Properties Of Cell Wall Materials Research Articles

Atomic Force Microscopy in the Characterization of the Structure of Cell Wall Components.

Atomic force microscopy (AFM) has broken boundaries in the characterization of the supramolecular architecture of cell wall assemblies and single cell wall polysaccharides at the nanoscale level. Moreover, AFM provides an opportunity to evaluate the mechanical properties of cell wall material which is not possible with any other method. However, in the case of plant tissue, the critical step is a smart sample preparation that should not affect the polysaccharide structure or assembly and on the other hand should consider device limitations, especially scanner ranges. In this chapter, the protocols from the sample preparation, including isolation of cell wall material and extraction of cell wall polysaccharide fractions, through AFM imaging of polysaccharide assemblies and single molecules until an image analysis to obtain quantitative data characterizing the biopolymers are presented.

The influence of microstructural characteristics and cell wall material properties on the mechanical behaviors of different tissues of sorghum stems

Sorghum stems comprise different tissue components, i.e., rind, pith, and vascular bundles in the rind and pith regions, of different cell morphologies and cell wall characteristics. The overall responses of stems to mechanical loadings depend on the responses of these tissues themselves. Investigating how each tissue deforms to various loading conditions will inform us of the failure mechanisms in sorghum stems when exposed to wind loadings, which can guide the development of lodging-resistant variants. To this end, numerical analyses were implemented to investigate the effects of cell morphologies and cell wall properties on the overall mechanical responses of the above four tissues under tension and compression. Microstructures of different tissues were constructed from microscopic images of the tissues using computer-aided design (CAD), which were then used for finite element (FE) analyses. Shell finite elements were used to model the cell walls, and the classical lamination model was used to determine the overall mechanical responses of cell walls having different fiber composite arrangements. The results from the numerical analyses helped explain how the loading (boundary) conditions, the cell microstructures, the mechanical properties of cell walls of different tissues, the cell wall thickness, the microfibril angle (MFA) of fiber composites of the cell walls, and the turgor pressure affected the overall mechanical responses of the tissues. Tissue stiffening or softening behaviors were attributed to different microstructural deformations, i.e., local or global buckling of cell walls, cell collapse, densifications of cells, or reorientation and rearrangement of cells. The mechanical properties and thickness of cell walls only affected the stiffness and load-bearing ability of the tissues. The turgor pressure affected the compressive responses but its effect on tensile responses was negligible. The MFA had a significant influence on the stiffness and load-bearing ability when the tissues were loaded along their longitudinal axis, but it had an insignificant effect on loading in the transverse direction. Tissues with smaller cell sizes and denser cells were stronger and stiffer than those with larger cell sizes. The numerical simulations also revealed that rind and rind vascular bundles were stiffer and had higher load-bearing ability than pith and pith vascular bundles.

Bacterial cell wall material properties determine E. coli resistance to sonolysis

The applications of bacterial sonolysis in industrial settings are plagued by the lack of the knowledge of the exact mechanism of action of sonication on bacterial cells, variable effectiveness of cavitation on bacteria, and inconsistent data of its efficiency. In this study we have systematically changed material properties of E. coli cells to probe the effect of different cell wall layers on bacterial resistance to ultrasonic irradiation (20 kHz, output power 6,73 W, horn type, 3 mm probe tip diameter, 1 ml sample volume). We have determined the rates of sonolysis decay for bacteria with compromised major capsular polymers, disrupted outer membrane, compromised peptidoglycan layer, spheroplasts, giant spheroplasts, and in bacteria with different cell physiology. The non-growing bacteria were 5-fold more resistant to sonolysis than growing bacteria. The most important bacterial cell wall structure that determined the outcome during sonication was peptidoglycan. If peptidoglycan was remodelled, weakened, or absent the cavitation was very efficient. Cells with removed peptidoglycan had sonolysis resistance equal to lipid vesicles and were extremely sensitive to sonolysis. The results suggest that bacterial physiological state as well as cell wall architecture are major determinants that influence the outcome of bacterial sonolysis.

