Membrane Transport Mechanisms

Cell Membrane & Transport - Gatekeeper's Guide

• Structure: Phospholipid bilayer (amphipathic), proteins (integral, peripheral), cholesterol. Fluid mosaic model.

• Transport Classification:

• Passive: Down concentration gradient.

  • Simple Diffusion: Small, lipid-soluble (O₂, CO₂).
  • Facilitated: Protein-assisted (glucose via GLUT, ions via channels).
  • Osmosis: Water via aquaporins.

• Active: Against gradient, requires energy.

  • Primary: Direct ATP use. E.g., Na⁺/K⁺ pump (3 Na⁺ out, 2 K⁺ in).
  • Secondary: Uses gradient from primary. E.g., SGLT1 (Na⁺-glucose symport).

⭐ The Na⁺/K⁺-ATPase is electrogenic, contributing to the resting membrane potential, and is inhibited by cardiac glycosides like digoxin. 📌 Passive = Pushes itself (no energy); Active = Always needs ATP (energy).

Passive Transport - Downhill Cruisin'

• No ATP used; substances move down their electrochemical or concentration gradient.
• Simple Diffusion:
  • Direct passage via lipid bilayer (e.g., O₂, CO₂, fatty acids, steroids) or un-gated protein channels/pores.
  • Rate factors: $\uparrow$ concentration gradient, surface area, lipid solubility; $\downarrow$ membrane thickness, molecular size.
  • Fick's Law: $J = PA \Delta C$ (P=permeability, A=area, $\Delta C$=concentration difference).
• Facilitated Diffusion:
  • Carrier-mediated (e.g., GLUT transporters for glucose, amino acid transporters) or Channel-mediated (e.g., aquaporins for H₂O, specific ion channels).
  • Characteristics: Specificity, Saturation (exhibits $V_{max}$), Competitive inhibition.
• Osmosis:
  • Net movement of water across a selectively permeable membrane from high water potential (low solute concentration) to low water potential (high solute concentration).
  • Osmotic pressure ($\pi$): Pressure required to stop net water flow. Van't Hoff's Law: $\pi = iCRT$.
  • Tonicity (effect on RBCs): Isotonic (no net change), Hypotonic (cell swells, may lyse), Hypertonic (cell shrinks, crenates).

⭐ GLUT4 is the primary insulin-sensitive glucose transporter, found in skeletal muscle and adipose tissue. Insulin promotes its translocation from intracellular vesicles to the cell membrane, thereby increasing glucose uptake. oka

Active Transport - Uphill Battle

• Movement against electrochemical gradient; needs energy (ATP). "Uphill" battle.
• Maintains cellular gradients for homeostasis.
• Types:
  • Primary Active Transport: Directly uses ATP.
    • Na+/K+ ATPase (Pump): Pumps 3 Na+ out & 2 K+ in per ATP. 📌 PUMPKIN: PUMPs K+ IN. Maintains Na+ & K+ gradients.

    +   Examples: Ca2+ ATPase (SERCA, PMCA), H+/K+ ATPase (gastric cells).
-   **Secondary Active Transport:** Indirectly uses ATP. Energy from ionic gradient (e.g., Na+) via primary transport.
    +   **Cotransport (Symport):** Solute & ion same direction (e.g., SGLT1: Na+-glucose in kidney/intestine).
    +   **Counter-transport (Antiport):** Solute & ion opposite directions (e.g., Na+/Ca2+ exchanger).

⭐ Ouabain (a cardiac glycoside) specifically inhibits the Na+/K+ ATPase pump by binding to the K+ site on its extracellular side when K+ is also bound or translocating from the outside to the inside. This prevents K+ from being released intracellularly, effectively locking the pump in a K+-bound state and halting its cycle. It is often used experimentally to block the pump and in the past was used to treat heart failure (digitalis glycosides have similar effects).

Bulk & Special Transport - Big Gulp Moves

• Bulk Transport: Moves large molecules/particles across membrane using vesicles; requires energy (ATP).
  • Endocytosis: Internalization.
    • Phagocytosis: "Cell eating"; e.g., macrophages engulf bacteria. Involves pseudopods, forms phagosome.
    • Pinocytosis: "Cell drinking"; non-specific uptake of ECF.
    • Receptor-Mediated Endocytosis (RME): Specific; uses receptors (e.g., LDL receptor), clathrin-coated pits.
  • Exocytosis: Export; vesicles fuse with plasma membrane to release contents (e.g., hormones, neurotransmitters).
    • Constitutive (continuous) vs. Regulated (triggered).

⭐ Clathrin is crucial for receptor-mediated endocytosis, forming coated pits that invaginate to become vesicles.

High‑Yield Points - ⚡ Biggest Takeaways

• Simple diffusion is passive, moves down an electrochemical gradient, and is not saturable.
• Facilitated diffusion requires a carrier protein, is saturable, and passive.
• Primary active transport directly uses ATP for uphill movement (e.g., Na⁺/K⁺ pump, Ca²⁺ pump).
• Secondary active transport uses an electrochemical gradient from primary active transport (e.g., SGLT1, Na⁺/Ca²⁺ exchanger).
• Osmosis is the net movement of water across a selectively permeable membrane driven by solute concentration differences.
• Ion channels are selective, protein-lined pores that can be gated, allowing rapid ion flux.
• Fick's Law describes factors affecting simple diffusion rate: surface area, concentration gradient, and permeability coefficient.