3.4 Bio-Membranes, Fluidity & Transport

The Fluid Mosaic Model & Membrane Architecture

The Fluid Mosaic Model describes the biological plasma membrane as a dynamic, two-dimensional fluid matrix of phospholipids in which proteins, steroids, and carbohydrates are embedded.

Membrane Components & Asymmetry:

1. Phospholipid Bilayer: Forms a continuous hydrophobic core (approximately 3–4 nm thick) flanked by hydrophilic polar head groups. Lipid composition is asymmetric: phosphatidylcholine and sphingomyelin predominate in the outer extracellular leaflet, whereas phosphatidylethanolamine and phosphatidylserine reside in the inner cytoplasmic leaflet.
2. Integral (Transmembrane) Proteins: Pass entirely through the lipid bilayer, anchored by hydrophobic $\alpha$-helices or $\beta$-barrels interacting with fatty acid tails (e.g., ion channels, GPCRs, transport pumps).
3. Peripheral Proteins: Bound non-covalently (electrostatic or hydrogen bonding) to integral proteins or polar head groups on either side of the membrane; easily extracted by altering ionic strength or pH.
4. Lipid-Anchored Proteins: Covalently attached to membrane lipids (e.g., GPI anchors on outer leaflet; prenyl or palmitoyl chains on inner leaflet).
5. Glycoproteins & Glycolipids: Carbohydrate chains attached exclusively to the extracellular surface, forming the glycocalyx for cell-cell recognition, protection, and immune signaling.
6. Lipid Rafts: Dynamic microdomains enriched in sphingolipids, saturated phospholipids, cholesterol, and signaling proteins (such as Src family kinases). Lipid rafts are thicker and more rigid than the surrounding liquid-disordered lipid bilayer, serving as platforms for signal transduction.

---

Determinants of Membrane Fluidity

Membrane fluidity must be tightly regulated to maintain structural integrity while permitting lateral protein diffusion and vesicular transport. Fluidity is dictated by three factors:

1. Temperature: As temperature increases, thermal kinetic energy increases, elevating membrane fluidity. At low temperatures, membranes undergo a phase transition into a rigid gel-like state.
2. Fatty Acid Conformation: Higher concentrations of unsaturated fatty acids with $\textit{cis}$ double bonds increase membrane fluidity because their hydrocarbon kinks disrupt tight lipid packing. Conversely, saturated fatty acids decrease fluidity.
3. Cholesterol (The Fluidity Buffer):
   • At High Temperatures (e.g., $37^\circ\text{C}$): Cholesterol's rigid steroid ring system intercalates between fatty acid tails, restricting excessive phospholipid thermal motion, decreasing fluidity, and stabilizing the bilayer.
   • At Low Temperatures: Cholesterol prevents saturated fatty acid tails from packing tightly into a solid crystalline lattice, increasing fluidity and preventing membrane freezing.

---

Passive Transport Dynamics ($\Delta G < 0$)

Passive transport processes drive net solute movement down a concentration gradient (from high to low concentration) without requiring chemical energy input ($\Delta G < 0$).

1. Simple Diffusion

Small, nonpolar, uncharged molecules diffuse directly through the hydrophobic core of the phospholipid bilayer. Rate of diffusion follows Fick's Law: $J = -D \frac{dC}{dx}$.

• Permeable Molecules: $\text{O}_2$, $\text{CO}_2$, $\text{N}_2$, steroid hormones, fat-soluble vitamins (A, D, E, K).
• Impermeable Molecules: Ions ($\text{Na}^+$, $\text{K}^+$, $\text{Cl}^-$), polar sugars (glucose), and amino acids.

2. Facilitated Diffusion

Polar, charged, or large molecules require transmembrane channel or carrier proteins to cross the hydrophobic core down their electrochemical gradient.

