2.1 Membrane Composition and the Fluid Mosaic Model

Core Explanation

Biological membranes are ~5 nm-thick assemblies of amphipathic lipids — molecules that possess both a hydrophilic (water-loving) head group and hydrophobic (water-fearing) hydrocarbon tails. When placed in aqueous solution, these lipids spontaneously organize into bilayers: the polar heads face outward toward water on both sides, while the hydrophobic tails pack together in the interior, shielded from water. This arrangement is driven by the hydrophobic effect: sequestering nonpolar tails away from water increases the entropy of the surrounding water molecules, which is thermodynamically favorable.

The classical fluid mosaic model, proposed by S. Jonathan Singer and Garth Nicolson in 1972, described the membrane as a two-dimensional fluid in which lipids and proteins diffuse laterally. This was a radical departure from earlier "protein-lipid-protein sandwich" models and correctly captured two essential truths: (1) the bilayer core is a fluid hydrocarbon environment, and (2) proteins are embedded within — not merely plastered onto — this fluid sheet.

Since 1972, however, the model has been substantially refined. We now know that membranes are heterogeneous: proteins are far more abundant than the original model implied (some membranes are >50% protein by mass), lipid and protein diffusion is constrained by the underlying cortical cytoskeleton (the "picket-and-fence" model), and the bilayer is not a featureless solvent — it contains dynamic, cholesterol-enriched nanodomains (lipid rafts) that compartmentalize signaling.

Major Molecular Components

1. Phosphoglycerides (Glycerophospholipids)

These are the most abundant membrane lipids. Their scaffold is glycerol-3-phosphate, esterified with two fatty-acid chains at the sn-1 and sn-2 positions and a polar head group attached to the phosphate. Common head groups include:

The sn-1 fatty acid is typically saturated; the sn-2 is often unsaturated. This creates a "kink" that prevents tight packing and maintains fluidity.

2. Sphingolipids

Instead of a glycerol backbone, sphingolipids are built on sphingosine, an amino alcohol with a long hydrocarbon chain. The simplest is ceramide (sphingosine + fatty acid via amide linkage). Adding phosphocholine yields sphingomyelin, the only phospholipid not built on glycerol. Sphingolipids tend to have long, saturated acyl chains and pack tightly — they contribute to thicker, more ordered membrane regions and are enriched in the outer leaflet.

3. Cholesterol

Cholesterol is a sterol with a rigid four-ring planar structure, a small polar hydroxyl group, and a short hydrocarbon tail. It intercalates between phospholipids with its hydroxyl near the head-group region and its rings embedded among the acyl chains. Cholesterol has a dual effect on fluidity:

• At high temperatures: it restrains acyl-chain motion, reducing fluidity (stiffening).
• At low temperatures: it disrupts tight crystalline packing of saturated chains, preventing gel-phase transition (fluidizing). This is why cholesterol is often called a fluidity buffer — it broadens and lowers the phase-transition temperature.

4. Glycolipids

Found exclusively in the exoplasmic (outer) leaflet, glycolipids carry one or more sugar residues attached to the lipid head group. They contribute to the glycocalyx — a carbohydrate coat that protects the cell surface and mediates cell–cell recognition. Gangliosides (glycolipids with sialic acid) are particularly enriched in neuronal membranes.

Leaflet Asymmetry

The two halves of the bilayer have distinct lipid compositions:

• Exoplasmic leaflet (outside): enriched in phosphatidylcholine, sphingomyelin, and glycolipids.
• Cytoplasmic leaflet (inside): enriched in phosphatidylethanolamine, phosphatidylserine, and phosphatidylinositol.

This asymmetry is established and maintained by ATP-dependent flippases (P4-ATPases that move aminophospholipids inward), floppases (ABC transporters that move lipids outward), and scramblases (Ca²⁺-activated, bidirectional, collapse asymmetry during apoptosis). Asymmetry matters: PS exposure on the outer leaflet is an "eat-me" signal for macrophages; PI(4,5)P₂ in the inner leaflet anchors signaling proteins and regulates the actin cytoskeleton.

The Fluid Mosaic Model: Then and Now

Original (1972)

• Lipids and proteins form a "mosaic" of structures floating in a fluid bilayer.
• Both lipids and integral proteins are free to diffuse laterally.
• The membrane is thermodynamically driven to form a bilayer, and proteins are dissolved within it.

Modern Refinements

1. Protein density: The classic textbook drawings grossly underrepresent protein. In the inner mitochondrial membrane, protein:lipid ratios approach 3:1 by mass. The membrane is better described as a "protein-rich fluid" than a lipid sea with occasional protein islands.

1. Cytoskeleton coupling ("picket-and-fence" model): The cortical actin-spectrin meshwork (the "fence") and transmembrane proteins anchored to it (the "pickets") restrict free diffusion. Single-particle tracking shows that many membrane proteins undergo hop diffusion — they diffuse freely within a compartment (~30–200 nm in diameter) and then "hop" to an adjacent compartment, as if constrained by fences.

