How big is a Phospholipid?

A phospholipid is the kind of fat-like molecule that cell membranes are mainly built from. It has a head that is attracted to water and two tails that avoid water. From the outer edge of its head to the tips of its tails, one phospholipid is about 2.5 nanometers long, and a nanometer is a billionth of a meter.

Size

The head of a phospholipid is a phosphate-containing group attached to a small molecule called glycerol. The two tails are fatty acids, long chains of carbon and hydrogen atoms, usually 10–20 carbon atoms long. Each carbon adds about 0.13 nanometers to a tail's length.

The exact length depends on the tails and how they lie. In a living membrane the tails are fluid, bending and wriggling. A reference book on cell biology gives 2 nanometers from head to tail for a typical phospholipid.

Measurements reviewed in 2000 found that a layer of phosphatidylcholine, a common phospholipid, is about 2.2–2.3 nanometers thick when fluid, and 2.6 nanometers in a cooler, ordered form with straighter tails. The layer is one molecule deep, so that is how far one molecule reaches from the middle of a membrane to its surface. Each molecule in a fluid layer takes up about 0.6–0.7 square nanometers of the surface. Estimates of the length therefore run from about 2 to 2.6 nanometers.

Size comparisons

• Two layers of phospholipids, tail to tail, make up a phospholipid bilayer, the sheet at the heart of nearly all cell membranes.
• A phospholipid is about twice as long as an alpha helix is wide. An alpha helix is a common part of proteins.
• It is about as long as the DNA double helix is wide, which is 2 nanometers.
• Between about 30,000 and 50,000 phospholipids laid end to end would span the width of a human hair, which is 80,000–100,000 nanometers wide.
• A single layer of phospholipids covering one square millimeter would hold about 1.5 trillion molecules.

How phospholipids build membranes

Because their heads are attracted to water and their tails are not, phospholipids in water arrange themselves with their tails facing each other and their heads facing out. They form a double layer that separates the water on one side from the water on the other. Heated in water, phospholipids usually form tiny spheres on their own, with the heads outside and the tails inside.

The shape of the tails helps keep membranes fluid. A tail with only single bonds between its carbon atoms is fairly straight. A double bond puts a bend of about 30 degrees in the tail, and these kinks push neighboring molecules apart. That keeps a membrane from stiffening in the cold.

Oil on a pond

Benjamin Franklin poured no more than a teaspoon of oil onto a pond near London and watched it spread over perhaps half an acre. The oil formed a layer one molecule thick. Its molecules are similar in character to those that make up each layer of a cell membrane.

A teaspoon holds about 5 cubic centimeters, and half an acre is about 2,000 square meters, so the layer was roughly 2.5 nanometers thick. Agnes Pockels later invented a way to make such single layers in the laboratory, which let scientists settle the size of molecules precisely.

Sources

• Components and Structure. In Biology 2e, 2nd edition. OpenStax, 2018.
• Lipids. In Biology 2e, 2nd edition. OpenStax, 2018.
• Structure of Lipid Bilayers. Nagle and Tristram-Nagle, Biochimica et Biophysica Acta, 2000.
• What Is the Thickness of the Cell Membrane?. In Cell Biology by the Numbers. Milo and Phillips, Garland Science, 2015.
• Predicting Helical Topologies in RNA Junctions as Tree Graphs. Laing et al., PLoS ONE, 2013.
• DNA Structure and Sequencing. In Biology 2e, 2nd edition. OpenStax, 2018.
• Just How Small Is "Nano"?. National Nanotechnology Initiative.