In general water insoluble compounds composed of carbon, hydrogen, oxygen and other elements like phosphorus are called as lipids. They included fats, steroids, prostaglandins, eicosanoids, etc.
Mainly lipids are composed of a glycerol backbone bonded with one, two, or three fatty acids. They act as a fuel as well as structural unit in cell membrane.
Glycerol is a trihydric alcohol with three carbon atoms while fatty acids have long non-polar hydrocarbon chain with polar carboxylic group. The hydrocarbon chain can be saturated or unsaturated in nature which affects the physical and chemical properties of fats. Unsaturated fats have a tendency to metabolized and mostly derived from plant oils. They exist in liquid state at room temperature compare to fat which are solid under normal conditions.
On the basis of their structural components, lipids can be classified in three types.
- Phospholipids: These lipids are made up of glycerol, fatty acids, phosphate group with some other groups.
- Triglycerides: They are formed by the esterification of glycerol and fatty acids.
- Steroids: Have a tetracyclic skeleton which consists of three fused six-member with one five-member ring.
The main component of cell membranes is Phospholipids because of their ability to form lipid bilayers. They are composed of glycerol, fatty acids and a phosphate moiety which further esterified with a simple organic molecule like serine and ethanolamine. The first phospholipid identified by Theodore Nicolas Gobley, a French chemist and pharmacist, in 1847 in biological tissues as lecithin or phosphatidylcholinein the egg yolk.
Cell membrane is a semi-porous barrier between the cell contents and outside environment which permeable to some specific molecules only. It regulates the transportation of different molecules in cell. Fluid mosaic model proves that this membrane is composed of a double layer or bilayer of lipids as oily substances found in all cells. These bilayers of lipids can be described as phospholipids.
- Triglycerides are esters of glycerols, formed by fatty acids which can be saturated or unsaturated.
- Three hydroxyl groups of glycerol react with three molecules of fatty acids to form trigylcerides.
- Phospholipids are diglycerids composed of one glycerol (1,2,3-propantriol) molecule boded with two fatty acids at the 1 and 2 positions on glycerol through ester bond.
- The third hydroxyl group of glycerol attached with a phosphoric acid through a phosphate ester bond.
- Further the phosphate moiety bonded with a complex amino alcohol by a second phosphate ester bond. These complex amino alcohols can be choline, ethanol amine or an amino acid like serine.
- Hence the chemical and physical properties of a phospholipid are depending upon the type of fatty acid chain and on the amino alcohol. Just like triglyceride, phospholipids also have polar and non-polar end.
- The long hydrocarbon chains of the fatty acids are non-polar in nature while phosphate group is a polar end with negatively charged oxygen and positively charged nitrogen.
There are many phospholipids involve in various biochemical reactions. They differ from each other in the presence of different organic molecules which bonded on phosphate group. For example Phosphatidylethanolamine or cephalin also abbreviated as PE is found in biological membranes and composed of ethanolamine bonded to phosphate group on diglyceride.
This phospholipid generally found in nervous tissue of bacteria such as in white matter of brain, spinal cord, nerves and neural tissue.
Another example is Phosphatidylcholine or lecithin or PC which has chloline bonded with phosphate group and glycerophosphoric acid with different fatty acids like palmitic or hexadecanoic acid, margaric acid, oleic acid. It is a major component of cell membrane and mainly present in egg yolk and soy beans.
Some other examples of phospholipids are as follows.
1. Phosphatidic acid (phosphatidate) (PA)
It consists of a glycerol with one saturated fatty acid bonded to carbon-1 of glycerol and an unsaturated fatty acid bonded to carbon-2 with a phosphate group bonded to carbon-3.
2. Phosphatidylserine (PS)
This phospholipid contains serine as an organic compound with other main components of phospholipids. Generally it found on the cytosolic side of cell membranes.
It is a group of phospholipids which are negatively charged and act as a a minor component in the cytosolic side of eukaryotic cell membranes. On the basis of different number of phosphate groups they can be different types like phosphatidylinositol phosphate (PIP), phosphatidylinositol bisphosphate(PIP2) and phosphatidylinositol trisphosphate (PIP3). PIP, PIP2 and PIP3 and collectively termed as phosphoinositide.
Similar to triglycerides, Phospholipids also have a polar and a non-polar end which enables them to act as barriers between aqueous solutions. The molecules of phospholipids line up in such a way that their polar "heads" which composed of glycerol and fatty acid end oriented toward the water molecules. While another non-polar tails which contains a long fatty acid end oriented away from the water. In other words, the polar "heads" is hydrophilic or water loving and the non-polar "tails" is hydrophobic or water fearing in nature.
Hence when phospholipids placed in an aqueous environment, spontaneously the hydrophobic portions stick together, as do the hydrophilic and form a very stable form. This arrangement of molecules is called as lipid bilayer or phospholipid bilayer. The spontaneously arrangement of phospholipid molecules in bilayer is make a lowest free-energy configuration in which the hydrophobic regions moves away from water, while the hydrophilic regions interact with water.
Phospholipid molecules involve in the formation of almost all living cell membranes due to their ability to arrange in bilayer form which provide membranes stability and specialized their functions. The bilayer membrane of cell is largely composed of phospholipids with many other components like cholesterol which contribute to its structural integrity. There are many protein channels which permit the transport of different chemical species in and out of the cell.
This bilayer membrane structure can also found in some aggregate structures which are known as liposomes. Liposomes formed during the vigorous shaking of phospholipid molecules with water. Liposomes are differing from micelle as they consist of aqueous interior and exterior. They are microscopic vesicles which contain an aqueous core enclosed in one or more phospholipid layers in circular manner.
Some important functions of phospholipids are as follow.
- There are two important phospholipids, Phosphatidylcholine and Phosphatidylserine found the cell membrane without which cell cannot function normally.
- Phospholipids are also important for optimal brain health as they found the cell membrane of brain cells also which help them to communicate and influence the receptors function. That is the reason food stuff which is rich in phospholipids like soy, eggs and the brain tissue of animals are good for healthy and smart brain.
- Phospholipids are the main component of cell membrane or plasma membrane. The bilayer of phospholipid molecules determine the transition of minerals, nutrients, and drugs in and out of the cell and affect various functions of them.
- As phospholipids are main component of all cell membrane, they influence a number of organs and tissues, such as the heart, blood cells and the immune system. As we grown up the amount of phospholipids decreases and reaches to decline.
- Phospholipids present in cell membrane provide cell permeability and flexibility with various substances as well its ability to move fluently. The arrangement of phospholipid molecules in lipid bilayer prevent amino acids, carbohydrates, nucleic acids, and proteins from moving across the membrane by diffusion. The lipid bi-layer is usually help to prevent adjacent molecules from sticking to each other.
- The selectivity of cell membrane form certain substances are due to the presence of hydrophobic and hydrophilic part molecules and their arrangement in bilayer. This bilayer is also maintained the normal pH of cell to keeps it functioning properly.
- Phospholipids are also useful in the treatment of memory problem associated with chronic substances as they improve the ability of organism to adapt the chronic stress.