A typical cell contains many different types of membranes, each with unique properties derived from its particular mix of lipids and proteins. The data in Table 1 illustrate the variation in lipid composition in different biomembranes. Several phenomena contribute to these differences. For in stance, the relative abundances of phosphoglycerides and sphingolipids differ between membranes in the endoplasmic reticulum (ER), where phospholipids are synthesized, and the Golgi complex, where sphingolipids are synthesized. The proportion of sphingomyelin as a percentage of total mem brane lipid phosphorus is about six times as high in Golgi membranes as it is in ER membranes. In other cases, the movement of membranes from one cellular compartment to another can selectively enrich certain membranes in lipids such as cholesterol. In responding to differing environments throughout an organism, different types of cells generate membranes with differing lipid compositions. In the cells that line the intestinal tract, for example, the membranes that face the harsh environment in which dietary nutrients are digested have a sphingolipid-to-phosphoglyceride-to cholesterol ratio of 1:1:1, rather than the 0.5:1.5:1 ratio found in cells subject to less stress. The relatively high con centration of sphingolipids in these intestinal membranes may increase their stability because of extensive hydrogen bonding by the free –OH group in the sphingosine moiety (see Figure 1).

Table1. Major Lipid Components of Selected Biomembranes

Fig1. Three classes of membrane lipids. (a) Most phosphoglycerides are derivatives of glycerol 3-phosphate (red), which contains two esterified fatty acyl chains that constitute the hydrophobic “tail” and a polar “head group” esterified to the phosphate. The fatty acids can vary in length and be saturated (no double bonds) or unsaturated (one, two, or three double bonds). In phosphatidylcholine (PC), the head group is choline. Also shown are the molecules attached to the phosphate group in three other common phosphoglycerides: phosphatidyl ethanolamine (PE), phosphatidylserine (PS), and phosphatidylinositol (PI). Plasmalogens contain one fatty acyl chain attached to glycerol by an ester linkage and one attached by an ether linkage; they contain the same head groups as other phosphoglycerides. (b) Sphingolipids are derivatives of sphingosine (red), an amino alcohol with a long hydrocarbon chain. Various fatty acyl chains are connected to sphingosine by an amide bond. The sphingomyelins (SM), which contain a phosphocholine head group, are phospholipids. Other sphingolipids are glycolipids in which a single sugar residue or branched oligosaccharide is attached to the sphingosine backbone. For instance, the simple glycolipid glucosylcerebroside (GlcCer) has a glucose head group. (c) The major sterols in animals (cholesterol), fungi (ergosterol), and plants (stigmasterol) differ slightly in structure, but all serve as key components of cellular membranes. The basic structure of sterols is a four-ring hydrocarbon (yellow). Like other membrane lipids, sterols are amphipathic. The single hydroxyl group is equivalent to the polar head group in other lipids; the conjugated ring and short hydrocarbon chain form the hydrophobic tail. See H. Sprong et al., 2001, Nature Rev. Mol. Cell Biol. 2:504.

The degree of bilayer fluidity depends on lipid composition, the structure of the phospholipid hydrophobic tails, and temperature. As already noted, van der Waals interactions and the hydrophobic effect cause the nonpolar tails of phospholipids to aggregate. Long, saturated fatty acyl chains have the greatest tendency to aggregate, packing tightly together into a gel-like state. Phospholipids with short fatty acyl chains, which have less surface area and therefore fewer van der Waals interactions, form more fluid bilayers. Likewise, the kinks in cis-unsaturated fatty acyl chains result in their forming less stable van der Waals interactions with other lipids, and hence more fluid bilayers, than do straight saturated chains, which can pack more tightly together.

Cholesterol is important in maintaining the appropriate fluidity of natural membranes, a property that appears to be essential for normal cell growth and reproduction. Cholesterol restricts the random movement of phospholipid head groups at the outer surfaces of the leaflets, but its effect on the movement of long phospholipid tails depends on its con centration. At the cholesterol concentrations normally present in the plasma membrane, the interaction of the steroid ring with the long hydrophobic tails of phospholipids tends to immobilize those lipids and thus decreases biomembrane fluidity. It is this property that can help organize the plasma membrane into discrete subdomains of unique lipid and protein composition. At lower cholesterol concentrations, however, the steroid ring separates and disperses phospholipid tails, causing the inner regions of the membrane to become slightly more fluid.

