Phospholipids and Biological Membranes

Phospholipids are amphipathic molecules with a polar phosphate head and two nonpolar fatty acid tails; in water they spontaneously form a bilayer about 5 nm thick, which is the basis of every biological membrane. In brief: the membrane of a red blood cell, with a surface of about 135 µm², holds roughly 400 million lipid molecules, and the potential difference of 70 mV across the 5 nm bilayer is an electric field of 14 million volts per meter.

Membranes enclose every cell and its organelles, controlling what goes in and out. They are not just passive walls: they host enzymes, receptors and pumps, and they generate the electrical signals of nerves. This article describes the structure of phospholipids, explains the formation of the bilayer, the fluid mosaic model of the membrane with a table of its components, the ways in which substances cross, and the calculations of lipid numbers and electric fields, with a short history.

What is the structure of a phospholipid?

The most common phospholipids are phosphoglycerides. A glycerol backbone carries two fatty acids (joined by ester bonds), and a phosphate group on the third carbon, to which a small polar group, such as choline, ethanolamine, serine or inositol, is attached. Phosphatidylcholine (lecithin) is the most abundant in animal membranes. Typically one of the fatty acids is saturated and the other unsaturated (with a cis kink). The molecule is amphipathic: it has a hydrophilic head (charged and polar, which interacts with water) and two hydrophobic tails (which are repelled by water).

Another important class is the sphingolipids, based on the amino alcohol sphingosine, for example sphingomyelin in the myelin sheath of nerves. Cholesterol, a steroid, is also a membrane lipid in animals.

Why do they form a bilayer?

In water, the hydrophobic tails disrupt the network of hydrogen bonds of water, which costs entropy, so the system gains stability when the tails are hidden from water. This is the hydrophobic effect. For phospholipids, the two tails give the molecule a cylindrical shape, and the lowest energy is a double layer (bilayer) in which the heads face the water on both sides and the tails meet in the middle, away from water. Single-tailed lipids such as soaps are cone-shaped and form spherical micelles, while the phospholipids form sheets. The sheets close up into spheres (liposomes or vesicles), since open edges would expose tails to water. The bilayer assembles spontaneously, without any enzymes, and it can reseal itself if it is punctured. The thickness of the hydrophobic core is about 3 nm, and with the head groups, about 5 nm.

What is the fluid mosaic model?

According to the fluid mosaic model of Singer and Nicolson (1972), the membrane is a fluid bilayer of lipids in which proteins are embedded or attached, like icebergs floating on a sea. The lipids and proteins can move sideways in the plane of the membrane, with diffusion coefficients of about 10⁻⁸ cm²/s for lipids (a lipid moves a distance of about 2 µm in one second), but they very rarely flip from one side to the other without the help of enzymes (flippases).

• Integral proteins span the bilayer, with hydrophobic α-helices in the membrane (channels, carriers, receptors).
• Peripheral proteins are attached to the surface.
• Cholesterol fits between the phospholipids, makes the membrane stiffer at high temperature, and prevents the tight packing of the chains at low temperature, so that the fluidity stays about the same over a range of temperatures.
• Glycolipids and glycoproteins on the outer surface have carbohydrate chains that are used for recognition between cells (as in blood groups).
• The two leaflets have different compositions (asymmetry): phosphatidylserine is on the inner leaflet, and its appearance on the outside signals a cell undergoing apoptosis (programmed cell death).

Components of the membrane

How do substances cross the membrane?

The hydrophobic interior forms a barrier that is easy to cross for small nonpolar molecules (O₂, CO₂, steroid hormones) and for very small uncharged molecules (water, slowly), but is almost impermeable to ions and large polar molecules such as glucose. The cell uses proteins to move these across:

• Simple diffusion: from high to low concentration, with no help.
• Facilitated diffusion: through channels (such as aquaporins for water, ion channels) or carriers, down the concentration gradient, without energy.
• Active transport: against the gradient, with energy from ATP or from another gradient. The sodium–potassium pump (Na⁺/K⁺-ATPase) moves 3 Na⁺ out and 2 K⁺ in for each ATP used, and consumes a large part (up to 20–40 %) of the ATP in some cells, such as nerves.
• Bulk transport: endocytosis and exocytosis, by which vesicles take up or release large particles.

Types of membrane transport

Worked examples

Example 1: number of lipids in a red blood cell

A red blood cell has a surface area of about 135 µm². In a bilayer there are two leaflets, so the total lipid area is 270 µm². Each phospholipid takes about 0.65 nm² = 6.5 × 10⁻⁷ µm². The number of lipid molecules is 270 / 6.5 × 10⁻⁷ = 4.2 × 10⁸, about 400 million. The cholesterol and proteins take up part of the area, so the real number is somewhat lower.

Example 2: the electric field across the membrane

The potential difference across the plasma membrane of a resting cell is about 70 mV (inside negative) over a thickness of 5 nm. E = V/d = 0.070 V / 5 × 10⁻⁹ m = 1.4 × 10⁷ V/m. That is a field of 14 MV/m, which is of the order of the field at which air breaks down in a spark (about 3 MV/m), but the membrane withstands it.

Example 3: the area of a monolayer (Gorter and Grendel’s experiment)

The lipids extracted from red blood cells and spread out as a one-molecule-thick layer on water covered an area twice the surface of the cells from which they were taken. If the extract of 4.2 × 10⁸ lipids per cell covers 4.2 × 10⁸ × 0.65 nm² = 2.7 × 10⁸ nm² = 273 µm², this is about twice the cell surface (135 µm²), which suggests that the lipid in the cell forms a double layer.

Example 4: how fast do lipids move in the membrane?

For diffusion in two dimensions, the mean square displacement is ⟨r²⟩ = 4Dt. With D = 1 × 10⁻⁸ cm²/s and t = 1 s, r = √(4 × 10⁻⁸) = 2 × 10⁻⁴ cm = 2 µm. A lipid can thus travel across a bacterial cell (about 2 µm long) in one second, but flipping from one leaflet to the other takes hours without a flippase enzyme.

Example 5: the ion exchange of the sodium–potassium pump

For each cycle of the pump, 3 Na⁺ leave and 2 K⁺ enter, so a net charge of one positive unit moves out. The pump therefore contributes directly to the negative potential inside the cell, in addition to keeping the Na⁺ concentration low inside (about 12 mM) and the K⁺ concentration high (about 140 mM).

How was the membrane structure discovered?

Ernest Overton found in the 1890s that substances that dissolve in lipids enter cells more easily, which suggested that the membrane contains lipids. In 1925 Evert Gorter and François Grendel extracted the lipids from red blood cells and showed that they cover twice the surface area of the cells, and concluded that the membrane is a bilayer. Hugh Davson and James Danielli proposed in 1935 a model with proteins coating the surfaces of the bilayer. Seymour Singer and Garth Nicolson proposed the fluid mosaic model in 1972, on the basis of electron microscopy and biochemical studies. Peter Agre received the Nobel Prize in Chemistry in 2003 for the discovery of aquaporins, and Jens Skou in 1997 for the sodium–potassium pump.

Which mistakes are common?

• Placing the heads inside the bilayer: the polar heads face the water, the tails the interior.
• Calling the membrane rigid: it is fluid and dynamic.
• Thinking that ions pass through the lipid part: they need channels or pumps.
• Mixing up facilitated diffusion and active transport: only the second needs energy.
• Forgetting that cholesterol is an essential part of animal membranes: it controls fluidity.