Shea Kathleen Guinto, Peter Julius Dale Inocando*, SooHyun Jung, Jurilla Camille Louise Department of Biology, University of Santo Tomas, Manila, Philippines
In this experiment an egg yolk will serve as the sample to isolate lipids and separate them into two classes which is the phosphorylated lipid and the non-phosphorylated lipid. The isolated lipids are then characterized through a series of color chemical tests along with the given standards.
Lipids (Greek: lipos, fat) are the fourth major group of molecules found in all cells. Unlike nucleic acids, proteins, and polysaccharides, lipids are not polymeric. However, they do aggregate, and it is in this state that they perform their central function as the structural matrix of biological membranes. Substances of biological origin those are soluble in organic solvents such as chloroform and methanol. Hence, they are easily separated from other biological materials by extraction into organic solvents (Voet, D., Voet, J.G., Pratt, C.W. 2012).
Lipids can be categorized to two classes depending on the presence of phosphate. Firstly are the Phosphorylated lipids, also known as Phospholipids which are amphipathic molecules (contains hydrophobic and hydrophilic domains). So that when phospholipids are suspended in water, they spontaneously rearrange into ordered structures exposing the hydrophilic polar head to the water while the hydrophobic groups are buried in the interior to exclude water that makes phospholipids the key structural component of membranes. Most phospholipids contain a diglyceride (glyceride consisting of two fatty acid chains covalently bonded to a glycerol molecule through ester linkages.), a phosphate group, and a simple organic molecule. Phospholipids can also be separated into two types: phosphoglycerides and sphinomeylins. Phosphoglycerides are molecules that contain glycerol, fatty acids, phosphate, and an alcohol while the sphinomeylins only differ that they contain sphingosine instead of glycerol (Timberlake, K.C. 2008).
On the other hand, Glycolipids which is also known as glycosphingolipids differs only from sphingomeylin in that they contain no phosphate, thus it is non-phosphorylated rather it has a carbohydrate attached (Timberlake, K.C. 2008). Their role is to provide energy and also serve as markers for cellular recognition. Glycolipids a monosaccharide, disaccharide, or oligosaccharide is attached to a ceramide, composed of sphingosine and a fatty acid, through an o-glycosidic linkage.
To proceed with the experiment one must know first of the composition of that sample that is going to be used and why. Egg yolk is a rich source of a variety of biochemically important compounds such as proteins and lipids (glycerides or fats, cholesterol, cholesterol esters, and phospholipids) and it easy to obtain one (DeKoning, A.J, J 1974). Egg yolk is also a good source of lecithin as well as egg oil. The yellow color is due to lutein and zeaxanthin, which are yellow or orange carotenoids known as xanthophylls.
Cholesterol, Lecithin and Galactocerebroside are the three standards that will be used in the experiment side by side with the isolated lipids from the egg yolk. Cholesterol is an organic chemical substance classified as a waxy steroid of fat and is required to establish proper membrane permeability and fluidity. Lecithin composed of phosphoric acid, choline, fatty acids, glycerol, glycolipids, triglycerides, and phospholipids. Can be acquired on egg yolk. Depending on hydration and temperature, in aqueous solution, its phospholipids can form liposomes, bilayer sheets, micelles, or lamellar structures. Galactocerebroside the most common example of this class, are almost entirely found in the cell membranes of the brain. It is a type of cerebroside that are sphingolipids in which the head group is monosaccharide with a galactose residue.