Membrane Transport
Core concepts
Transport splits into passive (no energy input) and active (energy input). Passive includes simple diffusion through the bilayer (small nonpolar molecules like O2, CO2) and facilitated diffusion through channels or carriers down a gradient. Active transport moves solutes against a gradient, using ATP directly (primary, e.g., Na+/K+ ATPase) or a stored ion gradient (secondary, e.g., SGLT symporters).
Concepts to name
- Concentration gradient, electrochemical gradient, driving force
- Simple vs facilitated diffusion; channel vs carrier
- Primary vs secondary active transport; symport vs antiport
- Osmosis, tonicity (hypertonic, hypotonic, isotonic), aquaporins
- Na+/K+ ATPase stoichiometry: 3 Na+ out, 2 K+ in per ATP
Traps
- Equating "facilitated" with "active" - facilitated diffusion is passive.
- Forgetting that carriers can saturate (Michaelis-Menten-like kinetics) while channels generally do not in the same way.
- Mixing up symport (same direction) and antiport (opposite directions).
- Ignoring the electrochemical gradient: charge matters, not just concentration.
- Confusing osmosis (water movement) with solute diffusion.
Worked example / rule of thumb
Rule: "Ask what powers it." If the answer is ATP hydrolysis at the protein, it is primary active. If it is a pre-existing ion gradient, it is secondary active. If it is the gradient alone with no energy coupling, it is passive. Example: glucose uptake in the intestine via SGLT uses the Na+ gradient set by the Na+/K+ ATPase - secondary active transport, powered indirectly by ATP.