11.1 Eukaryotic Organelles, Membrane Transport & Cytoskeletal Dynamics
Key Takeaways
The plasma membrane is an asymmetric fluid mosaic containing phospholipids, cholesterol (a bidirectional fluidity buffer restricting mobility at high temperatures and preventing crystallization at low temperatures), and lipid rafts enriched in sphingolipids and GPI-anchored proteins.
Membrane transport spans simple diffusion (lipophilic gases, steroids), facilitated diffusion (saturable aquaporins, ion channels, and GLUT uniporters), and active transport: primary active transport via the electrogenic Na+/K+-ATPase (3 Na+ out / 2 K+ in per ATP) establishes resting potentials and powers secondary active symporters (SGLT1) and antiporters (NCX).
Vesicular trafficking coordinates endocytosis (phagocytosis, pinocytosis, clathrin-mediated endocytosis via dynamin) and exocytosis mediated by v-SNARE and t-SNARE complexes; lysosomal acid hydrolases receive mannose-6-phosphate (M6P) tags in the cis-Golgi for targeting to acidic lysosomes maintained at pH ~4.5–5.0 by V-ATPase.
The eukaryotic cytoskeleton comprises microfilaments (polar F-actin filaments undergoing treadmilling and driving cytokinesis), intermediate filaments (non-polar keratin and lamins providing high tensile strength), and microtubules (alpha/beta-tubulin heterodimers exhibiting GTP-dependent dynamic instability, organized in 9 triplets in centrioles and a 9+2 axoneme with dynein arms in motile cilia).
Microtubule motor proteins exhibit strict directional polarity: kinesin drives ATP-dependent anterograde transport toward the plus (+) end at cell peripheries or axon terminals, whereas cytoplasmic dynein drives retrograde transport toward the minus (-) end anchored at the centrosome.
11.1 Eukaryotic Organelles, Membrane Transport & Cytoskeletal Dynamics
Note
Foundational Standard: The eukaryotic cell segregates biochemical pathways into membrane-bound organelles while maintaining selective exchange with the extracellular environment. The MCAT emphasizes membrane thermodynamics, active and passive transport kinetics, endomembrane protein sorting pathways, and cytoskeletal motor mechanisms.
Membrane Architecture: Bilayers, Cholesterol & Lipid Rafts
The eukaryotic plasma membrane is a selectively permeable lipid bilayer described by the Fluid Mosaic Model.
Bilayer Organization & Dynamics
Phospholipids assemble into bilayers driven by hydrophobic exclusion (ΔS > 0). Saturated acyl chains pack tightly to decrease fluidity, while cis-double bonds disrupt packing and lower transition temperatures (Tm).
- Bilayer Asymmetry: The outer leaflet concentrates phosphatidylcholine and sphingomyelin; the inner leaflet concentrates phosphatidylethanolamine and negatively charged phosphatidylserine (PS). Transverse flip-flop is catalyzed by flippases (outer-to-inner, ATP-dependent), floppases (inner-to-outer, ATP-dependent), and Ca²⁺-activated scramblases (bidirectional). Externalized PS serves as an apoptotic "eat-me" signal for macrophages.
- Cholesterol Fluidity Buffer: At high temperatures (>37°C), cholesterol's planar steroid rings restrict acyl chain mobility to prevent leakage; at low temperatures, they prevent crystallization to preserve fluidity.
- Lipid Rafts: Rigid outer-leaflet microdomains enriched in cholesterol, sphingolipids, and GPI-anchored proteins. Floating in a liquid-ordered state, they cluster signaling receptors and mediate endocytosis.
Membrane Transport Mechanisms
- Simple Diffusion: Passive movement directly across the bilayer (ΔG < 0). Nonpolar, lipophilic solutes permeate readily: respiratory gases (O₂, CO₂), steroid hormones (cortisol, testosterone, estrogen), and fat-soluble vitamins (A, D, E, K). Charged ions and polar molecules are impermeable.
- Facilitated Diffusion: Protein-mediated passive transport displaying saturation kinetics (Vmax, Km):
- Aquaporins: Tetrameric channels conducting single-file water flux (~3 × 10⁹ molecules/sec) while excluding ions.
- Ion Channels: Gated pores (voltage-, ligand-, or mechanically-gated).
- GLUT Uniporters: GLUT1 (erythrocytes, BBB), GLUT2 (hepatocytes, β-cells), and insulin-sensitive GLUT4 (muscle, adipose).
