membrane transport

Meaning: The lipid bilayer lets small nonpolar molecules (O₂, CO₂) cross readily but largely blocks ions and large polar molecules. Passive transport moves solute down its gradient with no cellular energy: simple diffusion through the bilayer, facilitated diffusion through channels or carriers, and osmosis for water. Active transport moves solute against its gradient. Primary active transport uses ATP directly (the Na⁺/K⁺ pump moves 3 Na⁺ out and 2 K⁺ in). Secondary active transport uses a gradient built by a pump, as in Na⁺-glucose symport.

Equation: J = P × A × (C₁ − C₂), where J is net flux (mol/s), P is membrane permeability (m/s), A is membrane area (m²), and C₁ − C₂ is the concentration difference across the membrane (mol/m³). This describes simple diffusion.

On the MCAT: A common figure plots transport rate against solute concentration. Simple diffusion rises linearly, while carrier-mediated transport levels off at a maximum when carriers saturate. Passages may add a pump inhibitor or ATP depletion and ask which process stops or which gradient collapses.

Common confusion: Facilitated diffusion uses a protein but is still passive, because solute moves down its gradient with no energy input. Secondary active transport uses no ATP directly, yet it is active because it depends on a gradient that ATP-driven pumps maintain.