Inverse identification of cell-wall material properties of closed-cell aluminum foams based upon Vickers nano-indentation tests

This study aims to develop an inverse approach to identifying the cell-wall material properties of closed-cell aluminum (Al) foams, which is based upon indentation tests, numerical simulation and optimization technique. Vickers nano-indentation tests are conducted on the cell-wall of foams, and the residual surface deformation is measured by a high definition confocal microscopy. To enhance identification efficiency, a combined 2D with 3D micromechanics modeling strategy is proposed here, in which the load-depth curve and residual imprint (pile-up) of the indentation tests are considered. In this study, the cell-wall material properties such as the yield strength and strain-hardening exponent are identified using this proposed inverse approach, while the Young's modulus is obtained directly for the unloading part of the indentation load-depth curves. The identification results indicate that the proposed inverse procedure can provide a well-posed solution to the cell-wall material properties by introducing the pile-up information. Microscopic computed tomography (μCT) technique is also utilized to reconstruct accurate 3D image-based representative volume element (RVE) model of foam structure, through which the numerical simulation is conducted for uniaxial compression. The identified cell-wall material parameters are validated by comparing the numerical simulation with the experimental data in terms of the deformation/failure modes and quantitative results. It is noted that the cell-wall of aluminum foams that are commonly made of melt foaming processes exhibits a quite different stress-strain relation when compared with the raw material of cell-wall. This study is expected to provide an effective approach for identifying the material properties that may be not easily determined through conventional testing methods.

Analysis of Changes in Plant Cell Wall Composition and Structure During Cold Acclimation.

The cell wall has a crucial influence on the mechanical properties of plant cells. It therefore has a strong impact on the freezing behavior and very likely also the freezing tolerance of plants. However, not many studies have addressed the question how cell wall composition and structure impact plant freezing tolerance and cold acclimation. In this chapter, we describe a comprehensive workflow to extract total cell wall material from leaves of Arabidopsis thaliana and to separate this material into fractions enriched in crystalline cellulose, pectins, and hemicelluloses by sequential fractionation. We further describe methods for the analysis of chemical structure, monosaccharide composition, and cellulose and uronic acid contents in the total cell wall material and the fractions in response to cold acclimation. Structural properties of cell wall material are analyzed by attenuated total reflectance-Fourier-transform infrared spectrometry (ATR-FTIR) and monosaccharide composition by gas chromatography-mass spectrometry (GC-MS) after isolation of alditol acetate derivatives of the sugars.

Nanostructure and poroviscoelasticity in cell wall materials from onion, carrot and apple: Roles of pectin

The hierarchical organisation of polysaccharides in primary plant cell walls is responsible for their unique mechanical properties, and in turn for the textural and rheological properties of plant-based foods and ingredients. It is expected that at the nano scale, the mechanical properties of cell wall materials arise from a combination of structural deformation of the polysaccharide networks and hydraulic properties of the continuous water phase, as has been shown for other cellulose-based composites. Pectin plays a key role in the load bearing properties of (bacterial) cellulose-pectin composites due to its contribution to both hydration structure and the dynamics of water movement. To investigate whether these features are also important in plant cell wall materials we have used a set of advanced characterisation techniques to elucidate cell wall structural features at different length scales (X-ray diffraction and small angle X-ray and neutron scattering) in cell walls from two dicotyledons (apple and carrot) and a non-commelinid monocotyledon (onion). The strength of isolated cell walls was measured under compression and fitted to a poroviscoelastic mechanical model, demonstrating that the mechanical properties of the isolated cell wall materials are directly linked to both polysaccharide networks and fluid flow through the networks. Our results show how pectin polysaccharides influence the viscoelastic behaviour of these materials and contribute to the texture of plant-derived food systems.

Effect of Properties and Turgor Pressure on the Indentation Response of Plant Cells

The indentation of plant cells by a conical indenter is modeled. The cell wall is represented as a spherical shell consisting of a relatively stiff thin outer layer and a softer thicker inner layer. The state of the interior of the cell is idealized as a specified turgor pressure. Attention is restricted to axisymmetric deformations, and the wall material is characterized as a viscoelastic solid with different properties for the inner and outer layers. Finite deformation, quasi-static calculations are carried out. The effects of outer layer stiffness, outer layer thickness, turgor pressure, indenter sharpness, cell wall thickness, and loading rate on the indentation hardness are considered. The calculations indicate that the small indenter depth response is dominated by the cell wall material properties, whereas for a sufficiently large indenter depth, the value of the turgor pressure plays a major role. The indentation hardness is found to increase approximately linearly with a measure of indenter sharpness over the range considered. The value of the indentation hardness is affected by the rate of indentation, with a much more rapid decay of the hardness for slow loading, because there is more time for viscous relaxation during indentation.

