Passive diffusion

Definition

Passive diffusion is the process by which molecules move across a cell membrane from an area of higher concentration to an area of lower concentration without the use of energy. This movement occurs until there is an equal distribution of molecules on both sides of the membrane and is a fundamental mechanism for drug absorption, distribution, and elimination in the body.

5 Must Know Facts For Your Next Test

1. Passive diffusion is crucial for the absorption of many drugs through biological membranes, especially in the gastrointestinal tract and blood-brain barrier.
2. Only small, non-polar molecules or lipid-soluble drugs can easily pass through cell membranes via passive diffusion.
3. The rate of passive diffusion is influenced by factors such as temperature, molecular size, and the surface area of the membrane.
4. Drugs that are weak acids or bases may have different absorption rates depending on the pH of the environment due to ionization effects.
5. In passive diffusion, once equilibrium is reached, molecules continue to move but at equal rates in both directions, maintaining balance.

Review Questions

• How does passive diffusion facilitate drug absorption in the body?
  • Passive diffusion allows drugs to move from areas of high concentration, such as after ingestion in the gastrointestinal tract, to areas of lower concentration in the bloodstream. This process does not require energy and relies on the concentration gradient. The ability of a drug to undergo passive diffusion directly impacts its bioavailability and how quickly it reaches systemic circulation.
• Compare and contrast passive diffusion with facilitated diffusion in terms of their mechanisms and implications for drug distribution.
  • Both passive diffusion and facilitated diffusion allow substances to cross cell membranes, but they operate differently. Passive diffusion does not require any protein assistance and depends solely on concentration gradients, making it suitable for small, non-polar molecules. In contrast, facilitated diffusion uses specific transport proteins to help larger or polar molecules cross membranes. This distinction is crucial for understanding how various drugs are absorbed and distributed within the body.
• Evaluate the role of passive diffusion in drug elimination and how it affects overall drug efficacy in treatment plans.
  • Passive diffusion plays a significant role in drug elimination as it governs how drugs exit cells and are filtered out by organs like the kidneys. Understanding this process helps in evaluating how quickly a drug will leave the system, which can influence dosing schedules and treatment efficacy. If a drug diffuses too slowly, it may lead to prolonged effects or toxicity; conversely, rapid elimination might necessitate frequent dosing to maintain therapeutic levels, thus impacting patient management strategies.

Related terms

Concentration gradient: The difference in concentration of a substance between two areas, which drives the process of diffusion.

Facilitated diffusion: A process similar to passive diffusion, but it requires specific transport proteins to help move substances across the cell membrane.

Bioavailability: The proportion of a drug or other substance that enters the circulation when introduced into the body and is available for action.