Endoplasmic Reticulum

• The endoplasmic reticulum (ER for short) is the site of the cell at or around which much of the lipids are synthesized. Beginning near the nucleus, the ER is a set of channels of flattened sacs and tubes that extends throughout the cytoplasm, the fluid interior of the cell.
  
  

ER Structure

• The ER is divided into two kinds: rough ER and smooth ER. The rough ER is studded with ribosomes, which synthesize proteins from amino acids, but smooth ER contains no ribosomes. It is in the smooth ER where the lipids are manufactured by enzymes which facilitate reactions that normally would happen slowly or not at all.
  
  

Types

• The types of lipids manufactured include phospholipids that compose most of the cellular membrane and cholesterol like hormones that provide a wide variety of functions for the organism. Phospholipids are composed of a backbone and two fatty acid tails, while cholesterol is made out of carbon rings.
  
  

Reactions

• In order to build larger macromolecules, reactions must be catalyzed from smaller constituents by the enzymes. For instance, phosphatidic acid is a very important intermediate in the synthesis of fats and phospholipids. Phosphatidic acid can be derived from a more common organic compound by the transfer of phosphate.
  
  

Phospholipids

• Phospholipids in particular use phosphatidic acid to synthesize diglycerides, which have the same backbone and two fatty acid tail structure. These fatty acid tails are elongated in the smooth ER. The major site of lipid synthesis is in the liver, so lipids have to be transferred to membranes in the same cell or to other tissue.
  
  

Triglycerides

• The process called lipogenesis is used to create fatty acids. A well-known product of lipogenesis is when insulin stimulates the conversion of glucose into fatty acids, which are subsequently transformed into triglycerides and packaged with very low density lipoprotein (VLDL) in the liver in order to store energy within the body.