Chapter 11: Membrane Structure, Lipid Bilayer Dynamics, and Membrane Protein Function

Membrane Lipid Structure, Fluidity, and Dynamics

• 11.1.a Plasma Membrane Thickness, Composition, and Functions

  • Thickness: The plasma membrane measures approximately $5\,nm$ to $10\,nm$ in thickness.
  • Main Constituents:
  • Lipids (phospholipids, glycolipids, cholesterol).
  • Proteins (transporters, anchors, receptors, enzymes).
  • Carbohydrates (attached to lipids and proteins forming the glycocalyx).
  • Main Functions:
  • Serves as a selective barrier separating the internal cell contents from the external environment.
  • Mediates cell-environment interactions and signal transduction.
  • Regulates transport of specific ions and molecules in and out of the cell.
  • Enables cell mobility, shape changes, and structural integrity.

• 11.1.b Lipid Solubility Characteristics

  • Cell membrane lipids are amphipathic molecules possessing both hydrophilic (water-loving) and hydrophobic (water-fearing) regions.
  • In soluble aqueous environments, membrane lipids are insoluble as free individual molecules due to their hydrophobic fatty acid tails.
  • Membrane lipids readily dissolve in nonpolar organic solvents (such as chloroform, ether, or benzene).

• 11.1.c Amphipathic Nature and Bilayer Assembly

  • Driving Force: Hydrophilic heads form electrostatic interactions and hydrogen bonds with water, while hydrophobic tails are repelled by water molecules, forcing surrounding water molecules into an energetically unfavorable cage-like structure.
  • Bilayer Formation: To minimize free energy, amphipathic lipids spontaneously assemble into a two-layered sheet (lipid bilayer), burying hydrophobic tails inside away from water while exposing polar heads to aqueous solution.
  • Formation of Closed Vesicles, Organelles, and Cells:
  • Planar bilayer edges expose hydrophobic tails to water, which is energetically unfavorable.
  • The bilayer spontaneously folds into sealed, closed spherical compartments (vesicles, cellular organelles, and intact cell membranes) to eliminate free edges.

• 11.1.d Coalescence of Neutral Fats in Water

  • Triacylglycerols are purely hydrophobic neutral fats lacking hydrophilic polar head groups.
  • When placed in water, triacylglycerols cannot interact favorably with polar water molecules.
  • Because they lack amphipathic structure, they cannot form lipid bilayers; instead, they coalesce into large, compact, hydrophobic fat droplets to minimize exposure to water.

• 11.1.e Structural Comparison of Membrane Lipids and Neutral Fats

  • Phospholipids: Composed of a hydrophilic head containing a phosphate group (often linked to a polar molecule like choline, ethanolamine, or serine), a glycerol or sphingosine backbone, and two hydrophobic hydrocarbon fatty acid tails.
  • Glycolipids: Composed of a sugar/carbohydrate head group directly linked to a hydrophobic lipid region (lacking a phosphate group).
  • Triacylglycerol: Composed of a glycerol backbone esterified to three hydrophobic fatty acid tails, completely devoid of polar head groups.
  • Cholesterol: Composed of a rigid four-ring steroid nucleus, a short nonpolar hydrocarbon tail at one end, and a single polar hydroxyl ($-\text{OH}$) group at the other end.

• 11.1.f Intramembrane Lipid Mobility

  • Lateral Diffusion: Rapid movement of lipid molecules swapping places side-to-side within the same monolayer plane.
  • Flexion: Rapid flexing and bending of hydrocarbon fatty acid tails.
  • Rotation: Rapid rotation of individual lipid molecules along their long vertical axis.
  • Transverse Flipping ("Flip-Flop"): Movement of a lipid molecule from one monolayer to the opposite monolayer; extremely rare spontaneously without enzymatic catalysis.

• 11.1.g Influence of Hydrophobic Tail Length and Saturation on Fluidity

  • Tail Length:
  • Shorter hydrocarbon tails decrease the surface area available for van der Waals interactions between neighboring tails, increasing membrane fluidity.
  • Longer hydrocarbon tails increase van der Waals interactions, decreasing membrane fluidity.
  • Tail Saturation:
  • Unsaturated tails contain one or more cis-double bonds, introducing rigid kinks that prevent tight packing of neighboring lipids, thereby increasing membrane fluidity.
  • Saturated tails contain no double bonds, allowing straight hydrocarbon chains to pack closely together, thereby decreasing membrane fluidity.

