Mediated transport is a biological process by which substances cross cellular membranes with the assistance of specific protein molecules, such as carrier proteins or transporters. Unlike simple diffusion, which occurs directly through the phospholipid bilayer, mediated transport requires a protein that interacts with the substrate to facilitate its movement across the membrane. This category of transport includes both passive and active mechanisms, distinguished by whether the movement of the substrate is driven by a concentration gradient or requires an external energy source.
Types of mediated transport
| Type | Energy requirement | Direction relative to gradient | Typical proteins involved |
|---|---|---|---|
| Facilitated diffusion (facilitated transport) | None (passive) | Down gradient (from higher to lower concentration) | Carrier proteins, channel proteins |
| Active transport (primary) | Direct energy input, usually ATP hydrolysis | Up gradient (against concentration) | ATP‑binding cassette (ABC) transporters, P‑type ATPases |
| Active transport (secondary) | Indirect energy via electrochemical gradient of another ion | Up gradient (against concentration) | Symporters (co‑transporters), antiporters (exchangers) |
Mechanistic overview
- Recognition and binding – The transport protein possesses a binding site with specificity for the substrate. Binding may induce a conformational change in the protein (alternating‑access model).
- Translocation – The conformational change reorients the binding site to the opposite side of the membrane, allowing the substrate to be released.
- Reset – The protein returns to its original conformation, ready for another transport cycle. In active transport, this reset often requires the hydrolysis of ATP or the flow of another ion down its gradient.
Physiological relevance
- Nutrient uptake: Glucose transporters (GLUT family) mediate facilitated diffusion of glucose into cells.
- Ion homeostasis: Na⁺/K⁺‑ATPase (a primary active transporter) maintains intracellular sodium and potassium concentrations essential for neuronal excitability and osmotic balance.
- pH regulation: H⁺/Cl⁻ exchangers (antiporters) help regulate intracellular pH by exchanging protons for chloride ions.
- Detoxification and drug resistance: ABC transporters export xenobiotics and metabolic waste from cells, contributing to multidrug resistance in cancer cells.
Regulation
Mediated transport can be modulated at multiple levels, including transcriptional control of transporter expression, post‑translational modifications (e.g., phosphorylation), allosteric regulation by metabolites, and changes in membrane lipid composition that affect protein activity.
Research and clinical implications
Abnormalities in mediated transport are implicated in various diseases, such as cystic fibrosis (defective CFTR chloride channel), diabetes mellitus (altered GLUT transporter function), and neurological disorders linked to dysfunctional ion pumps. Pharmacological agents often target specific transporters to modify substrate flux—for example, diuretics that inhibit Na⁺/K⁺‑ATPase or selective serotonin reuptake inhibitors (SSRIs) that block the serotonin transporter.
References
- Alberts, B. et al. Molecular Biology of the Cell, 6th ed., Garland Science, 2014.
- Hille, B. Ion Channels of Excitable Membranes, 3rd ed., Sinauer Associates, 2001.
- Lodish, H. et al. Molecular Cell Biology, 8th ed., W.H. Freeman, 2016.