| | Diet & Nutrition | Nutrition
Active Transport: Energy-Dependent Movement Against Concentration Gradients
Active transport moves solutes across a membrane against their electrochemical gradient, requiring direct energy input—typically ATP hydrolysis.
Key Features
- Direction: Low → high concentration (uphill).
- Energy: Primary active transport uses ATP directly (e.g., Na⁺/K⁺-ATPase, H⁺/K⁺-ATPase, Ca²⁺-ATPase, ABC transporters).
- Secondary active transport: Uses energy stored in ion gradients (e.g., Na⁺-glucose cotransporter SGLT1, Na⁺/H⁺ exchanger).
- Specificity: Transporter proteins exhibit substrate selectivity.
- Saturability: Follows Michaelis-Menten kinetics (Vmax, Km).
- Inhibitability: Competitive/non-competitive inhibitors (e.g., ouabain for Na⁺/K⁺-ATPase).
Physiological Examples
- Na⁺/K⁺-ATPase: Maintains resting membrane potential, cell volume.
- Intestinal/renal glucose absorption (SGLT1/2).
- Gastric acid secretion (H⁺/K⁺-ATPase).
- Neurotransmitter reuptake (SERT, DAT, NET).
- Multidrug resistance (P-glycoprotein/ABCB1).
Contrast with Passive Transport
| Feature | Passive (Diffusion/Facilitated) | Active |
|---|---|---|
| Energy | None (ΔG < 0) | ATP or ion gradient |
| Direction | Down gradient | Against gradient |
| Rate vs. [S] | Linear (diffusion) or hyperbolic | Hyperbolic (saturable) |