• 11.1.h Impact of Cholesterol on Animal Membrane Fluidity

  • Cholesterol molecules fill the space left by kinks in unsaturated hydrocarbon chains.
  • Modulates membrane fluidity by stiffening the lipid bilayer and making it less flexible and less permeable to small water-soluble molecules.
  • Prevents hydrocarbon chains from coming together and crystallizing at lower temperatures, maintaining optimal fluidity across physiological temperature ranges.

• 11.1.i Thermal Adaptation in Single-Celled Organisms

  • Microorganisms like yeast and bacteria adjust the lipid composition of their cell membranes in response to environmental temperature changes to maintain constant membrane fluidity (homeoviscous adaptation).
  • At Lower Temperatures: Cells synthesize lipids with shorter tails and a higher proportion of double bonds (higher unsaturation) to prevent gelation.
  • At Higher Temperatures: Cells synthesize lipids with longer tails and a lower proportion of double bonds (higher saturation) to prevent excessive membrane permeability and loss of structural integrity.

• 11.1.j Biological Importance of Regulating Membrane Fluidity

  • Enables rapid lateral diffusion of membrane lipids and proteins within the membrane plane to facilitate cellular signaling and enzymatic interactions.
  • Allows lipids and proteins to diffuse from their initial sites of insertion in the membrane to other cellular regions.
  • Permits crucial membrane fusion events, including vesicle budding, endocytosis, exocytosis, organelle division, and cell division.
  • Ensures membrane proteins can undergo required structural dynamic changes during ion/molecule transport and signal transduction.

Membrane Biosynthesis, Asymmetry, and Transport

• 11.1.k Phospholipid Production and Bilayer Growth

  • Site of Biosynthesis: New phospholipids are synthesized by enzymes bound to the cytosolic surface of the smooth endoplasmic reticulum (ER) membrane.
  • Mechanism for Even Growth: Newly synthesized phospholipids are inserted exclusively into the cytosolic monolayer. Scramblase enzymes in the ER membrane randomly transfer phospholipids between monolayers to balance lipid numbers and allow even growth of both sides of the bilayer.

• 11.1.l Establishment and Maintenance of Membrane Asymmetry

  • Establishment: Asymmetry is established in the Golgi apparatus. Flippase enzymes selectively transfer specific phospholipids from the non-cytosolic (lumenal) monolayer to the cytosolic monolayer.
  • Maintenance: Asymmetric distribution is preserved during membrane trafficking via vesicle budding and fusion events.

• 11.1.m Retention of Membrane Orientation During Vesicle Transport

  • Membrane orientation (topological symmetry) is strictly preserved during vesicular transport between cellular compartments.
  • The cytosolic face of the membrane always remains facing the cytosol during budding, transport, and fusion.
  • The lumenal face of organelle membranes corresponds to the non-cytosolic face and becomes the outer extracellular surface of the plasma membrane upon exocytosis.

• 11.1.n Comparison of Flippases and Scramblases

  • Scramblases:
  • Located in the endoplasmic reticulum (ER) membrane.
  • Non-selectively transfer random phospholipids between monolayers.
  • Function to equalize lipid distribution across monolayers without establishing asymmetry.
  • Flippases:
  • Located in the Golgi apparatus and plasma membrane.
  • Selectively transfer specific phospholipids (e.g., phosphatidylserine, phosphatidylethanolamine) from the non-cytosolic/extracellular monolayer to the cytosolic monolayer.
  • Function to establish and maintain characteristic lipid asymmetry.

• 11.1.o Asymmetric Distribution of Specific Lipid Classes

  • Glycolipids: Located exclusively in the non-cytosolic (extracellular) monolayer of the plasma membrane. Sugar groups are added inside the Golgi lumen and remain facing the extracellular space, facilitating cell recognition and protection.
  • Phosphatidylserine: Concentrated almost exclusively in the cytosolic monolayer of the plasma membrane. Possesses a net negative charge; exposure on the outer extracellular monolayer serves as a cell signal triggering phagocytosis during apoptosis.
  • Phosphatidylcholine and Sphingomyelin: Concentrated primarily in the non-cytosolic (extracellular) monolayer of the plasma membrane.

