Third major class of lipids. It is a compound that contains three cyclohexane rings A. Cholesterol The most abundant steroid in the human body. The most important steroid in the human body. Serves as a plasma membrane component in all animal cells. Red blood cells.

 Second most important function is to serve as a raw material for the synthesis of other steroids.  Sex and adrenocorticoid hormones and bile salt  It exists in both free form and esterfied with fatty acids.  Gallstones  Cholesterol is necessary for human life.  It circulates constantly in the blood.

 Since it is hydrophobic it needs a water soluble carrier to circulate in the aqueous medium of blood.

 Cholesterol and fat are transported by lipoproteins.  Most have a quart of hydrophobic lipid molecules surrounded by a shell of hydrophilic molecules.  Proteins and phospholipids

 Four kinds  High-density lipoprotein (HDL) “Good Cholesterol”- 33% protein and 30% cholesterol  Low-density lipoprotein (LDL) “Bad Cholesterol”- 25% protein and 50% cholesterol  Very-low-density lipoprotein (VLDL)-Carries triglycerides synthesized by the liver.  Chyomicrons- Carry dietary lipids synthesized in the intestines.

 Transport of cholesterol from the liver starts out as large as VLDL particle  Core of VLDL contains triglycerides and cholesteryl esters  Mostly cholesteryl linoleate  Surrounded by a polar coat of phospholipids and proteins  VLDL is carried into the serum  Capillaries reach muscle or fat tissues triglycerides and all proteins are removed  Diameter shrinks and its core only contains cholestery esters  The removal of fat increases its density and it becomes LDL  Stays in plasma for 2.5 days

 LDL carries cholesterol to the cells  LDL- receptor molecules line the cell surface in coated pits  ApoB-100 protein binds to the LDL-receptor molecules in the coated pits  LDL- is taken inside the cell and enzymes breakdown the lipoprotein  Micheal Brown and Joseph Goldstein discovered if the LDL- receptors are not sufficient in number cholesterol accumulates in the blood

 High-density lipoprotein transport cholesterol from peripheral tissues to the liver and transfers cholesterol to LDL  In the serum free cholesterols in HDL are converted to cholesteryl esters  HDL binds to the liver cell surface and transfers its cholersteryl ester to the cell  Depleted HDL reenters the circulation

 Cholesterol is insoluble in water  If its level is elevated in the blood serum plaque-like deposits may form on the inner surfaces of the arteries  Diameter of blood vessels decrease and blood flow decreases  Atherosclerosis  May lead to heart attack, stroke, kidney dysfunction

 Most cholesterol is transported by low- density lipoproteins  When concentration of cholesterol molecules is high,less LDL is taken into the cells from the plasma  LDL concentration in the plasma rises  When cholesterol level inside the cell is low synthesis of the LDL receptor increases

 When the cholesterol level inside the cells is low, the synthesis of the LDL receptor increases.  Familial Hypercholesterolemia  The cholesterol in the plasma may be as high as 680 mg/100mL, compared to 175mg/100mL  In general high LDL content means high cholesterol content in the plasma.  The serum cholesterol level controls the amount of cholesterol synthesized in the liver.

 Diets in low cholesterol and saturated fatty acids usually reduce the serum cholesterol level.  Drugs can block the synthesis of cholesterol inside the cells and stimulate the synthesis of LDL- receptor proteins.  It is generally considered desirable to have high levels of HDL and low levels of LDL in the blood stream.