Lecture 2 – Biological Membranes: Comprehensive Study Notes

Page 1 – Administrative Information

• Lecture title: Biological Membranes (Lecture 2)
• Instructor: Dr. Yalda Moayedi — email: ym2994@nyu.edu
• Context: Part of an introductory Cell Biology course. Builds on Lecture 1 (general cell structure) and forms conceptual basis for later lectures on transport, signaling, cytoskeleton, and energetics.

Page 2 – Big-Picture Outline

1. Biological membranes in cell function

2. Membrane structure – fluid mosaic model

3. Lipids & lipid bilayers

4. Membrane-protein classification (structure & function)

5. Lipid/protein asymmetry

6. Higher-order membrane organizations: lipid rafts, cortex, glycocalyx

Page 3 – Cell Membranes as Selective Barriers

• Universality: Both bacterial and eukaryotic cells are enclosed by a plasma membrane; eukaryotes possess additional internal membranes (nuclear envelope, ER, etc.).
• Selective permeability: Controls exchange of ions, metabolites, and information with environment or between organelles.
• Thickness: Typical bilayer ≈ $5\,\text{nm}$ (too thin to see by light microscopy; requires EM or AFM).
• Key term: Selective barrier – allows specific diffusion/transport while restricting others.

Page 4 – Biological Roles of Membranes

• Barrier & compartmentalization → maintains distinct ionic & molecular homeostasis.
• Signal transduction: Receptors convert extracellular cues into intracellular changes.
• Energy production: e.g., proton gradient across mitochondrial inner membrane drives ATP synthesis.
• Cell growth & motility: Membrane expansion, lamellipodia formation.
• Molecular synthesis: Lipid & protein modifications occur on/within membranes.

Ethical / clinical relevance: Drugs frequently target membrane enzymes or receptors (e.g., GPCRs, ion channels).

Page 5 – Functional Specialization of Internal Membranes

• Plasma membrane: Signal detection, nutrient import/export, cell adhesion & migration.
• Nuclear envelope: Double membrane continuous with ER; stores and protects DNA.
• Mitochondrial membranes: Inner membrane houses ETC & ATP synthase; regulates apoptosis; Ca²⁺ buffering.
• Endoplasmic reticulum (ER): Rough ER → protein synthesis; smooth ER → Ca²⁺ storage & lipid synthesis.
• Golgi apparatus: Post-translational modification, sorting, vesicle trafficking.
• Lysosomes / peroxisomes / vesicles: Controlled degradation or specialized metabolic reactions (peroxide detox).

Connection to earlier knowledge: Organelle-specific membranes create micro-environments enabling incompatible reactions to occur in one cell without interference.

Page 6 – Fluid Mosaic Model (1972, Singer & Nicolson)

• Membrane = two-dimensional fluid where lipids plus proteins can move laterally.
• “Mosaic” because proteins of diverse shapes/sizes are dispersed in a sea of lipids.
• Bilayer thickness ≈ $5\,\text{nm}$; hydrophobic core ~$3\,\text{nm}$.
• Experimental evidence: FRAP, cell-fusion assays (will revisit on Page 28).

Metaphor: Think of proteins as buoyant icebergs drifting in an oily sea.

Page 7 – Amphipathic Nature of Membrane Lipids

• Definition: Amphipathic = possessing both hydrophilic (polar) head and hydrophobic (non-polar) tail.
• Classes shown: LPC, PC, PE (differ only in head group).
• Importance: Drives spontaneous self-assembly into bilayers in aqueous environment—no energy input required.

Page 8 – Self-Assembly Demonstration

(A) Cartoon or simulation: random lipid molecules in water align such that tails avoid water, forming bilayer.
(B) TEM reveals ~$1\,\text{nm}$ electron-dense head regions separated by lipid core.

Hypothetical scenario: If you mechanically puncture a liposome, hydrophobic edges re-seal almost instantly to minimize free energy.

Page 9 – Energetics of Bilayer Closure

• Planar sheet with exposed edges = energetically unfavorable (hydrophobic tails contact water).
• Bilayer spontaneously folds/curves, forming a sealed vesicle (liposome) ⇒ energetically favorable state.
• Biological implication: Explains continuous membrane surfaces in cells; holes are transient.

Page 10 – Fundamental Bilayer Properties

1. Self-sealing: Micelles/liposomes close and repair.

2. Primary diffusion barrier: High dielectric constant interior resists ion passage → electrical resistance.

3. Fluidity: Individual molecules undergo lateral diffusion, rotation, and tail flexing.

4. Flexibility: Cell membranes can deform during cytokinesis or endocytosis.

Page 11 – Phosphatidylcholine (PC) – A Prototypical Phospholipid

• Contains choline head (positively charged), phosphate, glycerol backbone, two fatty acyl tails.
• One tail often saturated; the other frequently unsaturated (cis-double bond) producing a “kink.”
• Chemical shorthand: 1-Stearoyl-2-Oleoyl-sn-Glycero-3-Phosphocholine (18:0/18:1).
• “Kink” prevents tight packing → increases fluidity.