Membrane Proteins: Interaction, Structure, and Function

• 11.2.a Mass and Molecular Ratios of Membrane Components

  • Mass Proportion: Proteins account for approximately $50\%$ of the mass of a typical animal cell plasma membrane, with lipids accounting for the remaining $50\%$.
  • Molecular Ratio: Because lipid molecules are much smaller in molecular weight than protein molecules, there are approximately $50$ lipid molecules for every $1$ protein molecule in typical animal cell membranes.

• 11.2.b Functions of Plasma Membrane Proteins

  • Transporters and Channels: Selectively transport nutrients, metabolites, and inorganic ions across the hydrophobic bilayer (e.g., $\text{Na}^+$ pumps, glucose transporters).
  • Anchors: Mechanically attach the membrane to intracellular cytoskeletal filaments or extracellular matrix components (e.g., integrins, spectrin linkages).
  • Receptors: Detect extracellular chemical signals and transduce them into intracellular biochemical responses (e.g., PDGF receptor, G-protein coupled receptors).
  • Enzymes: Catalyze specific metabolic or signaling reactions at the membrane surface (e.g., adenylyl cyclase).

• 11.2.c Categorization of Membrane Proteins by Bilayer Interactions

  • Transmembrane Proteins: Extend entirely through the lipid bilayer, with regions exposed on both sides (can be single-pass or multipass $\alpha$-helices, or $\beta$-barrels).
  • Monolayer-Associated Proteins: Anchored in the cytosolic monolayer by an amphipathic $\alpha$-helix exposing hydrophobic side chains to the lipid interior and hydrophilic side chains to the cytosol.
  • Lipid-Linked Proteins: Located outside the bilayer, covalently attached to lipid molecules (such as GPI anchors or fatty acid chains) embedded in the membrane.
  • Protein-Attached Proteins: Bound to the membrane indirectly via non-covalent interactions with other membrane proteins.

• 11.2.d Integral vs. Peripheral Membrane Proteins

  • Integral Membrane Proteins: Directly attached to or embedded in the lipid bilayer (transmembrane, monolayer-associated, or lipid-linked). Can only be solubilized and extracted by disrupting the lipid bilayer with harsh detergents.
  • Peripheral Membrane Proteins: Bound to the exterior membrane surfaces through non-covalent interactions with integral membrane proteins. Can be detached using gentle chemical treatments (e.g., high salt concentrations or pH changes) that leave the lipid bilayer intact.

• 11.2.e Polypeptide Transmembrane Conformation

  • Polypeptide backbones are hydrophilic due to polar peptide bonds ($\text{C}=\text{O}$ and $\text{N}-\text{H}$ groups).
  • In the hydrophobic interior of the lipid bilayer, the polypeptide chain folds into an $\alpha$-helix.
  • Internal hydrogen bonds form between peptide bond atoms inside the helix, neutralizing polar backbone charges.
  • Nonpolar, hydrophobic amino acid side chains face outward toward the lipid tails to stabilize the helix in the membrane.

• 11.2.f Single-Pass vs. Multipass Transmembrane Proteins

  • Single-Pass Transmembrane Proteins: Cross the lipid bilayer a single time using a single $\alpha$-helix. Function primarily as cell surface receptors for signal reception and transduction.
  • Multipass Transmembrane Proteins: Cross the lipid bilayer multiple times using multiple $\alpha$-helices. Hydrophobic side chains contact lipid tails on the outer surfaces, while hydrophilic side chains line an interior aqueous channel or pore for solute transport.

• 11.2.g Structure of Transmembrane $\beta$-Barrels

  • Constructed from a large $\beta$-sheet rolled into a closed cylindrical barrel structure.
  • Amino acid side chains facing outward toward the lipid bilayer core are strictly hydrophobic.
  • Amino acid side chains facing inward toward the central pore interior are hydrophilic.
  • Functions as aqueous pores or channels allowing selective passage of water and small solutes (e.g., porin proteins in bacterial and mitochondrial outer membranes).

• 11.2.h Extraction of Membrane Proteins Using Detergents

  • Detergent Properties: Amphipathic, single-tailed lipid-like molecules that aggregate into spherical micelles in water.
  • Extraction Mechanism: Hydrophobic ends of detergent molecules interact with hydrophobic regions of membrane proteins and lipid tails, disrupting the lipid bilayer.
  • Solubilizes membrane proteins by forming soluble protein-detergent complexes and lipid-detergent micelles.

