Phospholipids | Cholesterol >>
Phospholipids are amphipathic molecules that are made up of a hydrophilic head and a hydrophobic tail. The head group has a 'special' region that changes between various phospholipids. This head group will differ between cell membranes [types of cells] or different concentrations of specific 'head groups'. The fatty acid tails are also differ, but there is always one saturated and one unsaturated 'leg' of the tail.
Phospholipids are 2 fatty acids one saturated and one unsaturated (shown by the double bond) that are linked to a glycerol.
The Noncovalent Assemblies
For the phospholipid bilayer, even though it consists of hydrophilic heads on the outer membrane, the non-covalent hydrophobic tails of the inner membrane is the key to hold the entire membrane together because there are Van der Waals attractive forces within the cell membrane in which the hydrocarbon tails are closely packed together. With its non-covalent character inside the cell membrane, the hydrophobic molecules can easily pass through the cell membrane through passive diffusion. With this, the cell has control the molecules' movement through the transmembrane proteins complexes such as pores and gates. As for the hydrophilic molecules (such as ions, carbohydrates, proteins, amino acids, and nucleic acids), they require active diffusion in order to pass through the cell membrane because of their polarity and because they are hydrophilic (most non-covalent assemblies of the cell membrane are hydrophobic, such as hydrocarbon chains).
The non-covalent assemblies of the cell membrane can help give rise to bubbles such as liposome, or lipid vesicle, that can deliver drugs into a specific part of the body. The structure of liposome is very similar to the cell membrane's lipid bilayer, and the materials that compose the liposome is identical to cell membrane. Because liposome is a bubble, the structure is shaped like a ring, where the hydrophilic heads are the outer and inner ring while the hydrophobic tails are in between the hydrophilic heads' rings. Because of the ring structure, the liposomes are able to trap aqueous materials such as drugs into their rings. Once the materials are within the ring, the liposomes would deliver them to a specific location in the body, such as cancer cells.