Biochemistry/Complex Lipid Metabolism and Cholesterol

Introduction
Dietary fats serve a variety of function in the body. One of the most important function is as a metabolic fuel, per gram, fats yield twice as much energy as carbohydrate. All steroid hormones are derived initially from lipid, and these in turn are responsible for the regulation of body functions such as growth, development and the appearance of sexual characteristics as a result of sex hormones. Essential fatty acids, such as linoleic acid and α-linolenic acid are prostaglandin and ecosanoid precursors.

Lipid Droplets (LDs)
The LDs takes part of a hydrophobic phase in the cytosol and are composed of an organic phase of neutral lipids. This hydrophobic core stores metabolic energy and membrane components making them the central region for lipid metabolism. They also take part in protein storage, degradation, and viral replication. The formal definition for Lipid Droplets is "the cytoplasmic organelle composed of a hydrophobic core of neutral lipids bounded by a phospholipid monolayer and specific proteins" [1]

How were they Discovered
These droplets were found in the nineteenth century by the method of light microscopy of cellular organelles. Lipid Droplets were often called liposomes until the in vitro vesicles were generated in 1960's and this new vesicles were named liposomes. Since this moment the Lipid Droplets were referred as lipid bodies, fat bodies, spherosomes, or simply LDs. LDs were mainly ignored on research until a protein that specifically localized to LD surface was identified in 1991 [1]

Where are they found?
LDs are found in most cells. LDs can be found among bacterias and eukaryote, although their number and size vary greatly in different cell types.

What is their functions?
The LDs provide building blocks for membranes for energy metabolism. LDs have the ability to package hydrophobic lipids, such as TGs, without water, providing a very efficient form of energy storage. It has also been found that there are links between lipid synthesis pathways and LDs. LDs also serve as organizing center for specific lipids and have been linked to protein storage with the possibility of also storing unfolded membrane proteins. LDs have also been found to be involved in the hepatitis C virus assembly.

Digestion and Absorption of Lipids
Lingual Lipase

Short Chain Fatty Acids

Long Chain Fatty Acids

Bile
Primary Bile Acids Bile Salts Secondary Bile Acids

Chylomicron Formation
Micelles

Low Density Lipoprotein (LDL)
Low density lipoprotein (LDL) is known as bad cholesterol. When the level of LDL is high in the blood, it could slowly build up inside the arteries that pump the blood to the brain and the heart. LDL and other substances can cause atherosclerosis. Atherosclerosis is a disease that happened when the arteries became less flexible and narrow due to the plaque that LDL and other substances form.

High Density Lipoprotein (HDL)
Cholesterol can not dissolved in the blood there for it has to be transported in and out of the cells by carrier called lipoprotein. There are two type of lipoprotein: low-density lipoprotein ( LDL) and high-density lipoprotein HDL. high density lipoproteins (HDL) are known as good cholesterol because high level of HDL could prevent from heat attacks. Some scientists believe that high-density lipoprotein (HDL) slow the build up of plaque in which it removes cholesterol from arteries.

=Reference= [1] Walther, Tobias C., and Robert V. Farese, Jr. "Lipid Droplets and Cellular Lipid Metabolism." National Center for Biotechnology Information. U.S. National Library of Medicine, n.d. Web. 06 Dec. 2012. .