Viscoelastic properties of pectin/cellulose composites studied by QCM-D and oscillatory shear rheology

The interactions between cellulose and pectin polysaccharides in primary plant cell walls are not fully understood, although several recent studies indicate that they might play an important role in wall properties. Studying polysaccharide interactions in planta is challenging, due to the complexity and heterogeneity of plant materials. Therefore, to investigate these interactions and the implications for the rheological properties of cell walls, we have taken a bottom-up approach in which cellulose/pectin composites are created either by adsorption of pectin polysaccharides (arabinan, galactan, homogalacturonan DE 69, homogalacturonan DE 33 and pectin DE 33) on cellulose-coated sensors in a quartz crystal microbalance with dissipation monitoring (QCM-D) or by incorporation of pectin during in vivo cellulose synthesis by Komagataeibacter bacteria. The viscoelastic behavior of the adsorbed layers was analyzed by applying the Voigt model to the QCM-D data, whilst the bulk viscoelastic properties of bacterial cellulose/pectin composites were studied by small amplitude oscillatory shear rheology. Our results show that all of the pectin polysaccharides studied have the ability to adsorb on the cellulose surfaces. The viscoelastic properties of the adsorbed layer varied depending on the substitution and degree of esterification of the pectin polysaccharides. Additionally, oscillatory rheology results showed that all bacterial cellulose-pectin composites had a gel nature (G′ > G″) with moduli varying in line with QCM-D determined viscoelasticity. Our interpretation of the results provides a better understanding of pectin-cellulose interactions and has implications for primary plant cell wall material properties.

A computational approach for inferring the cell wall properties that govern guard cell dynamics.

SummaryGuard cells dynamically adjust their shape in order to regulate photosynthetic gas exchange, respiration rates and defend against pathogen entry. Cell shape changes are determined by the interplay of cell wall material properties and turgor pressure. To investigate this relationship between turgor pressure, cell wall properties and cell shape, we focused on kidney‐shaped stomata and developed a biomechanical model of a guard cell pair. Treating the cell wall as a composite of the pectin‐rich cell wall matrix embedded with cellulose microfibrils, we show that strong, circumferentially oriented fibres are critical for opening. We find that the opening dynamics are dictated by the mechanical stress response of the cell wall matrix, and as the turgor rises, the pectinaceous matrix stiffens. We validate these predictions with stomatal opening experiments in selected Arabidopsis cell wall mutants. Thus, using a computational framework that combines a 3D biomechanical model with parameter optimization, we demonstrate how to exploit subtle shape changes to infer cell wall material properties. Our findings reveal that proper stomatal dynamics are built on two key properties of the cell wall, namely anisotropy in the form of hoop reinforcement and strain stiffening.

On the micro-indentation of plant cells in a tissue context

The effect of geometry on cell stiffness measured with micro-indentation techniques has been explored in single cells, however it is unclear if results on single cells can be readily transferred to indentation experiments performed on a tissue in vivo. Here we explored this question by using simulation models of osmotic treatments and micro-indentation experiments on 3D multicellular tissues with the finite element method. We found that the cellular context does affect measured cell stiffness, and that several cells of context in each direction are required for optimal results. We applied the model to micro-indentation data obtained with cellular force microscopy on the sepal of A. thaliana, and found that differences in measured stiffness could be explained by cellular geometry, and do not necessarily indicate differences in cell wall material properties or turgor pressure.

Material properties of the cell walls in nanofibrillar cellulose foams from finite element modelling of tomography scans

The mechanical properties of the nanofibrillar cellulose foam depend on the microstructure of the foam and on the constituent solid properties. The latter are hard to extract experimentally due to difficulties in performing the experiments on the micro-scale. The aim of this work is to provide methodology for doing it indirectly using extracted geometry of the microstructure. X-ray computed tomography scans are used to reconstruct the microstructure of a nanofibrillar cellulose foam sample. By varying the levels of thresholding, structure of differing porosities of the same foam structure are obtained and their macroscopic properties of the uni-axial compression are computed by finite element simulations. A power law relation, equivalent to classical foam scaling laws, are fit to the data obtained from simulation at different relative densities for the same structure. The relation thus obtained, is used to determine the cell wall material properties, viz. elastic modulus and yield strength, by extrapolating it to the experimental porosity and using the measured response at this porosity. The simulations also provide qualitative insights into the nature of irreversible deformations, not only corroborating the experimental results, but also providing possible explanation to the mechanisms responsible for crushable behaviour of the nanofibrillar cellulose foams in compression.