The lipid composition of a bilayer also influences its thickness, which in turn may influence the distribution of other membrane components, such as proteins, in a particular membrane. It has been argued that relatively short transmembrane segments of certain Golgi-resident enzymes (glycosyltransferases) are an adaptation to the lipid composition of the Golgi membrane and contribute to the retention of these enzymes in the Golgi apparatus. The results of biophysical studies on artificial membranes demonstrate that sphingomyelin associates into a more gel-like and thicker bi layer than phosphoglycerides do (Figure 2a). Cholesterol and other molecules that decrease membrane fluidity also increase membrane bilayer thickness. Because sphingomyelin tails are already optimally stabilized, the addition of cholesterol has no effect on the thickness of a sphingomyelin bilayer.

Fig2. Effect of lipid composition on bilayer thickness and curvature. (a) A pure sphingomyelin (SM) bilayer is thicker than one formed from a phosphoglyceride such as phosphatidylcholine (PC). Cholesterol has a lipid-ordering effect on phosphoglyceride bilayers that increases their thickness, but it does not affect the thickness of the more ordered SM bilayer. (b) Phospholipids such as PC have a cylindrical shape and form essentially flat monolayers, whereas those with smaller head groups, such as phosphatidylethanolamine (PE), have a conical shape. (c) A bilayer enriched with PC in the exoplasmic leaflet and with PE in the cytosolic face, as in many plasma membranes, would have a natural curvature. See H. Sprong et al., 2001, Nature Rev. Mol. Cell Biol. 2:504.

Another property dependent on the lipid composition of a bilayer is its curvature, which depends on the relative sizes of the polar head groups and nonpolar tails of its constituent phospholipids. Lipids with long tails and large head groups are cylindrical in shape; those with small head groups are cone-shaped (Figure 2b). As a result, bilayers composed of cylindrical lipids are relatively flat, whereas those containing large numbers of cone-shaped lipids form curved bilayers (Figure 2c). This effect of lipid composition on bilayer curvature may play a role in the formation of highly curved membranes, such as sites of viral budding and of formation of internal vesicles from the plasma mem brane, and in specialized stable membrane structures such as microvilli. Several proteins bind to the sur face of a phospholipid bilayer and cause the membrane to curve; such proteins are important in formation of transport vesicles that bud from a donor membrane.

اخر الاخبار

اخبار العتبة العباسية المقدسة

قسم الشؤون الفكرية ينظّم برنامجًا تدريبيًّا لعدد من ملاكات المؤسسات الثقافية في ذي قار

إقامة المحاضرة الرابعة من محفل نهج البلاغة في مقام الإمام المهدي (عجل الله فرجه)

للعام التاسع.. المجمع العلمي يطلق 29 ختمة قرآنية رمضانية في النجف الأشرف

قسم الصيانة يجري أعمال الإدامة الدورية لجدران حرم مرقد أبي الفضل العباس (عليه السلام)

المزيد

الآخبار الصحية

مفاجأة.. تحسين القدرة على التحمل لا يعتمد العضلات فقط

المرتفعات والأكسجين.. دراسة "مبشرة" لمرضى السكري

علماء يبتكرون "آلية" تتوقع موعد ظهور أعراض الزهايمر

ماذا يحدث لجسمك عند التوقف عن تناول الخضراوات والفواكه؟

المزيد

الآخبار التكنلوجية

رصد "القرش النائم" في أحد أقسى الأماكن على وجه الأرض

أسماك أعماق البحر تكشف نظاما بصريا يتجاوز "المألوف"

زوكربيرغ أمام القضاء في قضية إدمان المراهقين

"علي بابا" تكشف عن نموذج جديد للذكاء الاصطناعي

المزيد

مواضيع ذات صلة

The Activity of Antithrombin, an Inhibitor of Thrombin, Is Increased by Heparin

Conversion of Fibrinogen to Fibrin Is Catalyzed by Thrombin

Factor Xa Leads to Activation of Prothrombin to Thrombin

Most Transmembrane Proteins Have Membrane Spanning α Helices

Lipid Composition Is Different in the Exoplasmic and Cytosolic Leaflets

The Intrinsic Pathway Also Leads to Activation of Factor X

The Extrinsic Pathway Leads to Activation of Factor X

Both Extrinsic & Intrinsic Coagulation Pathways Result in the Formation of Fibrin

Most Lipids and Many Proteins Are Laterally Mobile in Biomembranes

Biomembranes Contain Three Principal Classes of Lipids

Platelet Aggregation Requires Outside-In and Inside-Out Transmembrane Signaling

Phagocyte-Derived Proteases can Damage Healthy Cells