The Model
Fluid mosaic model — Singer and Nicolson, 1972
Singer and Nicolson proposed the fluid mosaic model in 1972: the membrane is a fluid phospholipid bilayer with proteins embedded in it like icebergs in an ocean. Two words capture everything. 'Fluid' — phospholipids and proteins move laterally within the bilayer continuously. 'Mosaic' — a diverse mixture of different proteins, lipids, and carbohydrates scattered throughout, not a uniform surface.
This model replaced the earlier sandwich model and was confirmed by cell fusion experiments where fluorescent membrane proteins from two different cells mixed throughout the combined membrane within minutes of cell fusion.
💡 Selective Permeability — The Rules of What Crosses
Two rules determine whether a molecule crosses the membrane:
Small + nonpolar = crosses freely by simple diffusion: O₂, CO₂, steroid hormones, fat-soluble vitamins, most lipid-soluble drugs, ethanol.
Large or charged = needs protein assistance: Ions (Na⁺, K⁺, Ca²⁺, Cl⁻), glucose, amino acids, ATP — all require protein channels or carriers.
The critical clinical implication: Steroid hormones (cortisol, testosterone, estrogen, aldosterone) are lipid-soluble → cross freely → bind INTRACELLULAR receptors → change gene expression (slow, hours to days, long-lasting). Peptide hormones (insulin, glucagon, growth hormone) are water-soluble → cannot cross → bind SURFACE receptors → trigger second messengers (fast, seconds to minutes). This distinction appears on virtually every biology and pharmacology exam.
PL
The phospholipid bilayer — the foundation
Each phospholipid is amphipathic — it has a hydrophilic (water-loving) head (glycerol + phosphate + polar group) and two hydrophobic (water-fearing) fatty acid tails. In water, phospholipids spontaneously form a bilayer: the hydrophobic effect drives the tails together in the interior, while heads face the aqueous environment on both sides.
Tail saturation controls fluidity: saturated fatty acids (no double bonds, straight chains) pack tightly → less fluid. Unsaturated fatty acids (double bonds create kinks) cannot pack tightly → more fluid. Cold-water fish have highly unsaturated membranes to remain fluid in cold temperatures — a key ecological adaptation.
Memory trick: Phospholipid = a lollipop with two sticks. Head = candy (loves water, faces out). Tails = sticks (hate water, hide in the bilayer interior).
Chol
Cholesterol — the membrane fluidity thermostat
Cholesterol is interspersed among phospholipids throughout the bilayer. It acts as a fluidity buffer in both directions: at low temperatures, it disrupts tight packing of saturated fatty acid tails, preventing solidification. At high temperatures, it restrains movement of unsaturated tails, preventing the membrane from becoming too fluid and leaky. Mammalian membranes contain 25–40% cholesterol — reflecting how critical this buffering function is.
Cholesterol also concentrates in lipid rafts — specialized microdomains enriched in cholesterol and sphingolipids where certain signaling proteins cluster together.
Memory trick: Cholesterol = thermostat built into the membrane. Prevents freezing in cold. Prevents melting in heat. Keeps fluidity in the Goldilocks zone.
Prot
Membrane proteins — where all function lives
Integral (transmembrane) proteins span the entire bilayer, anchored by hydrophobic amino acid stretches. They include ion channels, transporters, receptors, and enzymes. Many receptors (GPCRs) span the membrane seven times. Peripheral proteins attach to the surface without penetrating the hydrophobic core — these include cytoskeletal anchor proteins and some signaling molecules.
Integral proteins require detergent to remove (embedded in lipid). Peripheral proteins can be removed with high salt or pH changes. This difference is exploited in protein isolation experiments.
Memory trick: Integral = INTO the bilayer (need detergent to remove). Peripheral = on the surface periphery (can be washed off).
Glyco
The glycocalyx — carbohydrate coat and cell identity
Carbohydrate chains attached to glycoproteins and glycolipids on the outer membrane surface form the glycocalyx — a fuzzy carbohydrate coat projecting outward. Functions: cell-cell recognition (ABO blood type antigens are glycolipids on red blood cells), immune self/non-self distinction, cell adhesion, and mechanical protection. Cancer cells often have an altered glycocalyx, which helps them evade immune surveillance.
🔬 Clinical Scenario — Membrane Properties in Drug Design and Disease
Membrane permeability directly determines how drugs are designed and why some diseases disrupt cellular function:
A
Lipid solubility and oral drug absorption. Most oral drugs must be lipid-soluble enough to cross the intestinal epithelial membrane, capillary endothelium, and target cell membrane. Aspirin, ibuprofen, and steroids are lipid-soluble. Aminoglycoside antibiotics (gentamicin) cannot cross membranes freely — they must work extracellularly or against surface structures.
B
Steroid vs peptide hormone mechanism — membrane determines everything. Cortisol (steroid, lipid-soluble) crosses the plasma membrane → binds cytoplasmic glucocorticoid receptor → complex translocates to nucleus → changes gene expression → anti-inflammatory effect in hours. Insulin (peptide, water-soluble) cannot cross → binds insulin receptor tyrosine kinase on cell surface → GLUT4 vesicles fuse with membrane → glucose uptake within minutes.
C
Anesthetics and membrane fluidity. General anesthetics (sevoflurane, older agents like chloroform) are highly lipid-soluble and dissolve into neuronal membranes → increase membrane fluidity → alter ion channel function → disrupt nerve signal transmission → loss of consciousness. The Meyer-Overton rule: anesthetic potency correlates almost perfectly with lipid solubility.
D
The FRAP experiment — proof of membrane fluidity. Fluorescence Recovery After Photobleaching: a spot of fluorescent membrane proteins is bleached with a laser → fluorescent proteins from adjacent areas diffuse back into the bleached spot within seconds → proves lateral movement is fast and continuous. This experiment confirmed the fluid mosaic model experimentally.
✓ Quick Self-Test
1. What does 'fluid' and 'mosaic' mean in the fluid mosaic model?
2. Why do phospholipids spontaneously form a bilayer in water?
3. Why do steroid hormones have intracellular receptors while peptide hormones have surface receptors?
4. Name three factors that increase membrane fluidity.
5. What is the glycocalyx and what are its functions?
Answers:
1. Fluid = phospholipids and proteins move laterally within the bilayer. Mosaic = the membrane contains a diverse mixture of different phospholipids, proteins, cholesterol, and glycolipids.
2. Phospholipids are amphipathic. In water, the hydrophobic effect drives the tails together to minimize water contact, while hydrophilic heads face outward. Bilayer formation is thermodynamically spontaneous.
3. Steroid hormones are lipid-soluble → cross the membrane freely → bind intracellular receptors → change gene expression. Peptide hormones are water-soluble → cannot cross → bind surface receptors → trigger second-messenger cascades.
4. Unsaturated fatty acids (kinked tails prevent tight packing), higher temperature (increased kinetic energy), shorter fatty acid chains.
5. The glycocalyx is a carbohydrate coat on the outer cell surface formed by oligosaccharide chains on glycoproteins and glycolipids. Functions: cell recognition (ABO blood type), immune self/non-self distinction, cell adhesion, and protection.