Lipid bilayer

Definition

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 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. A phospholipid is a two-tailed lipid. Note however that this two describes two tails of each single molecule and is distinct from the two that is part of the bilayer definition.

Also, when people talk of double membranes (as in the case of the nucleus, where the membrane is folded onto itself, and the mitochondrion, where there is a distinct inner mitochondrial membrane and outer mitochondrial membrane), this refers to an additional doubling over and above the bilayer, so that there are effectively four layers of molecules.

Summary

Item	Value
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 $10^{-9}$ 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).

Size

Thickness

The bilayer is only two molecules thick, so its thickness is a few nanometers (nm), where a nanometer is $10^{-9}$ 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.

Physical structure

Outer leaflet

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.