Lipids are found in all living molecules and play an essential role in he maintenance of life. Unlike carbohydrates and proteins, lipids are highly polymorphic and structurally difficult to define. As a result they are classified according to their major feature as water insoluble (or mostly insoluble) organic compounds found in biological systems. Lipids can be either amphipathic (having polar and nonpolar qualities) or hydrophobic (nonpolar).
Even though lipid structures can be relatively complex, there are a few common structural themes. The simplest lipids are the fatty acids with the general formula R-COOH, where R is the hydrocarbon tail. Fatty acids can also be constituents of more complex lipds such as triacylglcerols, glycerophospholipids, and sphingolipids, as well s waxes and eicosanoids. On the other hand there are structurally distinct lipids like steroids and lipid vitamins which are derived from a five carbon molecule called isoprene. Below is a figure outlining the most important types of lipids and their relations to one another.
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All major classes of lipids are depicted. Notice that lipids containing phosphate moieties are collectively called phospholipids, lipids with carbohydrate moieties are called glycolipids, and lipids derived from isoprene are called polyprenyl compounds (or isoprenoids).
Due to their diverse structures, lipids also have diverse biological functions. The most common role of amphipathic lipids is as structural components of all biological membranes. However, fats and oils can also act a way to store metabolic energy for some organisms. In addition, these fats and oils can serve as important heat insulating elements for animals. Lipids like the steroids hormones are associated with highly specialized functions like controlling host metabolic activities, while eicosanids regulate blood pressure in mammals. And finally, lipids can act as individual molecules or in concert with other molecules like proteins and carbohydrates. This allows lipids to become integral for complex aggregates such as a plasma membrane which is in fact made up of a variety of proteins, lipids, and carbohydrates.
Propanol Based Lipids
Propanol based lipids have an even simpler structure as a backbone:
Since these structures have only one or two hydroxyl groups respectively, they can interact with only one or two fatty acid chains. Some examples of the propanol based lipids are listed below. Cat. No. - Product Name H-1870
- 1-O-Hexadecyl-1,3-propandiol M-9550
- 1-Myristoyl-1,3-propandiol P-1090
- N-Palmitoyl-3-aminopropan-1-ol P-1160
Glycerol Based Lipids
The simplest of the glycerol based lipids are those where fatty acids are linked to a glycerol via ester bonds. Fatty acids are compounds such as palmitic acid that has a carboxylic acid attached to a long chain of hydrocarbons. Fatty acids are important fuel molecules. Oxidation of fatty acids yields more energy (~9kcl/g) than the oxidation of protein or carbohydrates (~4kcal/g). Fatty acids are stored as complex lipids called triacylglycerols. This refers to three fatty acids esterified to a glycerol (three-carbon alcohol) backbone.
Triacylglycerols are neutral and nonpolar lipids which allow them to be stored in anhydrous environments and means they will not expand due to uptake of water. Consequently they are space saving efficient molecules for storing energy.
Fatty acids can be released from their triacylglycerols via lipase activity. During digestion, lipases release fatty acids this way in the gastrointestinal tract. The strong lipase activity that occurs during digestion allows for the generation of monoacylglycerols, which are somewhat polar due to their two hydroxyl groups. This means they can form stable micelles which will aid in further lipid digestion and absorption from the small intestine.
There are also more complex glycerol based lipids. These include the phospholipids and the glycolipids. All of these diverse structures are built upon a glycerol bound to a fatty acid or other group (i.e.phosphate) via an ester or ether linkage. The table below lists just a few of the glycerol based lipids offered by Biosynth. Cat. No. - Product Name D-4250
- 1,2-Dilauroylethyleneglycol D-6079
- 1,2-Dipalmitoylglycol H-1825
- 1-O-Hexadecylglycol L-1476
- 1-Lauroylglycol M-9520
As presented on the diagram above, the lipids are a diverse family of molecules. The many members of the lipid family also include members in which the backbone alcohol on which they are built is not glycerol. Some lipids, phospholipids, and glycolipids have a sphingosine platform which is a more complex alcohol. In conjunction with carbohydrate structures these sphingolipids can give rise to more complex glycolipids. In animal cells glycolipids are derived from sphingosine. The simplest glycolipid, called a cerebroside, contains a single sugar residue, either glucose or galactose. More complex glycolipids, such as gangliosides, contain a branched chain of as many as seven sugar residues. Glycolipids contain a carbohydrate instead of a phosphate ester. Cerebrosides and gangliosides are important cell surface components in nerve cells.