cholesterol

Examples of cholesterol in the following topics:

• Cholesterol Metabolism

  • However, most ingested cholesterol is esterified; 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.
  • Microscopic appearance of cholesterol crystals in water.

• Regulation of Blood Cholesterol Levels

  • Cholesterol is transported through the blood by lipoproteins which direct cholesterol to where it is needed.
  • The cholesterol within all the various lipoproteins is identical, although some cholesterol is carried as the "free" alcohol and some is carried as fatty acyl esters referred to as cholesterol esters.
  • LDL molecules, therefore, are the major carriers of cholesterol in the blood, and each one contains approximately 1,500 molecules of cholesterol ester.
  • When the cell has abundant cholesterol, LDL receptor synthesis is blocked so new cholesterol in the form of LDL molecules cannot be taken up.
  • Also, HDL particles are thought to transport cholesterol back to the liver for excretion or to other tissues that use cholesterol to synthesize hormones in a process known as reverse cholesterol transport (RCT).

• Steroids

  • Steroids, like cholesterol, play roles in reproduction, absorption, metabolism regulation, and brain activity.
  • All steroids have four linked carbon rings, and many of them, like cholesterol, have a short tail.
  • Cholesterol is the most common steroid and is mainly synthesized in the liver; it is the precursor to vitamin D.
  • Cholesterol is also the precursor to bile salts, which help in the emulsification of fats and their absorption by cells.
  • Steroids, such as cholesterol and cortisol, are composed of four fused hydrocarbon rings.

• Gallstones

  • To be classified as such, they must be 80% cholesterol by weight.
  • They contain less than 20% of cholesterol.
  • Mixed gallstones typically contain 20%–80% cholesterol.
  • Cholesterol gallstones develop when bile contains too much cholesterol and not enough bile salts.
  • Besides a high concentration of cholesterol, two other factors are important in causing gallstones.

• Connecting Lipids to Glucose Metabolism

  • The lipids that are connected to the glucose pathways are cholesterol and triglycerides.
  • Cholesterol contributes to cell membrane flexibility and is a precursor to steroid hormones.
  • The synthesis of cholesterol starts with acetyl groups, which are transferred from acetyl CoA, and proceeds in only one direction; the process cannot be reversed.
  • Thus, synthesis of cholesterol requires an intermediate of glucose metabolism.

• Membrane Fluidity

  • The mosaic nature of the membrane, its phospholipid chemistry, and the presence of cholesterol contribute to membrane fluidity.
  • In animals, the third factor that keeps the membrane fluid is cholesterol.
  • Cholesterol extends in both directions the range of temperature in which the membrane is appropriately fluid and, consequently, functional.
  • Cholesterol also serves other functions, such as organizing clusters of transmembrane proteins into lipid rafts.
  • The plasma membrane is a fluid combination of phospholipids, cholesterol, and proteins.

• Importance of Carbohydrates

  • Fiber also helps to remove excess cholesterol from the body.
  • Fiber binds and attaches to the cholesterol in the small intestine and prevents the cholesterol particles from entering the bloodstream.
  • Then cholesterol exits the body via the feces.

• Fluid Mosaic Model

  • The fluid mosaic model describes the plasma membrane structure as a mosaic of phospholipids, cholesterol, proteins, and carbohydrates.
  • The fluid mosaic model describes the structure of the plasma membrane as a mosaic of components —including phospholipids, cholesterol, proteins, and carbohydrates—that gives the membrane a fluid character .
  • The fluid mosaic model of the plasma membrane describes the plasma membrane as a fluid combination of phospholipids, cholesterol, and proteins.
  • The principal components of a plasma membrane are lipids (phospholipids and cholesterol), proteins, and carbohydrates attached to some of the lipids and some of the proteins.

• Lipid-Derived, Amino Acid-Derived, and Peptide Hormones

  • Most lipid hormones are derived from cholesterol, so they are structurally similar to it .
  • These peptide hormones are much larger than those derived from cholesterol or amino acids.
  • The structures shown here represent (a) cholesterol, plus the steroid hormones (b) testosterone and (c) estradiol.

• Diagnostic Blood Tests

  • Blood tests focusing on cholesterol levels can determine LDL and HDL cholesterol levels, as well as triglyceride levels.
  • Some blood tests, such as those that measure glucose, cholesterol, or for determining the existence or lack of STD, require fasting eight to twelve hours prior to the drawing of the blood sample.