Mesoscale investigation of the structural properties of unrefined cell wall materials extracted from minimally processed salads during storage

Cell walls in plants are polymeric composite systems, which determine the textural attributes of vegetal tissue. In this paper, results are presented on the investigation on the structural properties of cell wall materials (CWMs), intended for use as structuring materials in manufactured products. Specifically, CWMs were extracted from fresh and ripened minimally processed Valerianella locusta Laterr. At the tissue level, the structural resilience inferred by CWM varies with storage time, as it was evaluated both quantifying the membrane degradation and the mechanical characteristics of salads texture wilting. Mechanical spectra of the rehydrated CWM gel show the increase of the elastic character with the salad aging time. Stress relaxation spectra deduced from rheological study on water suspension of the extracted cell wall polysaccharides, showed fast and slow relaxation modes, whose ratio changed with the aging of the salad tissue. The use of unrefined cell wall materials (CWMs) in food formulations may infer naturalness to food offerings and this paper shows how CWM extraction from salad that underwent different time storage histories, can provide modulation of the functional properties of these structures in final manufactured products.

Modeling of elastic properties of cell-wall material in nanoclay-reinforced foams

A new micromechanical model that predicts anisotropic elastic properties of cell-wall material in nanoclay-reinforced polymeric foams is proposed. The model accounts for the reorientation of nanoclays during the foaming process, due to the stretching of the foam cell walls. Elastic moduli of bulk nanocomposites were studied first. It was found that given the high aspect ratio of nanoclays, they cannot be randomly placed in a representative volume element without having them intersect each other at high weight fractions. At 1.5 wt% the nanoclays already align and form clusters resulting in a significant effect on the stiffness of the nanocomposite. The number of silicate layers per cluster stack was also found to influence the Young’s modulus of the nanocomposite. When reinforcing polypropylene (PP) with 2 wt% of nanoclays, the modulus of elasticity decreased by 17% when the number of clays in a cluster went from 1 (fully exfoliated system) to 5. The stretching of the nanocomposite was shown to reorient nanoclays and, consequently, to change the stiffness of the cell wall. For the PP reinforced with 1 wt% of nanoclays and a stretching ratio of 6.5, the modulus increased by 15%.

Strain-rate sensitivity of foam materials: A numerical study using 3D image-based finite element model

Realistic simulations are increasingly demanded to clarify the dynamic behaviour of foam materials, because, on one hand, the significant variability (e.g. 20% scatter band) of foam properties and the lack of reliable dynamic test methods for foams bring particular difficulty to accurately evaluate the strain-rate sensitivity in experiments; while on the other hand numerical models based on idealised cell structures (e.g. Kelvin and Voronoi) may not be sufficiently representative to capture the actual structural effect. To overcome these limitations, the strain-rate sensitivity of the compressive and tensile properties of closed-cell aluminium Alporas foam is investigated in this study by means of meso-scale realistic finite element (FE) simulations. The FE modelling method based on X-ray computed tomography (CT) image is introduced first, as well as its applications to foam materials. Then the compression and tension of Alporas foam at a wide variety of applied nominal strain-rates are simulated using FE model constructed from the actual cell geometry obtained from the CT image. The stain-rate sensitivity of compressive strength (collapse stress) and tensile strength (0.2% offset yield point) are evaluated when considering different cell-wall material properties. The numerical results show that the rate dependence of cell-wall material is the main cause of the strain-rate hardening of the compressive and tensile strengths at low and intermediate strain-rates. When the strain-rate is sufficiently high, shock compression is initiated, which significantly enhances the stress at the loading end and has complicated effect on the stress at the supporting end. The plastic tensile wave effect is evident at high strain-rates, but shock tension cannot develop in Alporas foam due to the softening associated with single fracture process zone occurring in tensile response. In all cases the micro inertia of individual cell walls subjected to localised deformation is found to have negligible effect on the macro strain-rate sensitivity of Alporas foam.

