Eggs are divided into 3 main parts: the egg shell, the egg yolk, and the egg white. The egg yolk is 31% of the composition of the whole egg. On average the egg yolk is 48% water, 17.5% protein, and 32.5% fat. These percentages are prone to change because osmosis of water between egg yolks and egg whites occurs. Older egg yolk are larger than new egg yolks because water from the egg whites enters the egg yolk be seeping through the semi-permeable membrane.
Egg yolks have two uses that separate them apart from the egg white. The first has to do with phospholipids present in the yolk. The phospholipid lecithin is found in the yolk. It is an emulsifier, which helps fats and water distribute evenly throughout sauces, baked goods, and any other items that use yolks. Lecithin is able to accomplish this because it has a hydrophilic end that is attracted to water via hydrogen bonding and a hydrophobic end that is attracted to fats via London dispersion forces. In this way, lecithin physically pulls fat and water molecules together.
The second use that egg yolks are commonly used for is custard creation. Custards use the ability of protein to coagulate (set) in order to bind liquid in custard in a protein matrix. Egg whites can also be used to make custards, but because it lacks the flavor and compaction of the egg yolk the product will not be as creamy or rich.