A lipid bilayer is a thin membrane (surrounding a cell or organelle) that is two molecules thick, with both molecules being lipids with a hydrophilic head and hydrophobic tail. The hydrophilic head of the outer molecule points outward to the (usually aqueous) external environment, and the hydrophilic head of the inner molecule points inward to the cell or organelle being surrounded by the bilayer. The hydrophobic tails both point inward toward each other.
Lipid bilayers constitute biological membranes, which, in addition to the lipid bilayer, contain embedded proteins called integral membrane proteins. Examples of biological membranes are cell membranes (in both prokaryotic cells and eukaryotic cells) as well as the membranes of cellular organelles (mostly in eukaryotic cells). The most typical examples of lipid bilayers that occur in cell membranes and organelle membranes are those where the lipid is a phospholipid.
Confusion with other twos
A phospholipid is a two-tailed lipid. Thus, a phospholipid bilayer has two twos in it: one describing the two tails of each molecule, and the other describing the fact that the layer is two molecules thick. The bilayer refers to the latter.
There are some contexts where biological membranes are double membranes:
- In the case of the nucleus, the membrane is folded on top of itself
- In the case of the mitochondrion, there is a distinct inner mitochondrial membrane and outer mitochondrial membrane)
The double here is distinct from the bilayer double. In the case of a double membrane where each membrane constitutes a bilayer, we expect a -molecule thick layer.
|Type of structures that have lipid bilayers as their membranes||both prokaryotic cells and eukaryotic cells have cell membranes that are made of lipid bilayers. In addition, the organelles in eukaryotic cells, such as the nucleus, mitochondrion, lysosomes, and others, have their own membranes which comprise lipid bilayers.|
|Size||Thickness: The bilayer is only two molecules thick, so its thickness is a few nanometers (nm), where a nanometer is meters.|
|Chemical constituents||The lipid molecules, which are usually phospholipids in the case of cell membranes and organelle membranes. Note that the lipid making up the outer leaflet may differ from the lipid making up the inner leaflet.|
|Function||Controls the entry and exit of materials between the cell or organelle and the environment. Specifically, the hydrophobic tails attempt to block water-soluble substances from crossing the lipid bilayer. There are a number of special mechanisms in use (depending on the cell or organelle) that are used to transport materials across the membrane).|
The bilayer is only two molecules thick, so its thickness is a few nanometers (nm), where a nanometer is meters.
The exact thickness depends on the specific choice of the lipid, and in particular the length of its hydrophobic tail.
Comparison with cell sizes
Comparison with prokaryotic cells: Prokaryotic cells have diameters in the 1000-10000 nm range, which is about 100-1000 times the thickness of the lipid bilayer.
Comparison with eukaryotic cells: Eukaryotic cells have diameters in the 10000-1000000 nm range, which is about 1000-10000 times the thickness of the lipid bilayer.
Comparison with wavelengths of light and implication for visibility under microscopes
The thickness of the lipid bilayer is considerably smaller than the wavelength of visible light (400-700 nm). Light microscopes have resolutions limited to about 200nm, and hence cannot be used to study these bilayers. Lipid bilayers can be studied using electron microscopes or fluorescence microscopes. However, the study of these bilayers is quite difficult due to their fragility.
The outer leaflet refers to the outer half of the bilayer which has a hydrophilic head pointing to the aqueous external environment and a hydrophobic tail pointing inward and facing the hydrophobic tail of the inner leaflet.
Note that the "tail" here refers to all the hydrophobic tails of the molecule. If the molecule is two-tailed (as is the case with phospholipids) the two tails both point in the same direction, toward the inner leaflet. If the molecule is three-tailed, the three tails point in the same direction, toward the inner leaflet.
The inner leaflet refers to the inner half of the bilayer which has a hydrophilic head pointing to the aqueous environment of the enclosed cell or organelle and a hydrophobic tail pointing outward and facing the hydrophobic tail of the outer leaflet.
Note that the "tail" here refers to all the hydrophobic tails of the molecule. If the molecule is two-tailed (as is the case with phospholipids) the two tails both point in the same direction, toward the outer leaflet.