Membranes: structure and fluidity

Every cell is wrapped in a membrane only about 7 to 10 nanometres thick, thinner than a soap bubble's film and roughly ten thousand times thinner than a sheet of paper. That film separates life from non-life: inside, the cell keeps its own chemistry; outside is the environment. Eukaryotic cells also use membranes to build their internal compartments. This lesson explains what membranes are made of, why they form spontaneously, why they are fluid, and how cells adjust that fluidity. Lesson 7 then shows how things cross them.

A brief history of the membrane model

Scientists inferred the structure of the membrane long before they could see it, by a chain of clever experiments. In 1925 Evert Gorter and François Grendel extracted the lipids from red blood cell membranes, spread them as a single layer on water, and found the area was about twice the surface area of the cells. They concluded that the membrane is a lipid bilayer. (Their measurements contained two errors that happened to cancel out, but the conclusion was right.) Because pure lipid bilayers did not fully explain how membranes behave, models with protein layers coating the lipid were proposed, but by the late 1960s electron microscopy and biochemistry showed that many proteins pass right through the bilayer.

In 1972 S. J. Singer and Garth Nicolson proposed the fluid mosaic model, which is still the basis of how we picture membranes: a fluid bilayer of lipids in which proteins float like icebergs, forming a mosaic. A famous experiment by Larry Frye and Michael Edidin in 1970 had already shown the fluidity. They fused a mouse cell with a human cell, each with its surface proteins labelled with a different colour of fluorescent antibody. At first the colours stayed on their own halves of the hybrid cell, but within about 40 minutes at 37 °C they had completely intermixed. At low temperature, mixing slowed dramatically. Membrane proteins drift sideways in a fluid layer.

Phospholipids and the bilayer

The main structural molecules of membranes are phospholipids (lesson 3). Each has a hydrophilic head, containing a negatively charged phosphate group and usually a small attached molecule such as choline, and two hydrophobic fatty acid tails. In water, phospholipids spontaneously arrange themselves so their tails avoid water: they form a bilayer, two layers with the tails facing each other in the middle and the heads facing the water on both sides. No energy or enzyme is needed; the hydrophobic effect drives it. A bilayer also tends to close on itself into a sealed sphere, because open edges would expose tails to water. That is why membranes form closed compartments.

The hydrophobic core, about 3 to 4 nm thick, is what makes a membrane a barrier. Small non-polar molecules such as oxygen, carbon dioxide and steroid hormones dissolve in it and pass through easily. Water passes slowly. Ions and large polar molecules such as glucose barely cross a pure bilayer at all; they need protein help (lesson 7).

# Relative permeability of a pure lipid bilayer (approximate, cm/s, log scale)
import math
P = {"oxygen O2": 2e1, "carbon dioxide CO2": 3e0, "water H2O": 3e-3,
     "urea": 4e-6, "glycerol": 5e-6, "glucose": 4e-10,
     "chloride Cl-": 1e-11, "potassium K+": 5e-14, "sodium Na+": 1e-14}
for name, p in P.items():
    bar = "#" * int(math.log10(p) + 15)
    print(f"{name:<20} {p:8.0e}  {bar}")

oxygen O2               2e+01  ################
carbon dioxide CO2      3e+00  ###############
water H2O               3e-03  ############
urea                    4e-06  #########
glycerol                5e-06  #########
glucose                 4e-10  #####
chloride Cl-            1e-11  ####
potassium K+            5e-14  #
sodium Na+              1e-14  #

The output spans more than 15 powers of ten. Oxygen crosses a bilayer roughly a million billion (10 to the power 15) times more easily than a sodium ion (the values are approximate and vary with lipid composition). The permeability gap is the whole point: by blocking ions and most polar molecules, the membrane lets the cell build up very different concentrations on each side and then control traffic with proteins.

The other membrane lipids

Membranes contain many kinds of phospholipids, differing in their heads and tails, and two other important classes of lipid:

The two layers of a membrane are not identical. Cells actively maintain different lipid compositions in the inner and outer layers, a property called asymmetry. Lipids drift freely sideways, exchanging places with neighbours millions of times a second, but they rarely flip from one layer to the other on their own, because the polar head would have to cross the hydrophobic core. Enzymes called flippases and scramblases move them when needed.

