Lipids can be divided in a simple way to three major classes of chemical compounds. These are: Triglycerides and phospholipids in addition to lipoproteins. Triglycerides are usually ingested in food rich in fat. They are composed of fatty acids that are linked chemically with one glycerol molecule.
These triglycerides which are ingested in the diet are degraded in the small intestine to fatty acids and glycerol with the help of the enzyme lipase which is specific for triglycerides and which is secreted by the pancreatic cells of the exocrine pancreas. These degraded compounds are then absorbed into the circulation where they are used for different metabolic processes such as energy production.
Phospholipids are the other type of lipids which are abundant in the body especially in the cellular membrane. It is synthesized from fatty acids by enzymatically catalyzed reactions. Phospholipids form part of the cellular membrane in the various cells and are hydrophobic compounds that repell water.
Lipoproteins are the third type of lipids that are found in the body. They are chemical compounds between lipids and proteins. Lipids are usually non-polar compounds that are insoluble in water and therefore are hydrophobic. In contrast proteins are polar compounds that are due to the presence of charged amino acids residues on their structure. Thus they are water soluble so they are hydrophilic.
Lipoproteins are classes of compounds which are divided based on the percent of proteins in its structure. This division is based on how much protein in there in the compound and how much lipids is there in the same compound. Chylomicron is the first type of lipoproteins that is discussed here.
Chylomicrons contain approximately 99% lipids and 1% proteins. Therefore, this structure of lipoproteins is not water soluble and is harmful to the body in large quantities. The second type of lipoproteins is the very low density lipoproteins. The density refers to the amount of protein in the structure of the lipoprotein. It contains up to 10% of proteins and the rest is lipids. This is also a lipoprotein which is water insoluble and is bad in high concentration in the blood. This type of lipoprotein can precipitate in the blood arteries and can cause atherosclerosis in high concentration.
The third type of lipoproteins is the low density lipoprotein and includes up to 20% of proteins in its structure. It is higher density in proteins in comparison with the very low density lipoproteins. It contains up to 50% of cholesterol. This type of lipoprotein can also deposit cholesterol in the arteries and muscle tissues.
The best type of lipoproteins for the body is the high density lipoprotein. It contains up to 50% of protein in its structure in addition to 20% of cholesterol. This type of lipoprotein is the healthiest for the body among all types of lipoproteins. There is no risk of deposition of cholesterol in the arteries in this type of lipoprotein and there is no risk for atherosclerosis. It removes excessive amount of cholesterol from the blood and deposits it to the liver. High density lipoprotein is known as the good cholesterol for its chemical character as being water soluble.
Cholesterol is another lipid inside the body which is useful in small quantities but can be harmful in large quantities due to its potential to precipitate in blood vessels causing its blocking. It is usually a constituent of the phospholipid bilayer of the cellular membrane. It is usually embedded in the phospholipid bilayer imparting hydrophobicity to its structure.
Cholesterol is synthesized from fatty acids in the body in a metabolic pathway that is enzymatically catalyzed. Certain drugs that are used to lower cholesterol level in the blood function by inhibiting key enzymes in the process of cholesterol synthesis in the body. An example of such a drug is called simvastatin.
Cholesterol is also used by the bode cells for the synthesis of bile acids and steroid hormones such as estrogen and testosterone. Therefore another drug that is used to reduce cholesterol level in the blood works by binding to bile acids in the intestine thus forcing more cholesterol in the direction of bile acids synthesis. This eventually reduces the amount of cholesterol in the body.