The Effect of Cholesterol on Membrane Fluidity

The cytoplasmic membrane is a two-layer structure composed mainly of phospholipids characterized by its fluidity. Membrane fluidity and its effect on lipid and protein composition and temperature (Purdy et al., 2005) is regulated by its cholesterol concentration (Harby 2001, McLaurin 2002). Cholesterol is a special kind of lipid called steroid formed by polar OH end groups and individual hydrocarbon tails (Wikipedia 2005, Diwan 2005). Like its membrane lipids, cholesterol is aligned in the same direction; its polar head is aligned with the polar head group of the phospholipid molecule (Spurger 2002).

Steroids and their metabolites often act as signaling molecules (most notably steroid hormones) which are components of cell membranes. Steroids such as cholesterol can lower the fluidity of the membrane. Like lipids, steroids are highly concentrated energy reservoirs. However, they are not usually energy sources; in mammals they are usually metabolized and excreted. Steroid biosynthesis is an assimilation pathway that produces steroids from simple precursors. Following the unique biosynthetic pathway in animals (as compared to many other organisms), this pathway is a common target for antibiotics and other antiinfective agents. Human steroid metabolism is also a target of cholesterol lowering drugs such as statins.

Steroids are biologically active organic compounds with four rings arranged in a specific molecular configuration. Steroids have two major biological functions: as an important component of the cell membrane to alter membrane fluidity; and as a signal molecule. Hundreds of steroids have been found in plants, animals and fungi. All steroids are made from cells of sterol oxists (opisthokonts) or cyclic enols (plants). Lanolinol and cycloartenol are derived from the cyclization of triterpene squalene

In animal cells, it has been found that cholesterol disperses throughout the cell membrane to varying degrees within an irregular space between the hydrophobic tails of membrane lipids, where it gives the membrane a hardening and strengthening effect. In addition, the amount of cholesterol in the biofilm varies from organism, cell type and even individual cells. Cholesterol is the main component of animal membranes and regulates the fluidity of the entire membrane. In other words, cholesterol controls the amount of movement of various cell membrane components depending on its concentration. At high temperatures, cholesterol interferes with the movement of phospholipid fatty acid chains, resulting in reduced permeability to small molecules and lower membrane fluidity. The effect of cholesterol at low temperature is reversed. In response to low temperature, cholesterol production and concentration are upregulated (increased)