• 11.2.i Structure and Function of Bacteriorhodopsin

  • Structure: A multipass transmembrane protein found in Halobacterium halobium, containing seven membrane-spanning $\alpha$-helices covalently bound to a light-absorbing retinal chromophore molecule.
  • Function: Serves as a light-driven proton ($\text{H}^+$) pump. Light absorption induces a conformational change in retinal, causing the protein to pump protons ($\text{H}^+$) out of the bacterial cell to establish a transmembrane electrochemical proton gradient used to synthesize ATP.

Cell Cortex, Membrane Stabilization, and Protein Mobility

• 11.2.j Mechanical Stabilization in Animal vs. Plant Cells

  • Plant Cells: The plasma membrane is mechanically supported externally by a thick, rigid outer cell wall composed of cellulose microfibrils and extracellular carbohydrates.
  • Animal Cells: The fragile plasma membrane is mechanically supported internally by a fibrous protein network called the cell cortex, anchored to the cytosolic face of transmembrane proteins.

• 11.2.k Characteristics of the Red Blood Cell Cortex

  • Red Blood Cell Cortex Structure: Extremely simplified, highly ordered cell cortex dominated by the long, flexible protein spectrin.
  • Assembly: Spectrin forms a net-like meshwork attached to the plasma membrane via intracellular attachment proteins (such as ankyrin) bound to specific transmembrane proteins (e.g., glycophorin and Band 3).
  • Function: Confers mechanical strength and elastic resilience required for red blood cells to deform and squeeze through tiny capillaries without rupturing.
  • Comparison: Other animal cell cortical networks are far more dynamic, complex, and rich in actin and myosin filaments, enabling active cell shape changes and cell motility.

• 11.2.l Demonstration of Lateral Diffusion via Cell Fusion Experiments

  • Method: Human cells and mouse cells were artificially fused to generate human-mouse hybrid cells (heterokaryons).
  • Visualization: Specific human and mouse surface proteins were labeled using fluorescently tagged antibodies (e.g., green fluorescent tags for human proteins, red fluorescent tags for mouse proteins).
  • Observation: Immediately after fusion, human and mouse membrane proteins were restricted to separate halves of the hybrid cell. After incubation at $37\,^\circ\text{C}$, the fluorescently labeled proteins mixed completely over the entire cell surface within 40 minutes.
  • Conclusion: Confirmed that membrane proteins undergo continuous lateral diffusion in the plane of the lipid bilayer.

• 11.2.m Assessment of Fluidity via Fluorescence Recovery After Photobleaching (FRAP)

  • Method: Membrane proteins or lipids are uniformly tagged with a fluorescent marker. An intense, focused laser beam photobleaches a small region of the cell membrane, permanently destroying fluorescence in that defined area.
  • Measurement: The recovery of fluorescence within the photobleached area is monitored over time using fluorescence microscopy as unbleached fluorescent molecules diffuse laterally into the spot from surrounding unbleached regions.
  • Assessment: The rate of fluorescence recovery provides a quantitative measure of lateral mobility and membrane diffusion rates.

• 11.2.n Cellular Mechanisms Restricting Protein Lateral Mobility

  • Tethering membrane proteins to internal structures (the underlying cell cortex).
  • Tethering membrane proteins to external structures (extracellular matrix molecules).
  • Tethering membrane proteins to surface proteins of neighboring cells (intercellular cell-cell junctional complexes).
  • Creating specialized membrane domains using diffusion barriers, such as tight junctions in epithelial cells that prevent apical proteins from diffusing into basolateral domains.

• 11.2.o Tracking Single Molecules via Single-Particle Tracking (SPT) Microscopy

  • Method: Individual membrane protein molecules are labeled with antibody-coated gold nanoparticles or fluorescent beads.
  • Tracking: High-speed video microscopy tracks the individual trajectory path of single protein molecules over time.
  • Insights: Reveals distinct modes of protein movement, such as free lateral diffusion, confined diffusion within membrane microdomains, directed active movement, or complete immobility.

• 11.2.p Structure and Functions of the Glycocalyx

  • Structure: A outer sugar coating on the cell surface formed by carbohydrate chains attached to membrane glycolipids, glycoproteins, and proteoglycans.
  • Functions:
  • Protects the cell surface against mechanical damage and chemical stress.
  • Absorbs water to lubricate cell surfaces and prevent unwanted cell-cell adhesion.
  • Mediates cell-cell recognition and cell adhesion processes (e.g., lectin recognition during leukocyte extravasation at sites of infection).