Have you ever wondered how medicines reach their target? For example, how paracetamol goes from the guts in the digestive system to the brain passing through a lot of membranes and which are considered intact and impermeable to a lot of substances. Or how does oxygen pass from the air in the lungs into our bloodstream and then finally into the tissues and cells? One keyword can answer these questions: Transporters, ...well actually sometimes it doesn’t require a transporter. To know how this happens, First, let’s discuss the membranes. Cell membranes have two key characteristics: 1) Semi-permeability , where only certain materials may freely cross – large and charged substances are typically blocked. 2) Selectivity , where membrane proteins regulate the passage of material that cannot freely cross. These membrane proteins are called transporters. Thus, the passage of molecules and substances across a biological membrane may occur either passively or actively. 1) Passive Transport This type of transport is the most common mechanism of absorption for drugs. It is the passage of molecules from a high concentration location to a low concentration location (along the concentration gradient). This is just simply how nature works. And this happens so easily that it does not need the energy to do it. Thus, NO ATP hydrolysis is required. And that’s why it’s called “passive”. OKAY, Mohamad-Ali…. we now understand that no energy is needed, but how do molecules go from one side to another? a) Simple diffusion: Since the core of the cellular membrane is lipophilic (Figure.1), thus, any lipophilic molecule (like ciprofloxacin) will have no problem just diffusing through the membrane without any effort. The same happens with very small molecules like O 2 and CO 2.