1. Lipid rafts — dynamic nanoscale assemblies: In the 1990s–2000s, lipid rafts were envisioned as static, cholesterol- and sphingolipid-enriched "islands." Current understanding, driven by super-resolution microscopy and single-molecule tracking, is that rafts are highly dynamic, nanoscale (10–200 nm), transient (milliseconds to seconds) assemblies. They can be stabilized and coalesced by protein–protein and protein–lipid interactions (e.g., during T-cell receptor signaling) into larger functional platforms, but they are not pre-formed, stable structures.

1. Membrane curvature and lipid shape: Lipids are not just passive solvents. Cone-shaped lipids (PE, with small heads) promote negative curvature; inverted-cone lipids (lysophospholipids) promote positive curvature. Local lipid composition directly influences budding, fusion, and fission events.

Lateral Mobility

Lipids diffuse laterally with a diffusion coefficient D ≈ 10⁻⁸ cm²/s, meaning a single lipid can traverse the length of a bacterial cell (~2 μm) in about one second. Proteins diffuse more slowly (D ≈ 10⁻⁹ to 10⁻¹¹ cm²/s) due to their larger size, cytoskeletal tethering, and crowding.

FRAP (Fluorescence Recovery After Photobleaching) is the classic experiment: a spot of fluorescently labeled membrane is bleached with a laser; the rate at which fluorescence returns reflects the lateral mobility of the labeled species. The original Frye and Edidin (1970) experiment — fusing mouse and human cells and watching surface antigens intermix — provided some of the earliest evidence for membrane fluidity.

In contrast, flip-flop (transverse diffusion, from one leaflet to the other) is extremely slow (half-time of hours to days for phospholipids) because moving the polar head through the hydrophobic core is energetically costly. Flippases accelerate this process.

Membrane Fluidity: Controlling Factors

Cells regulate fluidity through homeoviscous adaptation: when grown at lower temperatures, they increase the proportion of unsaturated fatty acids in their membrane phospholipids to maintain optimal fluidity. This was first demonstrated in E. coli by Sinensky (1974).

Lipid Rafts: A Cautious, Current Perspective

Lipid rafts are membrane microdomains enriched in cholesterol, sphingolipids, and specific proteins (GPI-anchored proteins, Src-family kinases). The key concept is liquid-ordered (Lₒ) phase coexistence with the surrounding liquid-disordered (L_d) phase. In model membranes (giant unilamellar vesicles), Lₒ/L_d phase separation is easily visualized. In living cells, rafts are:

• Nanoscale (~10–200 nm), below the diffraction limit of conventional light microscopy.
• Dynamic: individual lipids and proteins exchange in and out on millisecond timescales.
• Stabilized by protein scaffolds: caveolins (in caveolae) and flotillins can nucleate and stabilize raft domains.
• Functionally significant: raft coalescence upon receptor crosslinking concentrates signaling molecules (e.g., in immune synapse formation).

Avoid the cartoon depiction of rafts as static "icebergs" floating in a "sea" of fluid lipid — that imagery is obsolete.

Experimental Evidence

1. Frye & Edidin (1970): Mouse–human heterokaryons demonstrated intermixing of surface antigens within ~40 minutes at 37°C, establishing lateral diffusion.
2. Freeze-fracture electron microscopy: Revealed intramembranous particles (integral proteins) distributed throughout the bilayer.
3. FRAP and single-particle tracking: Quantified diffusion coefficients and revealed hop diffusion constrained by the cytoskeleton.
4. Super-resolution microscopy (STED, PALM, STORM): Directly visualized nanoscale clustering of raft markers (e.g., GPI-anchored proteins) in living cells.
5. Detergent-resistant membrane (DRM) isolation: Biochemically isolated cholesterol- and sphingolipid-rich fractions — useful but artifact-prone (detergent itself can induce phase separation).

Comparison: Membrane Lipids at a Glance

Disease and Clinical Relevance

• Acanthocytosis (abetalipoproteinemia): Defect in microsomal triglyceride transfer protein → abnormal erythrocyte membrane lipid composition → spiky red blood cells (acanthocytes), neurodegeneration, fat malabsorption.
• Paroxysmal nocturnal hemoglobinuria (PNH): Somatic mutation in PIGA gene → defective GPI-anchor synthesis → erythrocytes lack complement-regulatory proteins (CD55, CD59) → complement-mediated hemolysis.
• Niemann–Pick disease types A/B: Deficiency in acid sphingomyelinase → sphingomyelin accumulation in lysosomes → hepatosplenomegaly, neurodegeneration.
• Statin drugs: Inhibit HMG-CoA reductase → lower cholesterol synthesis. Statins also have pleiotropic effects on membrane signaling by depleting raft cholesterol.

Common Misconceptions