A phospholipid bilayer is a sheet of phospholipid molecules two layers deep, and it is the basic structure of nearly all cell membranes. It is about 5 nanometers thick, measured from the outer edge of the molecules' heads on one side to those on the other. A nanometer is a billionth of a meter.

Size

Each phospholipid has a head that is attracted to water and two tails that avoid it. In a bilayer, the heads face the water on either side and the tails meet in the middle, forming a water-repelling core. Measurements reviewed in 2000 put the full thickness of bilayers made of phosphatidylcholine, a common phospholipid, at 4.4–4.7 nanometers in the fluid form found in living cells. In a cooler, more ordered form it reaches 5.2 nanometers.

The water-repelling core makes up 2.6–2.9 nanometers of the fluid bilayer's thickness. Real cell membranes mix many kinds of lipid. X-ray measurements published in 2004 found 3.6–4.3 nanometers between the phosphate groups on the two sides of membranes from rat liver cells, and the heads reach a little beyond the phosphates.

Membranes are also packed with proteins, which cover about half of a membrane's area, and many of the proteins stick out past the membrane on both sides. With them included, a whole membrane is often described as 5–10 nanometers thick.

Size comparisons

• Two phospholipids, each about 2–2.5 nanometers long, laid tail to tail span the bilayer.
• An alpha helix of 20–25 amino acids, 3–3.75 nanometers long, is long enough to cross the water-repelling core. Proteins that pass through membranes often do it this way.
• A porcine circovirus, one of the smallest viruses known as of 2026, is about three and a half times as wide as the bilayer is thick.
• About 20,000 bilayers stacked on top of each other would be as thick as a sheet of paper, about 0.1 millimeters.
• If a bilayer were enlarged to the thickness of a sheet of paper, a human hair would be 1.6–2 meters (5.2–6.6 feet) wide.

What the bilayer does

The bilayer separates the water and dissolved materials inside a cell from those outside. Its water-repelling core is a barrier to water and to substances whose molecules are charged or polar, meaning a polar molecule's electric charge is unevenly spread. Phospholipids form a bilayer on their own when placed in water, and the result mends itself. A very fine needle can pass through a cell membrane without bursting it, and the membrane flows back together when the needle is pulled out.

How it was discovered

Scientists identified the cell membrane in the 1890s and found in 1915 that it was made mainly of lipids and proteins. In 1935, Hugh Davson and James Danielli proposed that it was built like a sandwich, with lipids as the filling and proteins as the bread. In the 1950s, transmission electron microscopes showed that the membrane's core was a double layer rather than a single one.

In 1972, S. J. Singer and Garth Nicolson proposed the fluid mosaic model, which still best explains how membranes are built and work. It pictures a membrane as a mosaic of phospholipids, cholesterol, proteins and carbohydrates that float and shift past one another like loose tiles.

Sources

• Structure of Lipid Bilayers. Nagle and Tristram-Nagle, Biochimica et Biophysica Acta, 2000.
• Modulation of the Bilayer Thickness of Exocytic Pathway Membranes by Membrane Proteins Rather Than Cholesterol. Mitra et al., Proceedings of the National Academy of Sciences, 2004.
• What Is the Thickness of the Cell Membrane?. In Cell Biology by the Numbers. Milo and Phillips, Garland Science, 2015.
• Components and Structure. In Biology 2e, 2nd edition. OpenStax, 2018.
• α-helix. In Foundations of Protein Structure. EMBL-EBI Training.
• Conformational Changes and Nuclear Entry of Porcine Circovirus without Disassembly. Wang et al., Journal of Virology, 2019.
• Just How Small Is "Nano"?. National Nanotechnology Initiative.