Learning Objective
To compare the major modes of drug transport across biological membranes—passive diffusion, facilitated diffusion, and active transport—by understanding their direction of movement, energy requirements, carrier dependence, and saturation properties.
Modes of Drug Transport Across a Membrane
Drugs cross biological membranes by three fundamental mechanisms. These modes differ in whether they require energy, rely on carrier proteins, or can become saturated at high concentrations.
Three Basic Modes of Drug Transport Across a Membrane
| Mechanism | Direction of Movement | Energy Required | Carrier | Saturable |
|---|---|---|---|---|
| Passive Diffusion | Down concentration gradient | No | No | No |
| Facilitated Diffusion | Down concentration gradient | No | Yes | Yes |
| Active Transport | Against the concentration or the electrical gradient | Yes | Yes | Yes |
Key Points
- Passive diffusion is the most common mechanism; it requires no energy and is not saturable.
- Facilitated diffusion uses carrier proteins but still moves down a gradient and requires no ATP.
- Active transport requires energy (often ATP) to move substances against their gradients and is saturable due to carrier involvement.