Page 12 – Diversity of Membrane Lipids

• Phosphatidylserine (PS): Negatively charged, cytosolic leaflet; externalization marks apoptosis (eat-me signal).
• Cholesterol: Rigid sterol nucleus + hydroxyl head; modulates membrane order.
• Galactocerebroside: Glycolipid abundant in myelin; sugar faces extracellular side, contributes to cell recognition.

Clinical tie-in: MS involves myelin sheath damage, partially linked to altered glycolipid composition.

Page 13 – Comprehensive Lipid Map (van Meer & de Kroon, 2011)

• Shows organelle-specific lipid distribution:
– ER: PC > PE > PI; low cholesterol.
– Plasma membrane: High SM & cholesterol (rigid).
– Mitochondria: Unique cardiolipin (CL) critical for ETC.
• Synthetic & catabolic pathways compartmentalized; transporters (e.g., CERT, ABC transporters) shuttle lipids between leaflets/organelles.
• Sterol homeostasis governed by SREBP/SCAP/Insig feedback loop.

Equation worth noting: Cholesterol biosynthesis first committed step
$\text{Acetyl-CoA} \xrightarrow[HMG-CoA\,Reductase]{2\,NADPH} \text{Mevalonate}$
Targeted by statin drugs.

Page 14 – Classification Recap & Relative Abundance

• Mammalian membranes ≈ $65\%$ glycerolipids (PC, PE, PS, PI), $25\%$ sterols (cholesterol), $10\%$ sphingolipids.
• Mitochondria retain bacterial-type PG + CL, reflecting endosymbiotic origin.

Page 15 – Lipid Composition Varies by Organelle

Graph highlights:
• Plasma membrane richest in cholesterol & sphingomyelin → rigidity, raft formation.
• ER membrane most fluid (low cholesterol).
• Endosome/lysosome have BMP (bis-monoacyl-glycerophosphate) essential for lipid degradation.

Implication: Protein sorting signals often rely on these lipid micro-environments.

Page 16 – Nomenclature of Individual Lipids

• Format: [position-1 fatty acid]-[position-2 fatty acid]-glycero-3-[head group].
• Fatty acid shorthand: “18:1” = 18 carbons, 1 double bond.
• In biological glycerophospholipids, sn-2 position is usually unsaturated.

Mnemonic: “sn-2 unsaturated → membrane unsaturated.”

Page 17 – Types of Lipid Motion in Bilayer (Fig 11-15)

1. Lateral diffusion: Fast (≈ $10^{7}\,\text{times/s}$).

2. Rotation: ~$10^{7}\,\text{times/s}$.

3. Flexion: Tail wiggles.

4. Flip-flop: Rare (days) unless catalyzed by flippase/scramblase.

Experimental data: FRAP half-times seconds to minutes depending on temperature & composition.

Page 18 – Determinants of Membrane Fluidity

Factor | Effect on Fluidity
——— | ————————
↑ Unsaturation | ↑ Fluidity (double bond “kinks”)
↓ Chain length | ↑ Fluidity
↑ Cholesterol | ↓ Fluidity at high T; prevents freezing at low T (buffer)
↑ Protein crowding | Generally ↓ Fluidity (obstacles)

Analogous to antifreeze vs. wax.

Page 19 – Cis vs. Trans Unsaturated Fatty Acids

• Oleic acid (cis 18:1Δ9): Natural; introduces ~30° bend → loose packing.
• Elaidic acid (trans 18:1): Straight chain similar to saturated fat; tighter packing, raises LDL in diet.

Health note: Industrial hydrogenation produces trans-fats → cardiovascular risk.

Page 20 – Diet Alters Cellular Lipidome (J. Lipid Res 1985)

Table summary for endothelial cells:
• Supplementing with palmitic acid raises 16:0 content to $\sim25\%$.
• Oleic acid raises 18:1 to $\sim44\%$.
• Linoleic (18:2) or arachidonic (20:4) enrich respective PUFAs.

Practical implication: Dietary fats can remodel membrane fluidity & signaling precursors (e.g., eicosanoids).

Page 21 – Role of Cholesterol in Lateral Organization

• Creates “liquid-ordered” (L_o) domains by tightening packing of adjacent phospholipids.
• Promotes segregation of specific proteins (raft hypothesis).
• Acts as a spacer at low T preventing gel; as rigidifier at high T.

Simplified physical model: Adds rigid planar rings that constrain adjacent acyl chains.

Page 22 – Asymmetric Lipid Distribution (Plasma Membrane)

Outer leaflet: PC, SM, glycolipids (sugar head groups face ECM).
Inner leaflet: PS (negative), PI (signaling), PE.
Cholesterol ~equal on both sides.

Significance: PS externalization = apoptotic “eat-me,” PI (4,5)P₂ serves as signaling hub for PLC/PI3K pathways.

Page 23 – Enzymes Controlling Asymmetry

• Scramblase: Bidirectional, ATP-independent; equalizes leaflets in ER; activated during apoptosis.
• Flippase (P-type ATPase): ATP-dependent, moves PS/PE to cytosolic side in Golgi & PM.
• Floppase: Transfers PC/SM to exoplasmic leaflet.