Membrane fluidity and how cells control it

At body temperature a membrane behaves like a two-dimensional liquid with the consistency of light oil. Fluidity matters: membrane proteins must move and change shape to work, vesicles must fuse, and cells must divide. If a membrane is too fluid it leaks; if too stiff it cracks and its proteins stop working. Three factors set the fluidity:

Organisms that cannot control their body temperature adjust their membranes instead, a process called homeoviscous adaptation. Bacteria grown in the cold make more unsaturated and shorter fatty acids; grown warm, they make more saturated ones. Fish in Arctic waters have more unsaturated membrane lipids than tropical fish. Winter wheat and other cold-hardy plants increase membrane unsaturation in autumn. Reindeer even vary the fatty acid composition of cell membranes along their legs, with more unsaturated lipids near the cold hooves.

Membrane proteins

If lipids form the fabric of the membrane, proteins do most of the work. By mass, membranes are typically about half protein, though this varies from about 25 per cent in the myelin sheath around nerve fibres to about 75 per cent in the inner mitochondrial membrane, which is packed with the machinery of respiration. Proteins are classified by how they associate with the bilayer.

Integral proteins are embedded in the membrane. Most span it completely (transmembrane proteins), with hydrophobic stretches of about 20 amino acids that coil into alpha helices across the core and hydrophilic parts sticking out on each side. Some cross once; others, such as transporters and channels, weave back and forth many times. Peripheral proteins sit on one surface, attached to integral proteins or to lipid heads, and can be removed with gentle treatments such as salt solutions.

# How many amino acids does a membrane-spanning alpha helix need?
core_thickness_nm = 3.0      # hydrophobic core
rise_per_residue_nm = 0.15   # an alpha helix rises 0.15 nm per amino acid
print("Residues to span the core:", round(core_thickness_nm / rise_per_residue_nm))

# A protein that crosses the membrane 12 times (like many transporters)
crossings = 12
print("Minimum hydrophobic residues in transmembrane helices:", crossings * 20)

Residues to span the core: 20
Minimum hydrophobic residues in transmembrane helices: 240

Programs that predict membrane proteins from a gene sequence look for exactly these stretches of about 20 hydrophobic amino acids. Roughly a quarter to a third of human genes encode membrane proteins, and more than half of all medicines act on membrane proteins, particularly receptors, channels and transporters.

Proteins also move in the membrane, but many are not free to wander. They may be tethered to the cytoskeleton underneath, held by junctions to neighbouring cells, or gathered in particular regions. Cells lining the gut, for example, keep glucose transporters on the side facing the gut and different transporters on the side facing the blood, separated by tight junctions. Small, more ordered patches enriched in cholesterol and certain lipids, often called lipid rafts, may help cluster some proteins, though their size and nature are still debated.

The carbohydrate coat

On the outer surface of animal cells, short chains of sugars attached to proteins and lipids form a fuzzy layer called the glycocalyx. These sugar chains are like name tags: they help cells recognize each other, let sperm recognize eggs, guide white blood cells to sites of infection, and define blood groups. The A, B and O blood types depend on which sugar, if any, is added to the end of a chain on red blood cells. A blood transfusion with the wrong type triggers an immune attack on the foreign sugars.

Membranes across life

All cell membranes follow the same general plan, with interesting variations. Archaea, many of which live in hot or acidic places, have distinctive lipids with ether rather than ester links and branched tails; some even form a single layer whose molecules span the whole membrane, resisting heat. Plant cells use plant sterols in place of cholesterol. The inner mitochondrial membrane contains a special phospholipid, cardiolipin, shared with bacteria, another clue to its bacterial origin (lesson 1).

Membrane biology matters medically. Some antibiotics, such as polymyxins, disrupt bacterial membranes. Anesthetic drugs and alcohol interact with membranes and membrane proteins. Many viruses, including influenza and SARS-CoV-2, are wrapped in a membrane taken from their host cell and enter new cells by fusing their membrane with the cell's. Soap destroys such viruses by dissolving their membranes, which is why hand washing works so well.