- Primary Active Transport: Directly couples uphill transport (ΔG > 0) to ATP hydrolysis (ΔG°' = -30.5 kJ/mol):
- Na+/K+-ATPase: A P-type ATPase pumping 3 Na⁺ out and 2 K⁺ in per ATP. It is electrogenic, maintaining resting membrane potential and steep ion gradients ([Na⁺]out > [Na⁺]in; [K⁺]in > [K⁺]out).
- Pharmacology: Cardiac glycosides (digoxin, ouabain) inhibit the phosphorylated E2-P state of the Na+/K+-ATPase. Rising intracellular [Na⁺] diminishes the gradient driving the Na+/Ca²⁺ antiporter (NCX), accumulating cytosolic Ca²⁺ to enhance cardiac contractility.
- Secondary Active Transport: Couples downhill ion flux to uphill substrate translocation without direct ATP consumption:
- Symport: SGLT1 cotransports 2 Na⁺ down their gradient to import 1 glucose into enterocytes and renal tubules.
- Antiport: The Na+/Ca²⁺ exchanger (NCX) extrudes 1 Ca²⁺ in exchange for 3 Na⁺ entering cardiomyocytes.
- Bulk Vesicular Transport:
- Endocytosis: Phagocytosis (actin-driven particle engulfment >0.5 μm), pinocytosis (fluid drinking), and receptor-mediated endocytosis (clathrin-coated pits and dynamin pinching; defective in familial hypercholesterolemia).
- Exocytosis: Vesicle fusion driven by SNARE complexes: vesicle v-SNAREs (synaptobrevin) interlock with target t-SNAREs (syntaxin, SNAP-25). Ca²⁺ binds synaptotagmin to trigger release; tetanus and botulinum toxins cleave SNAREs to block transmission.
Organellar Structure & Functional Pathways
- Nucleus & Nucleolus: Enclosed by a double membrane with an inner nuclear lamina (lamins A/B/C) and pore complexes driven by importins and Ran-GTP. The nucleolus transcribes pre-rRNA via RNA Polymerase I and assembles 40S and 60S preribosomal subunits.
- Rough Endoplasmic Reticulum (RER): Ribosome-studded sheets. Hydrophobic N-terminal signal peptides bind SRP, docking at Sec61 translocons for cotranslational lumen import and core N-glycosylation (Asn-X-Ser/Thr).
- Smooth Endoplasmic Reticulum (SER): Synthesizes lipids and steroid hormones, detoxifies xenobiotics via Cytochrome P450, and stores Ca²⁺ (sarcoplasmic reticulum).
- Golgi Apparatus: Polarized cisternae receiving ER cargo via COPII vesicles at the cis-face (COPI vesicles retrieve KDEL-tagged ER proteins). Modifies glycans and attaches mannose-6-phosphate (M6P) tags to lysosomal enzymes. In I-cell disease, defective GlcNAc phosphotransferase eliminates M6P tags, causing default secretion of acid hydrolases into plasma.
- Lysosomes: Degradative organelles housing acid hydrolases operating at pH 4.5–5.0, maintained by an ATP-driven V-type H+-ATPase. Autophagy encloses organelles in autophagosomes that fuse with lysosomes. In Tay-Sachs disease, deficient hexosaminidase A causes GM2 ganglioside accumulation.
- Peroxisomes: Replicate by fission; import PTS1-tagged proteins. Perform beta-oxidation of very-long-chain fatty acids (VLCFAs, ≥22 carbons) and degrade H₂O₂ via catalase (2 H₂O₂ → 2 H₂O + O₂). In Zellweger syndrome, defective peroxin import accumulates VLCFAs.
The Cytoskeleton & Molecular Motor Proteins
- Microfilaments (Actin, ~7 nm): Assembled from G-actin polymerizing into helical F-actin. Exhibits polarity ((+) barbed vs (-) pointed end) and treadmilling (ATP-actin adds to (+) end while ADP-actin dissociates from (-) end). Drives lamellipodia motility, microvilli core structure, and the actomyosin contractile ring in cytokinesis.
- Intermediate Filaments (~10 nm): Non-polar, rope-like antiparallel tetramers lacking motor activity that provide high tensile strength. Comprise keratins (epithelia), vimentin (mesenchyme), desmin (muscle), neurofilaments (axons), and nuclear lamins.
- Microtubules (~25 nm): Hollow cylinders of 13 protofilaments built from alpha/beta-tubulin heterodimers. Exhibit polarity ((+) beta-tubulin, (-) alpha-tubulin) and dynamic instability governed by a GTP cap on beta-tubulin. Catastrophe occurs when GTP hydrolysis reaches the tip, triggering depolymerization.
- Centrosome & Centrioles: Animal MTOC. Centrioles display a 9 triplets (9 × 3) arrangement nucleating microtubules via gamma-TuRCs.
