Hydrophobic cuticle

If hair is further damaged to the point that the cuticle has split (Figure 10.3), the exposed hydrophilic cortex could strongly adsorb hydrophilic soils that would not bind as strongly to the intact hydrophobic cuticle layers. 

Foliar Uptake Adsorption and Absorption The leaf surface provides an extensive surface area that can interact with chemicals either in the vapor phase or through wet or dry deposition. The cuticle limits water loss and protects against infection by plant pathogens. Detailed descriptions of the plant cuticle are available " and a brief synopsis has been provided in the discussion of the sorption of chemicals by foliage (see Sorption, Chapter 3). While the hydrophobic cuticle... 

In fine wool such as that obtained from merino sheep, the cuticle is normally one cell thick (20 x 30 x 0.5 mm, approximate dimensions) and usually constitutes about 10% by weight of the total fiber. Sections of cuticle cells show an internal series of laminations (Figs. 1 and 2) comprising outer sulfur-rich bands known as the exocuticle and inner regions of lower sulfur content called the endocuticle (13). On the exposed surface of cuticle cells, a membrane-like proteinaceous band (epicuticle) and a unique hpid component form a hydrophobic resistant barrier (14). These hpid and protein components are the functional moieties of the fiber surface and are important in fiber protection and textile processing (15). 

The conquest of the land by plants necessitated the development of a coating, the cuticle, that would reduce water loss. Suberin and cutin vary in their proportion of fatty acids, fatty alcohols, hydroxyfatty acids, and dicarboxylic acids. The cuticle is synthesized and excreted by the epidermis of aerial portions of the plant, such as the primary stems, leaves, flower organs, and fruits. The two major hydrophobic layers that contribute to the cuticle are composed of phenolic molecules combined with lipid polymers. Cutin is a polymer found in the outer cell wall of the epidermis, which is... 

The hydrophobic waxy cuticle of plants can inhibit the movement and accessibility of nutrients to bacterial cells. However, biosurfactants produced by the majority of epiphytic Pseudomonas spp. decreases the water tension, enabling relatively free movement across the leaf surface to nutrient sources and natural openings such as stomata. Pseudomonas are also known to release a toxin called syringomycin that can produce holes in the plant cell membrane allowing access to intracellular nutrients without necessarily resulting in disease symptoms (Cao et al.r 2005). 

Airborne pheromone and other odors are hydrophobic and tend to adsorb onto the waxy surface of the insect cuticle. Body surfaces thus can collect odors and become sources of background noise if these odors are later released. Degradation of these surface-bound odor molecules might significantly reduce such signal noise. 

It occurred to have a broad primary specificity toward amino acids with hydrophobic side groups in synthetic ester and amide substrates. Pr2 exhibited rapid hydrolytic activity to casein and substrates containing Arg or Lys, while little or no activity was shown to cuticle, elastin, or substrates for chymotrypsin and elastase. The relation between Pr2 and trypsin was confirmed by the use of specific inhibitors. 

We know the most about cuticular hydrocarbons, because they are abundant and because it is relatively easy to isolate and identify them. They are also the most hydrophobic lipid components, and so should provide the best barrier to water-loss. -Alkanes isolated from insect cuticles typically have chain lengths of 20-40 carbons. These can be modified by insertion of cis double bonds, or addition of one or more methyl groups. Relatively polar surface lipids include alcohols, aldehydes, ketones and wax esters (see Chapter 9). Given this diversity, is it possible to predict lipid phase behavior (and, by extension, waterproofing characteristics) from composition alone If so, a large body of literature would become instantly interpretable in the context of water balance. Unfortunately, this is not the case. 

It is clear from the preceding discussion that insect cuticle can be considered a two-phase, lipophilic-hydrophilic system. The outermost phase is waxy and hence hydrophobic (i.e., lipophilic). Because most insecticides are nonpolar, this first barrier is advantageous to their contact action. Therefore, in insects, the contact toxicity of an insecticide is similar to the oral toxicity. In contrast, the acute oral toxicity is much higher in mammals than the contact toxicity because mammalian skin is relatively resistant to the entry of insecticides. 

The polarity of insecticides has been regarded as an important factor for cuticular penetration. As mentioned earlier, the typical insect cuticle should be considered a two-phase system, the outer layer (epicuticle) having hydrophobic properties and the inner layers (procuticle) having hydrophilic properties. Thus, whether the insecticide is lipid soluble or water soluble, its tendency to move through the cuticle as a whole depends on whether it can pass through the hydrophobic or hydrophilic barrier, whichever the case may be. The efficiency of such movement will probably depend on the oil-water partition coefficient of the insecticide, the nature of the surfactant or solvent—which may be a part of the insecticide formulation—and the nature of the cuticle itself (Terriere, 1982). 

A totally different rationale has to be applied to the transport of hydrocarbons. These extremely hydrophobic compounds seem to reside in the interior of the lipophorin particles. A mechanism involving uptake and degradation of the lipoprotein might be possible for the transport of hydrocarbons to the epidermal cells. This type of mechanism might be important in certain stages of insect development, when the lipoprotein could deliver amino acids and other lipid components necessary for the construction of the cuticle. A similar process may also exist for the delivery of carotenoids and sterols. 

The two major morphological parts in the structure of wool are cuticle and cortex. The epi-cuticle of wool fibres surrounds each cuticle, it consists of approximately one-quarter fatty acid and three-quarters protein by mass. The hydrophobic epiCLiticle acts as a barrier to dyes which enter the wool fibre between cuticle cells through the highly cross-linked cell membrane complex (CMC). Enzyme from the liquor can diffuse into the interior of the fibre and hydrolyse parts of the endocuticle and proteins in the cell membrane complex, completely damaging the fibre if not controlled. In contrast, the catalytic action of enzyme on cotton is confined to the surface and the amorphous region only. 

The dynamics of intact lime cuticle and its two major component polyesters, cutin and wax, have been studied by the MAS NMR experiment [134]. By the measurements of spin-lattice relaxation times and spin-lattice relaxation times in the rotating frame which characterize respectively the megahertz- and kilohertz-regime motions, it is indicated that motional restrictions are present at the crosslinks of the cutin polymer and along the alkyl chains of the wax. The values of relaxation times, which differ for analogous carbon sites of cutin and wax individually, approach common values for the two materials in the intact lime cuticle. These results are considered to provide evidence for hydrophobic association within the plant cuticle of the long aliphatic chains of cutin and wax. 

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