Membrane Transport Calculators

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Membrane transport is the movement of molecules and ions across biological membranes. Membranes are selectively permeable — small, nonpolar molecules (O₂, CO₂) cross freely; charged or large polar molecules require transport proteins. Passive transport (no energy required): simple diffusion, facilitated diffusion through channel proteins or carrier proteins, moving down the concentration gradient. Active transport (energy required): primary active transport uses ATP (Na⁺/K⁺-ATPase); secondary active transport uses the electrochemical gradient of one ion to drive another (e.g., Na⁺/glucose symporter). Osmosis is the passive movement of water through aquaporins down its concentration gradient.

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Types of Membrane Transport

  • Simple diffusion: Small nonpolar molecules (O₂, CO₂, lipids, ethanol) cross directly through the lipid bilayer; rate ∝ concentration gradient × membrane permeability
  • Facilitated diffusion: Polar or charged molecules cross via membrane proteins (channels or carriers) down their gradient; no energy required; GLUT2 (glucose), aquaporins (H₂O), ion channels (Na⁺, K⁺, Cl⁻)
  • Primary active transport: Uses ATP directly; pumps molecules against gradient; Na⁺/K⁺-ATPase (3 Na⁺ out, 2 K⁺ in per ATP); Ca²⁺-ATPase (SERCA)
  • Secondary active transport: Uses electrochemical gradient of one ion (usually Na⁺) to drive uphill transport of another; symport (same direction): Na⁺/glucose; antiport (opposite): Na⁺/H⁺

Nernst Potential and Driving Force

Driving force = membrane potential − Nernst potential. E_K ≈ −90 mV; E_Na ≈ +60 mV. At resting potential (−70 mV): Na⁺ driven inward (electrochemical gradient); K⁺ driven outward (open channels).

Glossary

Secondary Active Transport
Uses the electrochemical gradient of one ion (Na⁺) to drive uphill transport of another; symport (same direction — Na⁺/glucose); antiport (opposite — Na⁺/H⁺ exchanger); powered by Na⁺/K⁺-ATPase.
Na⁺/K⁺-ATPase
Primary active transporter moving 3 Na⁺ out and 2 K⁺ in per ATP; maintains Na⁺ and K⁺ gradients; drives secondary active transport; uses 20–40% of resting cellular ATP.
Aquaporins
Water channel proteins dramatically increasing membrane water permeability; selective for water over protons; AQP2 regulated by ADH in kidney; Peter Agre, 2003 Nobel Prize.

Frequently Asked Questions

Membrane permeability depends on molecule properties: Freely permeable (simple diffusion): small, nonpolar molecules: O₂, CO₂, N₂, ethanol, steroids, fatty acids. Cross through the lipid bilayer rapidly. Slowly permeable: small polar molecules: water (some crosses lipid bilayer; most through aquaporins), urea, glycerol. Impermeant (require transport proteins): charged ions: Na⁺, K⁺, Ca²⁺, Cl⁻, H⁺. Large polar molecules: glucose, amino acids, nucleotides, ATP. The phospholipid bilayer is fundamentally a barrier to charged and polar species, which drives the need for specialized transport proteins.

Both are forms of secondary active transport using the Na⁺ (or H⁺) electrochemical gradient: Symport (cotransport): two or more molecules transported in the same direction across the membrane. Na⁺/glucose symporter (SGLT): Na⁺ moves inward down its gradient; this drives glucose inward against its gradient. Na⁺/amino acid transporters. Na⁺/K⁺/2Cl⁻ cotransporter (NKCC). Antiport (exchange): two species transported in opposite directions. Na⁺/H⁺ antiporter: Na⁺ in; H⁺ out (pH regulation). Na⁺/Ca²⁺ exchanger (NCX): 3 Na⁺ in; 1 Ca²⁺ out (cardiac Ca²⁺ regulation). Anion exchanger (AE1 in RBCs): Cl⁻ in; HCO₃⁻ out (CO₂ transport).

The Na⁺/K⁺-ATPase (sodium-potassium pump) transports 3 Na⁺ out and 2 K⁺ in per ATP hydrolyzed: Mechanism: cytoplasmic Na⁺ binds (3) → ATP phosphorylates the pump → conformation change → Na⁺ released outside → extracellular K⁺ binds (2) → dephosphorylation → K⁺ released inside → reset. Importance: maintains the sodium gradient ([Na⁺] high outside, low inside) that drives secondary active transport (Na⁺-coupled glucose, amino acid, Ca²⁺ transporters). Maintains the potassium gradient (drives resting membrane potential ≈ −70 mV). Maintains cell volume (prevents osmotic swelling). Consumes ~20–40% of resting cellular ATP — more in brain and kidney (neurons: 50–60% of ATP).

Aquaporins are water channel proteins that dramatically increase membrane water permeability. Structure: tetrameric proteins with 4 water-conducting subunits; each subunit forms a narrow pore selective for water. Selectivity: the pore is narrow enough for single-file water molecules but excludes protons (H⁺) via electrostatic barriers — preventing dissipation of the proton gradient. Discovery: Peter Agre (2003 Nobel Prize in Chemistry). Key aquaporins: AQP1: red blood cells, kidney proximal tubule. AQP2: kidney collecting duct (regulated by ADH/vasopressin → controls water reabsorption). AQP3, AQP7: skin, kidney. AQP4: brain, astrocytes. Clinical: mutations in AQP2 cause nephrogenic diabetes insipidus (cannot concentrate urine); AQP4 involved in brain edema formation after stroke.