- Cilia & Flagella: Motile axonemes possess a 9 + 2 arrangement (9 outer doublets, 1 central pair) linked by nexin and inner/outer axonemal dynein arms. Dynein walks toward minus ends, bending the axoneme. In Kartagener syndrome, absent dynein arms cause immotile cilia, bronchiectasis, and situs inversus.
- Motor Proteins: Processive, ATP-dependent transport:
- Kinesin: Moves toward the plus (+) end (anterograde; soma to axon terminal, transporting vesicles and mitochondria).
- Cytoplasmic Dynein: Moves toward the minus (-) end (retrograde; axon terminal to soma, carrying endosomes, growth signals, and retrogradely transported pathogens like rabies virus and tetanus toxin).
Cytoskeletal Polymer Comparison
| Feature | Microfilaments (Actin) | Intermediate Filaments | Microtubules |
|---|---|---|---|
| Subunit | G-actin monomers (ATP) | Fibrous proteins (Keratin, Lamins) | Alpha/beta-tubulin dimers (GTP) |
| Diameter | ~7 nm | ~10 nm | ~25 nm (hollow cylinder) |
| Polarity | Polar: (+) and (-) ends | Non-polar (antiparallel) | Polar: (+) beta, (-) alpha |
| Motor Proteins | Myosins (Myosin II, V) | None | Kinesins (plus-end), Dyneins (minus-end) |
| Dynamic Behavior | Treadmilling | Stable tensile scaffold | Dynamic instability (GTP cap) |
| Cellular Roles | Cytokinesis furrow, microvilli core | Tensile strength, desmosomes, lamina | Mitotic spindle, organelle transit, 9+2 cilia |
| Inhibitors | Cytochalasin, Phalloidin | None (inert to tubulin drugs) | Colchicine, Paclitaxel/Taxol, Vincristine |
A 68-year-old female with congestive heart failure is prescribed digoxin. Digoxin exerts its clinical effect by directly binding the extracellular surface of the phosphorylated E2-P conformation of the Na+/K+-ATPase. Which set of intracellular ion alterations and downstream transport changes directly accounts for the resulting increase in cardiac contractile force?
Inhibition of the Na+/K+-ATPase increases intracellular sodium concentration, which decreases the driving force for the Na+/Ca2+ antiporter (NCX), leading to elevated cytosolic calcium and enhanced myocardial contractility.
Inhibition of the Na+/K+-ATPase hyperpolarizes the plasma membrane, opening voltage-gated L-type calcium channels and triggering direct calcium influx through SGLT1.
Inhibition of the Na+/K+-ATPase depletes intracellular sodium, accelerating secondary active transport via the Na+/Ca2+ antiporter to pump excessive calcium into the sarcoplasmic reticulum.
Inhibition of the Na+/K+-ATPase stimulates V-type H+-ATPases to acidify the cytosol, dissociating troponin C from tropomyosin and triggering actin-myosin cross-bridge cycling independent of calcium.
A neurobiologist investigates the axonal transport of neurotropic viruses. Rabies virus enters sensory neurons at peripheral axon terminals and travels toward the cell body to initiate central nervous system pathology. Which motor protein and cytoskeletal polarity mechanism mediates this retrograde transport?
Kinesin hydrolyzes ATP to step processively toward the minus (-) end of actin microfilaments oriented toward the cell body.
Myosin V hydrolyzes ATP to walk along intermediate neurofilaments toward the nucleus.
Cytoplasmic dynein hydrolyzes ATP to move toward the minus (-) end of microtubules anchored at the centrosomal microtubule-organizing center near the soma.
Kinesin hydrolyzes GTP to move toward the plus (+) end of microtubules projecting into the cell soma.
A 9-month-old infant presents with coarse facial features, skeletal dysostosis multiplex, severe psychomotor retardation, and elevated plasma levels of multiple acid hydrolases. Biochemical analysis of cultured fibroblasts reveals a total absence of mannose-6-phosphate (M6P) tags on lysosomal enzymes. Which cellular compartment and enzymatic defect is responsible for this condition?
Smooth endoplasmic reticulum failure of cytochrome P450 monooxygenation, preventing the synthesis of lysosomal membrane lipids.
Rough endoplasmic reticulum failure of signal peptidase, preventing the cotranslational translocation of acid hydrolases into the lumen.
Nucleolar defect in RNA Polymerase I transcription, preventing the translation of 28S and 18S ribosomal subunits required for lysosomal enzyme synthesis.
Golgi apparatus deficiency of GlcNAc phosphotransferase in the cis-Golgi, causing acid hydrolases to be secreted into the extracellular space instead of sorted to lysosomes.
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