Objective 6: Describe the basic properties and structures of lipids
About lipids: A group of naturally occurring compounds including fats, oil, steroids, and waxes. Relatively insoluble in water - Fats are composed mostly of non-polar bonds which don't form hydrogen bonds. As a result the fat molecules do not interact well with water molecules, so they are 'repelled' by the water and do not dissolve Soluble in non-polar solvents such as ether and chloroform Through our diet essential fatty acids [PUFA: linoleic, linolenic and arachodinic] are obtained and allows for the absorption of fat soluble vitamins [ A,D,E,K] Fat is stored in adipose tissue, protects internal organs from shock because of insulating effect [thermal insulator in subcutaneous tissues and around certain organs]. Non-polar lipids act as electrical insulators [ i.e. do not respond to an electric field and completely resists the flow of electric charge], thus allowing rapid propagation of depolarization waves along myelinated nerves- this allows for the smooth and rapid flow of impulses. [messengers: steroid hormones, prostaglandins]
Fatty Acids Fatty acids fill two major roles in the body: 1. as the components of more complex membrane lipids. 2. as the major components of stored fat in the form of triacylglycerols.
Triacylglycerides Also called triglycerides, these are fats and oils that comprise the major energy source for the body. Triacylglycerides are composed of a glycerol backbone to which 3 fatty acids are esterified.
Phospholipids Phospholipids are similar to fats but one of the fatty acid groups is replaced by a phosphate group Amphiphilic nature(hydrophilic heads and hydrophobic tails) form a bilayer arrangement essential in cell membrane structure Lecithin present in brain, nervous tissue, sperm and egg yolk Cephalins present in brain and erythrocytes
The building block of the phospholipids is phosphatidic acid which results when the X substitution in the basic structure shown in the Figure below is a hydrogen atom.
Glycolipids They contain a fatty acid, sphingosine (amino alcohol- contain both an amine functional group and an alcohol functional group), carbohydrate or carbohydrate derivative. Are an essential part of cell membranes. Cerebrosides present in white matter of brain and myelin sheath of nerves. Gangliosides are found in grey matter of the brain.
Eg. CerebrosideNB. O=C-R: fatty acidhead group attached to sphingosine
Lipoproteins A lipoprotein contains both proteins and lipids water-bound to the proteins. It’s structure consists of: a core consisting of droplets of triacylglycerols and/or cholesteryl esters a surface monolayer of phospholipid, cholesterol and specific proteins (apolipoproteins, e.g., apoprotein B-100 in low density lipoprotein). Many enzymes, transporters, structural proteins, antigens and toxins are lipoproteins. The different types of lipoproteins are 1] Chylomicron: Transports dietary triglyceride and cholesterol esters from intestine to peripheral tissues and liver 2] Very low density lipoprotein (VLDL):Transports endogenous triglyceride from liver to extrahepatic tissues. 3] Low density lipoprotein (LDL): Transports cholesterol from liver to extrahepatic tissues 4] High density lipoprotein (HDL): Transports cholesterol from extrahepatic tissues back to the liver in an esterified form Since lipids are water insoluble they are present in the blood in the form of lipoproteins which are water-soluble.