LEARNING OBJECTIVES
In this article, author has explained the introduction, structure, properties, classification and functions of phopholipids.
Definition
Table of Contents
The complex lipids which contain phosphoric acid in addition to fatty acids, nitrogenous bases, and alcohol are called phospholipids.
Phospholipids are the main component of the cell membrane. They are naturally occurring molecules and the major component of all living cells. They maintain the fluidity of the cell membrane.
Structure of phospholipids
A phospholipid is made up of two regions; head and tail. The Head is made up of a phosphate group while the tail is made up of two fatty acid chains. The phosphate group has a phosphorus molecule which is attached to four oxygen molecules. The Head and tail are connected by a glycerol molecule.
Depending upon the type of glycerol molecule, phospholipids can be of various types like phosphatidylinositol, phosphatidylcholine, phosphatidylserine, etc. Phospholipids can also form zwitter ions.
Properties of phospholipids
- Phospholipids are components of the lipid bilayer membrane.
- Phospholipids are amphipathic molecules means they have both hydrophobic and hydrophilic ends. One end is hydrophobic or non-polar while another end is hydrophilic or polar.
- They have weak hydrophobic interactions.
- They fix proteins within the cell membrane.
- They are signal mediators; they help in the transduction of signals.
- Phospholipids are also components of bile and lipoproteins.
- Phospholipids can move within the bilayer regulating the membrane fluidity and flexibility.
Classification of phospholipids
Phospholipids are classified into two classes:
1. Glycerophospholipids
The phospholipids which are derived from glycerol are called glycerophospholipids. Glycerophospsolipids consist of glycerol 3-phosphate esterified at its C1 and fatty acid esterified at C2. The saturated fatty acid is present at C1 while unsaturated fatty acid is present at C2.
Phosphatidic acid, lecithin, and lysolecithin are examples of glycerophospholipids. Phosphatidic acid is the simplest phospholipid.
2. Sphingolipids
Those lipids which are derived from sphingosine alcohol are called sphingolipids. Glycerol alcohol is not present in them. Sphingosine is attached by an amide linkage to a fatty acid to produce ceramide. Ceramide acts as a secondary messenger to regulate cell death (apoptosis), cell cycle, and cell differentiation. A ceramide consisting of 30 carbon fatty acids is a component of skin and regulates skin permeability.
Sphingomyelin is an example of sphingolipids. Sphingomyelin is a component of myelin and is found in the brain and nervous tissues.
Functions of phospholipids
- Phospholipids are components of the cell membrane. They made a cell membrane. Phospholipids make the cell membrane semi-permeable. Only specific substances can pass through it. Fat-soluble molecules can pass through it while that water-soluble cannot. Oxygen, carbon dioxide, and urea can pass through the cell membrane very easily. Large substances cannot pass through it. For example, glucose or sodium, and potassium ions cannot pass through the membrane easily. This makes the cell membrane function properly and separates the inside of the cell from the external environment.
- Phospholipids broke down in the cell to provide energy.
- Phospholipids break into smaller molecules called cytokines. Cytokines help to regulate various activities like the production of certain proteins. They also help in the movement of cells to different areas of the body.
- Phospholipids assist in lubrication. Phospholipids present in lungs and joints help in the lubrication of cells.
- In the pharmaceutical industry, phospholipids are used in drug delivery systems. In this way, drugs can be transported throughout the body and where it is needed. For example, valium is a medication that uses a phospholipid-based drug delivery system.
- Phospholipids have high bioavailability. This means they can easily be absorbed by the body.
- In the food industry, phospholipids are used as an emulsifier. This means they can disperse oil droplets so that oil and water do not form separate layers. Phospholipids are present in egg yolks. Egg yolks are used in mayonnaise to keep it from separating.
- Phospholipids regulate many cellular processes such as growth, synaptic transmission, and immune surveillance.
- Phospholipids that do not form bilayers help in the fusion and bending of the membrane.
- Phospholipids assembled circulating lipoproteins. These lipoproteins play role in the transport of lipophilic triglycerides and cholesterol in the blood.
- In the endoplasmic reticulum, phospholipids are used for non-vascular transport of lipids between ER and other organelles.
- In mitochondria, phospholipids play role in cellular homeostasis and other functions of mitochondria.
- Phospholipids also help in the absorption of fats from the intestine.
- In mitochondria, phospholipids help in the electron transport chain.
- They play role in the transport and removal of cholesterol from the cells.
- In the respiratory system, they act as surfactants.
- They are also involved in the coagulation of blood cells.
- The phospholipid phosphatidylcholine acts as a secondary messenger and helps in the activity of various hormones.