Amphipathic molecules fatty acids

Amphipathic lipids spontaneously form a variety of structures when added to aqueous solution. All these structures form in ways that minimize contact between the hydrophobic lipid chains and the aqueous milieu. For example, when small amounts of a fatty acid are added to an aqueous solution, a mono-layer is formed at the air-water interface, with the polar head groups in contact with the water surface and the hydrophobic tails in contact with the air (Figure 9.2). Few lipid molecules are found as monomers in solution. 

Further addition of fatty acid eventually results in the formation of micelles. Micelles formed from an amphipathic lipid in water position the hydrophobic tails in the center of the lipid aggregation with the polar head groups facing outward. Amphipathic molecules that form micelles are characterized by a unique critical micelle concentration, or CMC. Below the CMC, individual lipid molecules predominate. Nearly all the lipid added above the CMC, however, spontaneously forms micelles. Micelles are the preferred form of aggregation in water for detergents and soaps. Some typical CMC values are listed in Figure 9.3. 

Phospholipids are the most important of these liposomal constituents. Being the major component of cell membranes, phospholipids are composed of a hydrophobic, fatty acid tail, and a hydrophilic head group. The amphipathic nature of these molecules is the primary force that drives the spontaneous formation of bilayers in aqueous solution and holds the vesicles together. 

Commercially, the most important non-ionic surfactants (APEOs and AEOs) are amphipathic molecules consisting of a hydrophilic (ethylene oxide chains of various length) and a hydrophobic (alkyl phenols, fatty acids, long chain linear alcohols, etc.) part. The polyethoxylated... 

Figure 4-1. Structures of the membrane bilayer and an amphipathic phospholipid. The head group attachment, X, may be H as in phosphatidic acid or one of several substituents linked via phosphoesters in the glycerophospholipids. The nonpolar tail is depicted as composed of saturated fatty acids in this molecule. The overall length of the hydrocarbon chain of the fatty acids may vary from 14 to 20.

Some compounds, like short-chain fatty acids, are amphiphilic or amphipathic, i.e., they have one part that has an affinity for the nonpolar media (the nonpolar hydrocarbon chain) and one part that has an affinity for polar media such as water (the polar group). The energetically most favourable orientation for these molecules is at surfaces or interfaces so that each part of the molecule can reside in an environment for which it has the greatest affinity (see Figure 3.18). These molecules that form... 

Figure 1. A schematic representation of the cross section of the lipid-globular protein mosaic model of membrane structure. The globular proteins (with dark lines denoting the polypeptide chain) are amphipathic molecules with their ionic and highly polar groups exposed at the exterior surfaces of the membranes the degree to which these molecules are embedded in the membrane is under thermodynamic control. The bulk of the phospholipids (with filled circles representing their polar head groups and thin wavy lines their fatty acid chains) is organized as a discontinuous bilayer.

Fatty acids are an essential component of all living cells [28] and are used in several major cellular functions. Their structural roles include use in biological membranes, facilitating the compartmentalization that is a major feature of cellular organisms. Here, hydrophilic alcohols including choline, ethanolamine, and serine are esterified with fatty acids to create amphipathic molecules which are the primary components of biological membranes. Proteins may be acylated with fatty acids as a means of targeting these proteins to membrane-bound loca-... 

The digestion of triacylglycerols in adult nonruminant mammals has been described as initiated in the mouth by hngual lipase released in the sahva at the base of the tongue (52). Up to 6% of the fatty acids are hydrolyzed and initiate emulsion formation in the stomach. The digesta (called chyme at this location) is released from the stomach slowly into the duodenum to ensure complete mixing with the bile salts and emulsification. Lipolysis occurs by association of pancreatic lipase and co-lipase at the surface of the bile salt-stabihzed emulsion. Amphipathic molecules (fatty acids, sn-2 monoacylglycerols, and lysolecithins) are produced and associate with the bile salts to form water-soluble micelles from which absorption occurs. 

The most common phospholipid, phosphatidylcholine, contains two molecules of fatty acid and one molecule of choline phosphate attached to a glycerol backbone (see Figure 1.11), Like all other phospholipids, it is amphipathic. The water-soluble end features a phosphate group, an amino group, and two keto groups. 

FIGURE I.n Phosphatidylcholine an amphipathic molecule. Phosphatidylcholine is a diglyceride. It contairts a glycerol backbone, two molecules of fatty acid, and a molecule of choline phosphate. The choline phosphate group is not linked to the central carbon of glycenoL... 

Some lipophilic nutrients are amphipathic because their molecules contain water-soluble groups at one end, A fatty acid has a long alkane "tail" with a carboxyl group at one end. A bile salt has a large aromatic structure wdth a carboxyl group at one end. 

The Gibbs equation (see equation 19) predicts that a substance that reduces the surface (interfacial tension) will be the adsorbed at the surface (interface). Surface-active substances (especially long-chain fatty acids, detergents, and surfactants) decrease the surface (interfacial) tension. Amphipathic molecules (which contain hydrophilic and hydrophobic groups) become oriented at the interface. At solid-water interfaces, Ae orientation depends on the relative affinities of the adsorbate for water and the solid surface. The hydrophilic groups (sulfate carboxylate, hydroxyl, etc.) may—if the hydrophobic tendency is relatively small—interact coordinatively with the functional groups of the solid surface (Ochs et al., 1994 Ulrich et al., 1988). 

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