Sunday, June 15, 2014

Lipids

By Anonymous 12:51 AM
Lipids

Ingested lipids consist of a heterogeneous mixture of hydrophobic particles that includes the following: 
  • Fatty acids: These monocarboxylic acids, which contain hydrocarbon chains, are either saturated (zero double bonds), monounsaturated (one double bond), or polyunsaturated (two or more double bonds).
  • Triacylglycerols: These lipids are composed of three fatty acids attached to a glycerol backbone. They are stored in adipose tissue as fat.
  • Cholesterol and cholesterol esters: These hydrophobic molecules respectively consist of free cholesterol and a cholesterol molecule esterified to a fatty acid.
  • Membrane lipids: Phospholipids are found in cellular membranes and are composed of fatty acids attached to a glycerol backbone (glycerophospholipids) or a sphingosine backbone (sphingophospholipids). A phosphate group is esterified to the glycerol and sphingosine backbones. Other membrane lipids, called glycolipids, include those that contain a fatty acid chain attached to a sphingosine backbone, which is in turn attached to carbohydrate residues.
  • Lipid-soluble vitamins: These include vitamins A, D, E, and K.

Lipids Structure
Digestion and Absorption
 
The digestion of lipids is catalyzed by lipases (enzymes that hydrolyze ester bonds) and is aided by the lipid-emulsifying molecules in bile. 
  • Salivary (lingual) lipase and gastric lipases break down only short-chain and medium chain fatty acids. Short-chain fatty acids then diffuse through the stomach to enter the circulatory system.
  • Bile acids/salts are amphipathic molecules that emulsify dietary lipids in the duodenum, making it easier for digestive enzymes (released by the pancreas and cells lining the intestines) to hydrolyze them. Hydrolysis of ester bonds in triacylglycerols, cholesterol esters, and phospholipids are catalyzed by lipases, cholesterol esterases, and phospholipases, respectively.
The lipid-soluble vitamins, as well as the products of lipid digestion—free fatty acids, 2-monoacylglycerols, and cholesterol— are assembled into spherical particles called micelles. Monomeric lipids are absorbed from the micelles by the villi of the intestinal mucosa. In these cells, the fatty acids are reassembled into triacylglycerols phospholipids, and cholesterol esters. The reassembled lipids are then incorporated into large spherical lipoproteins called chylomicrons for delivery to the liver.   
Lipid Digestion
Function
 
Lipids are important long-term energy providers, yielding 9 kcal/g. They are also the major structural components of cell membranes (e.g., phospholipids, glycolipids, and cholesterol). Lipids are used to generate lipid-soluble molecules, such as steroid hormones, bile acids, and eicosanoids, and the lipid-soluble vitamins A, D, E, and K. Several enzymes require lipids for their activity. 
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