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Berg JM, Tymoczko JL, Stryer L. Biochemistry. Fifth edition. Brand-new York: W H Freeman; 2002.
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Lipids different markedly indigenous the other groups of biomolecules considered thus far. By definition, lipids room water-insoluble biomolecules that are very soluble in essential solvents such as chloroform. Lipids have actually a selection of biological roles: they offer as fuel molecules, highly concentrated energy stores, signal molecules, and also components the membranes. The very first three roles of lipids will be discussed in later on chapters. Here, our emphasis is top top lipids together membrane constituents. The three significant kinds of membrane lipids room phospho-lipids, glycolipids, and cholesterol. We begin with lipids discovered in eukaryotes and bacteria. The lipids in archaea room distinct, although castle have plenty of features concerned their membrane-forming role in common with lipids of various other organisms.
12.3.1. Phospholipids room the significant Class that Membrane Lipids
Phospholipids are numerous in all organic membranes. A phospholipid molecule is built from 4 components: fat acids, a communication to which the fat acids space attached, a phosphate, and an alcohol attached come the phosphate (Figure 12.3). The fatty acid components provide a hydrophobic barrier, whereas the remainder that the molecule has actually hydrophilic properties to permit interaction through the environment.
The communication on i m sorry phospholipids are built may be glycerol, a 3- carbon alcohol, or sphingosine, a more complex alcohol. Phospholipids acquired from glycerol are referred to as phosphoglycerides. A phosphoglyceride consists of a glycerol backbone to which 2 fatty acid chains (whose qualities were explained in section 12.2.2) and also a phosphorylated alcohol room attached.
In phosphoglycerides, the hydroxyl teams at C-1 and also C-2 of glycerol space esterified to the carboxyl teams of the 2 fatty acid chains. The C-3 hydroxyl team of the glycerol backbone is esterified come phosphoric acid. As soon as no further additions are made, the resulting link is phosphati-date (diacylglycerol 3-phosphate), the simplest phosphoglyceride. Only tiny amounts that phosphatidate are current in membranes. However, the molecule is a vital intermediate in the biosynthesis of the other phosphoglycerides (Section 26.1). The absolute configuration the the glycerol 3-phosphate moiety that membrane lipids is displayed in figure 12.4.
Structure that Phosphatidate (Diacylglycerol 3-Phosphate). The absolute configuration of the center carbon (C-2) is shown.
The major phosphoglycerides are derived from phosphatidate by the development of an ester bond between the phosphate group of phosphatidate and the hydroxyl team of among several alcohols. The typical alcohol moieties that phosphoglycerides room the amino acid serine, ethanolamine, choline, glycerol, and also the inositol.
The structural formulas of phosphatidyl choline and the other principal phosphoglycerides—namely, phosphatidyl ethanolamine, phosphatidyl serine, phosphatidyl inositol, and also diphosphatidyl glycerol—are given in number 12.5.
Sphingomyelin is a phospholipid uncovered in membranes the is not acquired from glycerol. Instead, the backbone in sphingomyelin is sphingosine, an amino alcohol that consists of a long, unsaturated hydrocarbon chain (Figure 12.6). In sphingomyelin, the amino team of the sphingosine backbone is attached to a fatty mountain by one amide bond. In addition, the major hydroxyl group of sphingosine is esterified come phosphoryl choline.
Structures that Sphingosine and also Sphingomyelin. The sphingosine moiety of sphingomyelin is highlighted in blue.
12.3.2. Archaeal membranes Are constructed from Ether Lipids with Branched Chains
The membrane of archaea different in ingredient from those of eukaryotes or bacteria in three necessary ways. 2 of this differences clearly relate come the hostile living conditions of plenty of archaea (Figure 12.7). First, the nonpolar chains space joined to a glycerol backbone by ether quite than ester linkages. The ether affiliation is much more resistant to hydrolysis. Second, the alkyl chains are branched rather than linear. Castle are built up from repeats the a fully saturated five-carbon fragment. This branched, saturation hydrocarbons are an ext resistant to oxidation. The ability of archaeal lipids to stand up to hydrolysis and also oxidation may aid these organisms to resist the extreme conditions, such as high temperature, low pH, or high salt concentration, under which some of these archaea grow. Finally, the stereochemistry of the central glycerol is inverted contrasted with that displayed in figure 12.4.
An Archaeon and Its Environment. Archaea can grow in habitats together harsh together a volcano vent. Here, the archaea type an orange mat surrounded by yellow sulfurous deposits.
12.3.3. Membrane Lipids Can also Include Carbohydrate Moieties
Glycolipids, together their name implies, room sugar-containing lipids. Choose sphingomyelin, the glycolipids in pet cells are acquired from sphingosine. The amino group of the sphingosine backbone is acylated through a fatty acid, together in sphingomyelin. Glycolipids different from sphingomyelin in the identification of the unit the is attached to the main hydroxyl team of the sphingosine backbone. In glycolipids, one or an ext sugars (rather 보다 phosphoryl choline) are attached to this group. The easiest glycolipid, called a cerebroside, includes a solitary sugar residue, either glucose or galactose.
More facility glycolipids, such together gangliosides, contain a branched chain of as countless as 7 sugar residues. Glycolipids space oriented in a fully asymmetric fashion with the street residues always on the extracellular next of the membrane.
12.3.4. Cholesterol Is a Lipid based upon a Steroid Nucleus
Cholesterol is a lipid through a structure quite various from the of phospholipids. It is a steroid, built from four linked hydrocarbon rings.
A hydrocarbon tail is linked to the steroid in ~ one end, and also a hydroxyl group is attached at the other end. In membranes, the molecule is oriented parallel come the fatty mountain chains of the phospholipids, and the hydroxyl group interacts with the surrounding phospholipid head groups. Cholesterol is missing from prokaryotes but is found to varying levels in basically all pet membranes. The constitutes virtually 25% the the membrane lipids in particular nerve cells however is essentially missing from some intracellular membranes.
12.3.5. A Membrane Lipid Is one Amphipathic Molecule containing a Hydrophilic and also a Hydrophobic Moiety
The arsenal of membrane lipids is extensive, perhaps also bewildering, at an initial sight. However, they possess a an essential common structure theme: membrane lipids are amphipathic molecules (amphiphilic molecules). A membrane lipid consists of both a hydrophilic and also a hydrophobic moiety.
Let united state look in ~ a version of a phosphoglyceride, such together phosphatidyl choline. Its overall shape is around rectangular (Figure 12.8A). The 2 hydrophobic fatty acid chains are roughly parallel to every other, whereas the hydrophilic phosphoryl choline moiety points in the contrary direction. Sphingomyelin has a similar conformation, as does the archaeal lipid depicted. Therefore, the complying with shorthand has been embraced to stand for these membrane lipids: the hydrophilic unit, additionally called the polar head group, is represented by a circle, whereas the hydrocarbon tails are illustrated by directly or wavy lines (Figure 12.8B).
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Representations the Membrane Lipids. (A) Space-filling models the a phosphoglyceride, sphingomyelin, and an archaeal lipid present their shapes and distribution that hydrophilic and also hydrophobic moieties. (B) A shorthand relenten of a membrane lipid.
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