Membrane Transport Basics - Cellular Gatekeepers
- Selective Permeability: The cell membrane acts as a crucial barrier, meticulously controlling substance passage. Lipid-soluble, small, uncharged molecules pass most easily.
- Driving Forces for Transport:
- Concentration Gradient: Substances move from an area of high concentration ($C_{high}$) to one of low concentration ($C_{low}$).
- Electrochemical Gradient: For ions, this is the sum of the concentration gradient and the electrical potential difference (voltage) across the membrane.
- Major Transport Categories:
⭐ Lipid-soluble substances (e.g., steroid hormones, O2, CO2) readily diffuse across the lipid bilayer, while charged ions and polar molecules require transport proteins.
Passive Transport - Downhill Cruising Crew
No ATP; movement down electrochemical gradient.
- Simple Diffusion:
- Lipid-solubles (O₂, CO₂), small uncharged polar (H₂O).
- No carrier, not saturable, low specificity.
- Facilitated Diffusion:
- Protein-mediated. Specific, saturable, competition.
- Channels: Ions (Na⁺, K⁺), H₂O (aquaporins). Faster.
- Carriers: Glucose (GLUTs), AAs. Conformational change.
- Osmosis:
- Water movement: low → high solute conc.
- Osmotic Pressure ($\pi = iCRT$): Prevents water influx.
- Tonicity: Cell volume (isotonic, hypotonic, hypertonic).
| Feature | Simple Diff. | Facilitated Diff. |
|---|---|---|
| Protein | No | Yes (Channel/Carrier) |
| Saturation | No | Yes (Vmax) |
| Specificity | Low | High |
| Competition | No | Yes (carriers) |
⭐ GLUT transporters (e.g., GLUT2 in liver & pancreatic β-cells, GLUT4 insulin-dependent in muscle & adipose) are key examples of facilitated diffusion for glucose.
Active Transport - Uphill Battle Brigade
Moves solutes against concentration/electrochemical gradient; requires energy.
| Feature | Primary Active Transport | Secondary Active Transport |
|---|---|---|
| Energy Source | Direct ATP hydrolysis | Uses electrochemical gradient (indirect ATP consumption) |
| Mechanism | Pump proteins directly use ATP. | Carrier proteins use gradient established by primary transport. |
| Examples | $Na^+/K^+$ ATPase, $Ca^{2+}$ ATPase, $H^+$ ATPase | Symport (e.g., SGLT1: $Na^+$/glucose), Antiport (e.g., $Na^+/Ca^{2+}$ exchanger) |
⭐ The Na+/K+-ATPase pump (3 Na+ out, 2 K+ in) maintains cellular electrochemical gradients and is inhibited by cardiac glycosides like digoxin.
Special Shuttles & Sick Gates
- Bulk Transport:
- Endocytosis: Phagocytosis (cell eating), Pinocytosis (cell drinking), Receptor-mediated (specific).
- Exocytosis: Cellular secretion.
- Channel-mediated Transport:
- Ion Channels:
- Voltage-gated (e.g., nerve impulses).
- Ligand-gated (e.g., neurotransmitter receptors).
- Aquaporins: Rapid H₂O transport.
- Ion Channels:
- Sick Gates (Clinical Links):
- Cystic Fibrosis: CFTR (Cl⁻ channel) defect.
⭐ Cystic Fibrosis is an autosomal recessive disorder caused by mutations in the CFTR gene, leading to defective chloride ion transport, primarily affecting respiratory and digestive systems.
- Glucose-Galactose Malabsorption: SGLT1 defect.
- Long QT Syndromes: Ion channelopathies.
- Cystic Fibrosis: CFTR (Cl⁻ channel) defect.
High‑Yield Points - ⚡ Biggest Takeaways
- Facilitated diffusion: Passive, carrier-mediated, saturable. Active transport: Requires ATP, against gradient.
- Primary active transport: Na+/K+ ATPase (3Na+ out, 2K+ in) directly uses ATP.
- Secondary active transport: Uses ion gradient (e.g., SGLT1 for Na+/glucose cotransport).
- GLUT4 (muscle, adipose) is insulin-dependent; GLUT2 (liver, pancreas) is insulin-independent.
- Ion channels (voltage/ligand-gated) mediate rapid, selective ion flux.
- Cystic Fibrosis: Defective CFTR protein (ATP-gated Cl- channel, an ABC transporter).
Continue reading on Oncourse
Sign up for free to access the full lesson, plus unlimited questions, flashcards, AI-powered notes, and more.
CONTINUE READING — FREEor get the app
