Membrane Transport: Diffusion, Carriers and Pumps
Core concepts on passive, facilitated, primary and secondary active transport and their energy sources.
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Questions Covered in This Set
10 cards to master
What are the only three energy sources for membrane transport?
(1) The solute's own electrochemical gradient (passive), (2) direct ATP hydrolysis (primary active), (3) the gradient of another solute built by a pump (secondary active).
State Fick's law of diffusion and define its terms.
J = D·A·(C₁ − C₂)/Δx — flux depends on diffusion coefficient, surface area, concentration difference, and membrane thickness.
How do emphysema, fibrosis and altitude each impair O₂ diffusion (in Fick's terms)?
Emphysema reduces surface area A; fibrosis/edema increases thickness Δx; altitude reduces the driving concentration/pressure difference ΔP.
Two features that distinguish simple diffusion from carrier-mediated transport?
It is linear with the gradient (no saturation) and cannot be competitively inhibited — there is no protein to compete for.
Write the Nernst equation at 37 °C and give E_K and E_Na.
E_ion = (61/z)·log([out]/[in]) mV; E_K ≈ −94 mV (4 out/140 in), E_Na ≈ +66 mV (145 out/12 in).
Channels vs carriers: turnover rates and mechanism?
Channels are aqueous pores, gated, 10⁶–10⁸ ions/s at the diffusion limit; carriers bind solute and change conformation, only 10²–10⁴/s.
What are the three kinetic signatures of carrier-mediated transport?
Saturation (Tmax/Vmax, Michaelis–Menten), stereospecificity (D- not L-glucose), and competition (galactose vs glucose on SGLT1).
Name the key GLUT transporters and their locations.
GLUT4 — muscle and adipose (insulin-recruited); GLUT2 — liver and enterocyte basolateral membrane; GLUT1 — erythrocytes and brain endothelium.
Why does ouabain shut down secondary active transport?
Ouabain poisons the Na⁺/K⁺-ATPase; the Na⁺ gradient dissipates, so every Na⁺-coupled cotransporter/exchanger loses its driving force.
Why is the Na⁺ gradient called the cell's 'second currency'?
It is a rechargeable battery kept charged by ATP via the Na⁺/K⁺-ATPase and spent by secondary active transporters.
