What is a Phospholipid?
by John O’ Connor
Phospholipids are a special subgroup of lipids that are essential to human health. Although important for a number of cell functions, their most vital role is determining the structure of the cell membrane.
Generally composed of a hydrophilic (“water-loving”) head and hydro-phobic (“water-fearing”) fatty acid tail, phospholipids are naturally amphipathic, meaning that these molecules have two different affinities as indicated. Each phospholipid slightly varies in structure, but its hydrophilic head generally contains a small molecule bound to a phosphate group, which is attached to glycerol. Glycerol (C3H8O3) is a trihydroxy sugar alcohol that forms the backbone of many lipids. In phospholipids, glycerol is bound to two fatty acid chains that compose the hydrophobic tail.
A cell membrane is a physical barrier composed of a phospholipid bilayer, which consists of two layers of phospholipids that separate the inside of the cell from the outside. Supporting a well-known concept that water and oil do not mix, phospholipids spontaneously form a bilayer in aqueous conditions, reducing the interaction of the opposing parts. Since their hydrophilic heads easily interact with aqueous environments and hydrophobic tail do not, the hydrophobic tails orient away from the aqueous environment found both outside and inside the cell. Structurally, this means that the hydrophobic tails are sandwiched between the hydrophilic heads.
A phospholipid bilayer allows the cell membrane to be semipermeable, meaning only select molecules can enter or exit the cell. This tight regulation is critical for the proper function of each cell. The inner hydrophobic region of the cell membrane largely determines a cell’s permeability. In general, small and/or hydrophobic molecules, such as water itself, lipids and gas molecules, diffuse across the cell membrane. Either increasing a molecules’ water solubility or increasing its lipid composition will allow it to more easily cross the cell membrane, thus improving its availability to the cell. Large and/or hydrophilic molecules, such as proteins and carbohydrates, have increasing difficulty crossing the cell membrane or do not cross at all. These molecules may be facilitated across the membrane through various cellular mechanisms, but this process requires additional cell energy.
Phospholipids have a primary role in creating the cell membrane, which is necessary for regulating the environment inside and outside of the cell. Besides its function in structuring the cell membrane, phospholipids have a variety of functions that are dependent upon the composition of the hydrophilic head and hydrophobic tail.