Diagram (A) ER: symmetric synthesis → scramblase.
Diagram (B) Golgi/PM: flippase establishes asymmetry.

Page 24 – Thickness Variation via Head Group & Acyl Chains

• LPC (lyso-PC) has one tail → inverted-cone shape; tends to form micelles or positive curvature.
• PC & PE differ in head size → influences local curvature (PE favors negative curvature).
• Bilayer thickness ranges $\sim2\text{–}4\,\text{nm}$ depending on lipid.

Relevance: Membrane-remodeling proteins (BAR, dynamin) sense or induce curvature using these lipid preferences.

Page 25 – Structural Classes of Membrane Proteins

• α-helix spanning segment ≈20 hydrophobic amino acids → crosses bilayer (~$30\,\text{Å}$).
• Integral (transmembrane) vs. peripheral vs. lipid-anchored.
• Multi-pass proteins weave through bilayer multiple times; β-barrel common in outer membranes of bacteria/mitochondria.

Page 26 – Functional Classification & Examples (Table 11-1)

Class | Example | Specific Function
——— | ———— | ————————
Transporter | Na⁺/K⁺ ATPase | $3\,\text{Na}^+\,\text{out} / 2\,\text{K}^+\,\text{in}$ per ATP
Anchor | Integrins | Link actin cytoskeleton → ECM
Receptor | PDGF Receptor | Tyrosine-kinase signaling → proliferation
Enzyme | Adenylyl Cyclase | Generates $\text{cAMP}$ second messenger

Principle: Protein function often requires specific lipid environment (e.g., PIP₂ for ion channels).

Page 27 – Protein : Lipid Mass Ratios Across Membranes

Membrane | Ratio (Prot/Lipid)
————— | ——————————
Myelin (insulation) | 0.23 (lipid-rich)
Plasma membrane (liver) | 1.1
Mitochondrial inner membrane | 3.2 (protein-rich for ETC)

Take-home: Composition correlates with function (electrical insulation vs. energy conversion).

Page 28 – Lateral Protein Mobility (Cell Fusion Assay)

• Hybrid human–mouse cell; proteins labeled with different fluorophores.
• After fusion at $37^{\circ}\text{C}$, fluorophores intermixed within $\sim40\,\text{min}$ ⇒ confirms fluid mosaic.
• Temperature dependence: mobility drops near phase transition.

Page 29 – Mechanisms Restricting Protein Diffusion

1. Anchoring to cell cortex (spectrin/actin).

2. Binding to extracellular matrix.

3. Cell–cell junctions forming diffusion barriers.

4. Partitioning into lipid rafts.

Analogy: Fence posts in a pond limiting how far boats (proteins) can drift.

Page 30 – Red Blood Cell Cortex (Spectrin Network)

• Spectrin tetra-mers form 2-D lattice with actin at junctional complexes (~$100\,\text{nm}$ spacing).
• Connected to membrane via ankyrin, band 3, glycophorin.
• Provides elasticity → RBC squeezes through $\sim3\,\text{µm}$ capillaries without rupturing.

Clinical note: Hereditary spherocytosis from spectrin/ankyrin mutations causes fragile RBCs.

Page 31 – General Functions of Cell Cortex

1. Mechanical strength – resists shear stress.

2. Shape changes & motility – cortical actin rearrangements drive lamellipodia.

3. Protein diffusion restriction – establishes membrane domains (e.g., immunological synapse).

Page 32 – Polarized Membranes in Epithelia

• Tight junctions act as “molecular Velcro” caulking seal that also segregates apical vs. basolateral proteins.
• Example: Intestinal enterocytes – glucose transporters on apical side import nutrients; Na⁺/K⁺ ATPase on basolateral side maintains gradient.
• Breakdown causes malabsorption or pathogen entry.

Page 33 – Glycocalyx – Carbohydrate Coat

• Composition: O- & N-linked oligosaccharides on proteins, glycolipids, proteoglycans.
• Functions:
– Protection against mechanical & chemical damage.
– Lubrication; reduces friction in blood vessels.
– Cell recognition (e.g., ABO blood groups).
– Initial cell–cell contacts and immune evasion.
• “Destruction” bullet refers to pathogens/enzymes that remove glycocalyx to facilitate invasion (e.g., neuraminidase).

Page 34 – Summary / Key Points

• Biological membranes are fluid lipid bilayers embedded with diverse proteins (fluid mosaic model).
• Lipids are amphipathic, self-assemble, & repair. Classes: glycerolipids, sphingolipids, sterols, glycolipids.
• Bilayer exhibits multiple motions; flip-flop is enzyme-assisted.
• Asymmetry: inner vs. outer leaflets differ in lipid & protein composition, maintained by flippases/floppases/scramblases.
• Protein diversity dictates membrane functions (transport, signaling, catalysis, adhesion).
• Cortex, ECM, and junctions create spatial domains; not all proteins diffuse freely.
• Glycocalyx adds a carbohydrate-rich surface for protection & recognition.
• Membrane composition varies by organelle and can be remodeled by diet, development, or disease.

End of Lecture 2 notes.