• Channel Proteins: Pore-like structures (e.g., $\text{K}^+$ leak channels, voltage-gated $\text{Na}^+$ channels, aquaporins for rapid water transport). Flux rates are extremely high and generally non-saturable under physiological conditions.
• Carrier Proteins: Transport solutes by undergoing conformational changes (e.g., GLUT4 glucose transporter). Facilitated transport via carriers exhibits saturation kinetics characterized by a maximum transport velocity ($V_{\max}$) and transport affinity ($K_m$).

3. Osmosis & Osmotic Pressure

Osmosis is the net diffusion of water across a semipermeable membrane toward a higher solute concentration. Osmotic Pressure ($\Pi$) is a colligative property calculated using the van 't Hoff equation:

$$\Pi = iMRT$$

Where (i) is the van 't Hoff factor (number of dissociated particles per solute formula unit), (M) is molar solute concentration, (R) is the ideal gas constant ($0.0821\text{ L}\cdot\text{atm/mol}\cdot\text{K}$), and (T) is absolute temperature in Kelvin.

• Hypertonic Environment: Extracellular $\Pi$ is higher than intracellular $\Pi$; water exits the cell, causing cell shrinking (crenation).
• Hypotonic Environment: Extracellular $\Pi$ is lower than intracellular $\Pi$; water enters the cell, causing swelling and osmotic lysis.

---

Active Transport Mechanisms ($\Delta G > 0$)

Active transport moves solutes against an electrochemical gradient (from low to high concentration), requiring energy input ($\Delta G > 0$).

1. Primary Active Transport

Couples solute transport directly to the hydrolysis of ATP.

• $\text{Na}^+/\text{K}^+$ ATPase (Sodium-Potassium Pump): An electrogenic P-type ATPase that hydrolyzes 1 molecule of ATP to pump 3 $\text{Na}^+$ ions OUT of the cell and 2 $\text{K}^+$ ions IN to the cell.

Physiological Significance of $\text{Na}^+/\text{K}^+$ ATPase:

1. Maintains high extracellular $[\text{Na}^+]$ ($\approx 145\text{ mM}$) and high intracellular $[\text{K}^+]$ ($\approx 140\text{ mM}$).
2. Generates the resting membrane potential ($\approx -70\text{ mV}$) essential for nerve impulse propagation.
3. Regulates intracellular osmotic volume (preventing cell swelling by pumping net positive charge out).

2. Secondary Active Transport (Cotransport)

Harnesses the potential energy stored in an electrochemical gradient (established by primary active transport) to drive another solute against its concentration gradient.

• Symport: Solutes move in the same direction. Example: SGLT1 (Sodium-Glucose Cotransporter 1) in intestinal enterocytes and renal proximal tubules, which uses $\text{Na}^+$ moving down its concentration gradient to pump glucose into the cell against a 1000-fold concentration gradient.
• Antiport: Solutes move in opposite directions. Example: $\text{Na}^+/\text{H}^+$ exchanger (NHE), which uses $\text{Na}^+$ influx to drive $\text{H}^+$ efflux, regulating cytosolic pH.

---

Membrane Trafficking: Endocytosis & Exocytosis

Bulk transport of macromolecular particles across the plasma membrane involves membrane-bound vesicles:

1. Endocytosis: Inward invagination of the plasma membrane to capture extracellular material.
   • Phagocytosis ("cell eating"): Engulfment of large solid particles or bacteria into phagosomes (e.g., macrophages, neutrophils).
   • Pinocytosis ("cell drinking"): Nonspecific invagination of extracellular fluid and dissolved solutes.
   • Receptor-Mediated Endocytosis: Selective uptake of specific ligands bound to cell-surface receptors, mediated by clathrin-coated pits. Classic example: Uptake of low-density lipoprotein (LDL) via LDL receptors. Mutations in the LDL receptor cause Familial Hypercholesterolemia.
2. Exocytosis: Fusion of intracellular secretory vesicles with the plasma membrane to release cargo into the extracellular space (e.g., neurotransmitter release at synapses mediated by $\text{Ca}^{2+}$-triggered SNARE complexes, insulin secretion by pancreatic $\beta$-cells).