip through a channel and a pump sends three sodium ions out for every two potassium ions in. A real channel can pass tens of millions of ions a second; here everything is slowed down enormously.
A film of phospholipids just two molecules thick, about 4 nanometres, wrapped around everything the cell is. Water-loving heads face out and in; oily tails hide in the middle, so ions and sugars cannot simply leak across. About a third of its lipid is cholesterol, which stiffens the film and makes it less leaky.
Proteins take up a quarter to half of its area: channels, pumps, receptors and sugar-coated ID tags that decide what enters, what leaves and who the cell talks to. Some cells also push it out into microvilli, fingers about 0.1 µm wide that multiply their surface.
- Thickness
- ≈ 4 nm bilayer
- Protein share
- ≈ ¼–½ of the area
- Lipid diffusion
- ≈ 1 µm² per second
- Model
- Fluid mosaic, 1972
Did you know?
In a liver cell, the plasma membrane is only about 2% of all the membrane. The rest is folded up inside, mostly as endoplasmic reticulum.
Sources
- The Fluid Mosaic Model of the Structure of Cell Membranes · Singer & Nicolson 1972 · Science
- Diffusion coefficient of lipid in liquid-crystalline, liquid-disordered membrane · BioNumbers · BNID 107244
- 5.3 Active Transport · OpenStax · Biology 2e
- Energetic optimization of ion conduction rate by the K+ selectivity filter · Morais-Cabral, Zhou & MacKinnon 2001 · Nature
- What is the thickness of the cell membrane? · Milo & Phillips · Cell Biology by the Numbers
- 5.1 Components and Structure · OpenStax · Biology 2e
- 5.2 Passive Transport · OpenStax · Biology 2e
- Fraction of total membrane lipids that constitute its components · BioNumbers · BNID 113404
- High Cholesterol/Low Cholesterol: Effects in Biological Membranes: A Review · Subczynski et al. 2017 · Cell Biochem Biophys
- Protein area occupancy at the center of the red blood cell membrane · Dupuy & Engelman 2008 · Proc Natl Acad Sci U S A
- Shaping the intestinal brush border · Crawley et al. 2014 · J Cell Biol
- How big is the endoplasmic reticulum of cells? · Milo & Phillips · Cell Biology by the Numbers