Sterol function plant cell membranes

There is ample evidence that the sterols of plant cells are localized in the membranes of the intracellular organelles. Evidence for their function in the membranes is still only indirect. The polyene antibiotic filipin increases the cellular... 

Some T. have major physiological significance. Thus, lanosterol is converted biosynthetically to cholesterol, the precursor of all steroid hormones, bile acids, and vitamin D3. In fungi, lanosterol is converted to er-gosterol (see sterols), an essential component of the fungal cell membrane. Plant cell membranes also incorporate steroids (phytosterols). In prokaryotes, the hopanoids take over the functions of steroids in the cell membranes. As a component of animal and plant waxes T. strengthen the structures. They protect the plant surface from desiccation and attack by microorganisms (e.g., betulin, lupeol, oleanolic acid, and ursolic acid). 

Unesterified sterols modulate the function of eukaryotic membranes. In human cells, sterol is esterified to a storage form by acyl-co-enz)me A (CoA) cholesterol acyltransferase (SGTase). In plants, free sterols are associated mainly with microsomal membranes, whereas the steryl esters are stored in lipid granules. The esterification process may, thus, allow regulation of the... 

Even though cholesterol (substituted by other sterols in plants) is chemically classified as a lipid, the molecule is better grouped it into a special category, which is distinct from membrane proteins and (phospho- and sphingo-) lipids, when discussing its role in biomembrane stmcture and function. Cholesterol constitutes between 25% and 40% of the total lipid plus cholesterol fraction of most typical cell membranes. These numbers translate into a 33-66% fraction of the noncholesterol lipids, or considering its smaller size, about a 10-20%... 

Lipids have a variety of functions in living organisms. Fats and oils are a convenient and concentrated means of storing food energy in plants and animals. Phospholipids and sterols, such as cholesterol, are major components of cell membranes see upid bilayer). Waxes provide vitai waterproofing for body surfaces. Terpenes include tamins A, E, and K, and phytol (a component of chlorophyll) and occur in essential oils, such as menthol and camphor. Steroids include the adrenal hormones, sex hormones, and bile acids. 

In most higher plants, sterols are present as a mixture of A -sterols. Sitosterol, stigmasterol and 24-methylcholesterol are usually cited as the typical plant sterols (Fig.l). These sterols are concentrated mainly in the plasma membrane (PM) (1). Using sterol biosynthesis inhibitors, we have shown that it is possible to obtain plant cell suspensions or whole plants with a completely modified sterol profile (2,3). Such plants constitute most suitable material for studying structural and functional roles of sterols, which are still largely unknown. 

---