Membrane transport is organized as a taxonomy governed by two variables: whether metabolic energy is expended, and the size of the transported species. Passive transport (diffusion, osmosis, filtration, and facilitated diffusion through a protein channel) moves solutes down an existing electrochemical gradient at no energy cost; active transport moves solutes against their gradient, either primary — coupling movement directly to ATP hydrolysis, as in an ATPase pump — or secondary, dissipating a previously established ion gradient to drive a second solute, in the same direction (symport) or the opposite direction (antiport); and vesicular transport (endocytosis and exocytosis) moves cargo too large for any channel by enclosing it in a membrane-bounded vesicle that fuses with the plasma membrane, at high energetic cost. The concept belongs to cell biology and physiology and supplies the mechanistic basis for nutrient uptake, waste elimination, gradient maintenance, secretion, and the vulnerabilities these systems present to disease.
Membrane transport is organized as a taxonomy governed by two variables: whether metabolic energy is expended, and the size of the transported species. Passive transport (diffusion, osmosis, filtrati…