Cholesterol, from the Greek chole- (bile) and stereos (solid) followed by the chemical suffix -ol for an alcohol, is an organic chemical substance classified as a waxy steroid of fat. It is an essential structural component of mammalian cell membranes and is required to establish proper membrane permeability and fluidity.
In addition to its importance within cells, cholesterol is an important component in the hormonal systems of the body for the manufacture of bile acids, steroid hormones, and vitamin D. Cholesterol is the principal sterol synthesized by animals; in vertebrates it is formed predominantly in the liver. Small quantities are synthesized in other cellular organisms (eukaryotes) such as plants and fungi. It is almost completely absent among prokaryotes, i.e. bacteria.
Although cholesterol is important and necessary for human health, high levels of cholesterol in the blood have been linked to damage to arteries and cardiovascular diseaseFrançois Poulletier de la Salle first identified cholesterol in solid form in gallstones, in 1769. However, it was only in 1815 that chemist Eugène Chevreul named the compound "cholesterine".
Physiology
Since cholesterol is essential for all animal life, it is primarily synthesized from simpler molecules within each cell, a complex 37 step process which starts with the intracellular protein enzyme HMG-CoA reductase. However, normal and especially high levels of fats (including cholesterol) within the blood circulation, depending on how it is transported within lipoproteins, are strongly associated with progression of atherosclerosis.For a person of about 68 kg (150 pounds), typical total body cholesterol synthesis is about 1 g (1,000 mg) per day, and total body content is about 35 g, primarily located within all the membranes of all the cells of the body. Typical daily dietary intake of additional cholesterol, in the United States, is 200–300 mg.However, most ingested cholesterol is esterified and esterified cholesterol is poorly absorbed. The body also compensates for any absorption of additional cholesterol by reducing cholesterol synthesis. For these reasons, cholesterol intake in food has little, if any, effect on total body cholesterol content or concentrations of cholesterol in the blood.
Cholesterol is recycled. It is excreted by the liver via the bile into the digestive tract, in a non-esterified form. Typically about 50% of the excreted cholesterol is reabsorbed by the small bowel back into the bloodstream.
If a person eats only plant based food, typical daily intake of cholesterol is zero. Plants do not make cholesterol, thus contain no cholesterol. However, with no intake, internal synthesis of cholesterol increases. Thus, again, quantity of cholesterol intake typically has little to no effect on body cholesterol content or blood cholesterol levels.
Plants manufacture phytosterols (substances chemically similar to cholesterol produced within plants), which can compete with cholesterol for reabsorption in the intestinal tract, thus potentially reducing cholesterol reabsorption.However, phytosterols are foreign to animal cells and, if absorbed, accelerate the progression of atherosclerosis. When intestinal lining cells absorb phytosterols, in place of cholesterol, they usually excrete the phytosterol molecules back into the GI tract, an important protective mechanism.
Function
Cholesterol is required to build and maintain membranes; it modulates membrane fluidity over the range of physiological temperatures. The hydroxyl group on cholesterol interacts with the polar head groups of the membrane phospholipids and sphingolipids, while the bulky steroid and the hydrocarbon chain are embedded in the membrane, alongside the nonpolar fatty acid chain of the other lipids. Through the interaction with the phospholipid fatty acid chains, cholesterol increases membrane packing, which reduces membrane fluidity In this structural role, cholesterol reduces the permeability of the plasma membrane to neutral solutes, protons, (positive hydrogen ions) and sodium ions .Within the cell membrane, cholesterol also functions in intracellular transport, cell signaling and nerve conduction. Cholesterol is essential for the structure and function of invaginated caveolae and clathrin-coated pits, including caveola-dependent and clathrin-dependent endocytosis. The role of cholesterol in such endocytosis can be investigated by using methyl beta cyclodextrin (MβCD) to remove cholesterol from the plasma membrane. Recently, cholesterol has also been implicated in cell signaling processes, assisting in the formation of lipid rafts in the plasma membrane. Lipid raft formation brings receptor proteins in close proximity with high concentrations of second messenger molecules.In many neurons, a myelin sheath, rich in cholesterol, since it is derived from compacted layers of Schwann cell membrane, provides insulation for more efficient conduction of impulses.Within cells, cholesterol is the precursor molecule in several biochemical pathways. In the liver, cholesterol is converted to bile, which is then stored in the gallbladder. Bile contains bile salts, which solubilize fats in the digestive tract and aid in the intestinal absorption of fat molecules as well as the fat-soluble vitamins, A, D, E, and K. Cholesterol is an important precursor molecule for the synthesis of vitamin D and the steroid hormones, including the adrenal gland hormones cortisol and aldosterone, as well as the sex hormones progesterone, estrogens, and testosterone, and their derivatives.Some research indicates cholesterol may act as an antioxidant.
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