In the Absorption & Distribution process, a drug has to move across various biological membranes like cell wall, blood-brain barrier etc. The biological membrane is made up of 2 layers of phospholipids with intermingled protein molecules. All Lipid-Soluble substances get dissolved in cell membrane & they are easily permeated into the cells.

To understand the mechanism behind it, you have to understand 3 concepts. Passive Transfer, Carrier Mediated Transport, Endocytosis & Exocytosis.

Passive Transfer

Passive Transfer of The Drug is movement of drug across a membrane without any need for energy. The energy currency of Cells is ATP, in passive transfer cells don’t have to spend any energy in form of ATP to transfer the drug. The drug is transferred in the direction of concentration gradient which means from higher concentration to lower concentration. Outside of cell membrane is higher concentration & inside is lower concentration hence the drug is transfer from outside to inside, & once there is equilibrium of concentration on both side the transfer is stopped automatically. Passive transfer of drug use the basic Law of Physics which is movement from higher concentration to lower concentration to achieve state of equilibrium.

in Passive Transfer, there are 2 types of it. Simple diffusion & Filtration.

Simple Diffusion

Drugs which are lipid soluble or unionized transfer via Simple diffusion. The speed of diffusion depends on the degree of concentration gradient, lipid solubility & ionization. Sometimes, simple diffusion is also known as lipid diffusion because most drugs follow this as they are lipid soluble.

Filtration

there are small Aqueous pores on cell membrane which are also a passage for drugs to transport. The diameter of these pores is 7 Armstrong & Aqueous Soluble drugs with molecular weight less than 100 can easily pass through it.

Now note the difference between these 2, both Simple Diffusion & Filtration transfer drug across the membrane in direction of concentration gradient, from higher to lower concentration but Simple diffusion is used for Lipid Soluble Drugs only & Filtration is used for water soluble Drugs only. In simple diffusion a lipid soluble drug are easily dissolved or permeated in cell membrane because Cell membrane & Drugs Both have Lipophilic properties, Cell membrane is made up of phospholipid which is a lipid. On the other hand, in Filtration, Water Soluble drugs are transfer with via Aqueous pores because cell membrane has Lipophilic as well as hydrophobic properties which means any water-soluble drug cannot interact with cell membrane & that’s why there are Aqueous pores which helps in transfer of drug.

Carrier Mediated Transport

There are some drugs or substances which can’t be transported by passive transfer because every drug is different in it’s chemical properties, they need a special carrier protein, which helps to transfer the drug. First the drug is attached to carrier protein & then carrier protein takes it inside the cell & release it & the process goes on-n-on, that’s how this mechanism works.

There are 2 types of Carrier Mediated transport, Active Transport, Facilitated Diffusion.

Active Transport  

Iin Active transport, the carrier protein has to actively use energy in form of ATP to transport Drug across cell membrane. Active transport is useful when drug or chemical substance has to go against the concentration gradient which means from low to high concentration to transport across cell membrane, Since this is going against the concentration gradient there is expense of energy in cell & energy currency of cell ATP so cells needs to spend some amount of ATP carry out this transport process.

In human body, Nature Metabolites like sugar, amino acids, iron etc use active transport.

Facilitated diffusion

In facilitated diffusion, the carrier protein doesn’t require any energy for transport & the transport is in the direction of concentration gradient which means from high to low. So ATP is not required & it’s energy independent.

The carrier protein is Highly specific which means the binding site on carrier protein is specifically designed to bind with a specific drug or chemical substance at its binding site & no other drug can bind with it if it’s not meant to be.

Example of this can be seen in glucose uptake of cells & absorption of vitamin b12 from intestine.

If you look at the difference between active transport & facilitated diffusion it’s very simple. Both have the same mechanism of work but the difference is active transport requires energy & facilitated diffusion doesn’t need energy. Active transport is for natural Metabolites & facilitated diffusion is for very specific in binding, for example: if this specific binding site is designed for Vitamin B12 then other chemical substance can’t be bind to it but in Active transport any chemical substance can bind to it if it belongs to natural metabolites.

Endocytosis & Exocytosis

Endocytosis means a drug or chemical substance is engulfed by cell Membrane & carried it into cell as a vesicle. When it reaches inside cell it Is broken down & get absorbed. That’s how endocytosis works. Some anti-cancer drugs use this method for drug delivery to cancer cells.

Exocytosis is reverse of endocytosis, instead of engulfing, in exocytosis the chemical substance is expelled out or released from cell. Exocytosis process is very common in neurons when they release neurotransmitters which are stored in nerve ending.