Expansion
Simple and facilitated diffusion, primary and secondary active transport, and vesicular transport
Expansion
Passive (no energy, down the gradient)
- Simple diffusion: lipid-soluble substances and gases, straight through the bilayer
- Facilitated diffusion: via carriers or channels; saturable and specific, for example GLUT transporters for glucose
Active (energy required, against the gradient)
- Primary: uses ATP directly. Sodium-potassium ATPase, calcium ATPase, proton pump
- Secondary: uses the gradient created by a primary pump. Symport (same direction, such as sodium-glucose cotransport) or antiport (opposite directions, such as sodium-calcium exchange)
Vesicular: endocytosis, exocytosis and transcytosis, for large molecules.
Osmosis is the movement of water down its own concentration gradient, through aquaporins.
The sodium-potassium ATPase deserves special mention: it consumes about a quarter of basal energy expenditure, maintains the resting potential, and creates the sodium gradient on which nearly all secondary active transport in the body depends.