2. Absorption and transmembrane transport of drugs.

Transmembrane transport of drugs.

Drug molecules must cross biological membranes to reach their site of action. The cell membrane consists of a lipid bilayer that acts as a barrier, allowing certain drugs to pass while restricting others. Drugs can cross membranes by passive or active transport mechanisms.

Passive Transport (No Energy Required)

1. Simple Diffusion

• Movement of drug molecules from an area of higher concentration to lower concentration across the lipid bilayer.
• No energy required and occurs directly through the membrane.
• Lipophilic (fat-soluble) and small nonpolar molecules use this route.
• Example: Oxygen, carbon dioxide, ethanol, and steroid hormones.

2. Passive Transfer

• A broad term for all energy-independent transport mechanisms.
• Includes simple diffusion, facilitated diffusion, and filtration.
• The driving force is the concentration gradient (higher to lower).

3. Filtration

• Drug molecules pass through pores in the membrane due to hydrostatic or osmotic pressure.
• Occurs mainly in capillaries, where small water-soluble drugs move through gaps between endothelial cells.
• Example: Glomerular filtration in the kidneys.

4. Facilitated Diffusion

• A type of passive transport where drug molecules move down their concentration gradient but require specific carrier proteins or channels in the membrane.
• No energy required, but transporters increase the rate of movement.
• Highly selective for certain molecules.
• Example: Glucose transport via GLUT transporters.

Active Transport (Energy-Dependent Mechanisms)

5. Specialized Transport

• Involves specific transport proteins or cellular mechanisms that selectively move drugs across the membrane.
• Includes facilitated diffusion, active transport, and receptor-mediated endocytosis.

6. Active Transport

• Moves drug molecules against their concentration gradient (low to high concentration).
• Requires energy (ATP) and is mediated by specific transmembrane proteins.
• Important for absorbing essential nutrients and removing toxic substances.
• Example: Na+/K+ ATPase pump, ATP-binding cassette (ABC) transporters.

Vesicular Transport Mechanisms

7. Endocytosis

• The cell membrane engulfs extracellular material to bring it inside the cell.
• Used for large molecules that cannot pass through the membrane via diffusion or transport proteins.

Types of Endocytosis:

1. Phagocytosis: The cell engulfs large solid particles (e.g., bacteria, cellular debris).
2. Pinocytosis: The cell engulfs liquids and small molecules dissolved in extracellular fluid.

• Example: Vitamin B12 absorption in the intestines via receptor-mediated endocytosis.

Recap Questions

1. What is the main difference between simple diffusion and facilitated diffusion?
2. How does active transport differ from passive transport?
3. What type of drug molecules typically use filtration as a transport mechanism?
4. What is the role of ATP-binding cassette (ABC) transporters in drug transport?
5. What are the two types of endocytosis, and how do they function?