Biological membranes:

• These are the selective molecule barriers to the body environment which segregate organ function, separate cells from one another and divide intracellular events into different categories.
• Regardless the route of administration, a drug must cross membrane before it reaches to its target site.
• Biological membranes are composed of several layer of cell, a single layer of cell or even less than one cell in thickness.
• Matrix of biological membrane are polar lipid molecules (phospholipids) arranged in a bilayer with their hydrophobic tail (fatty acid chains) extended inwards and hydrophilic head aligned at both surfaces.

Fig: Arrangement of biological membrane