Characterization of Nomex honeycomb core constituent material mechanical properties

Nomex honeycomb cores have been widely used in composite sandwich panels. To accomplish meso-scale finite element modeling of these cores, cell wall mechanical properties are required, for which limited data are available. In this work, tensile testing was performed on Nomex paper, phenolic resin, and Nomex paper coated with phenolic resin. Flatwise tension and compression tests were also performed on two types of Nomex honeycomb cores. Test results were calibrated in finite element modeling to account for strain gage local stiffening effects and thickness normalization. Identified cell wall material properties were implemented in a honeycomb core finite element model and further calibrated by matching simulation results to manufacturer test data. The cells’ double-wall thickness was also adjusted. These calibrations and adjustments led to an exact simulation match with test data. Numerically matching cell wall material properties depends on modeling sophistication and is subject to core test result variability and core construction differences among manufacturers.

Roles of xyloglucan and pectin on the mechanical properties of bacterial cellulose composite films

Xyloglucan and pectin are major non-cellulosic components of most primary plant cell walls. It is believed that xyloglucan and perhaps pectin are functioning as tethers between cellulose microfibrils in the cell walls. In order to understand the role of xyloglucan and pectin in cell wall mechanical properties, model cell wall composites created using Gluconacetobacter xylinus cellulose or cellulose nanowhiskers (CNWs) derived there from with different amounts of xyloglucan and/or pectin have been prepared and measured under extension conditions. Compared with pure CNW films, CNW composites with lower amounts of xyloglucan or pectin did not show significant differences in mechanical behavior. Only when the additives were as high as 60 %, the films exhibited a slightly lower Young’s modulus. However, when cultured with xyloglucan or pectin, the bacterial cellulose (BC) composites produced by G. xylinus showed much lower modulus compared with that of the pure BC films. Xyloglucan was able to further reduce the modulus and extensibility of the film compared to that of pectin. It is proposed that surface coating or tethering of xyloglucan or pectin of cellulose microfibrils does not alone affect the mechanical properties of cell wall materials. The implication from this work is that xyloglucan or pectin alters the mechanical properties of cellulose networks during rather than after the cellulose biosynthesis process, which impacts the nature of the connection between these compounds.

Xyloglucan endotransglucosylase/hydrolase (XTH) overexpression affects growth and cell wall mechanics in etiolated Arabidopsis hypocotyls

Growth and biomechanics of etiolated hypocotyls from Arabidopsis thaliana lines overexpressing xyloglucan endotransglucosylase/hydrolase AtXTH18, AtXTH19, AtXTH20, and PttXET16-34 were studied. Overexpression of AtXTH18, AtXTH19, and AtXTH20 stimulated growth of hypocotyls, while PttXET16-34 overexpression did not show this effect. In vitro extension of frozen/thawed hypocotyls measured by a constant-load extensiometer started from a high-amplitude initial deformation followed by a slow time-dependent creep. Creep of growing XTH-overexpressing (OE) hypocotyls was more linear in time compared with the wild type at pH 5.0, reflecting their higher potential for long-term extension. XTH-OE plants deposited 65-84% more cell wall material per hypocotyl cross-sectional area than wild-type plants. As a result, their wall stress under each external load was lower than in the wild-type. Growing XTH-OE hypocotyls had higher values of initial deformation·stress(-1) compared with the wild type. Plotting creep rates for each line under different loads against the respective wall stress values gave straight lines. Their slopes and intercepts with the abscissa correspond to ϕ (in vitro cell wall extensibility) and y (in vitro cell wall yield threshold) values characterizing cell wall material properties. The wall material in XTH-OE lines was more pliant than in the wild type due to lower y values. In contrast, the acid-induced wall extension in vitro resulted from increasing ϕ values. Thus, three factors contributed to the XTH-OE-stimulated growth in Arabidopsis hypocotyls: their more linear creep, higher values of initial deformation·stress(-1), and lower y values.

Dynamic Responses of Sandwich Beams with Gradient-Density Aluminum Foam Cores

The dynamic responses of end-clamped sandwich beams comprised of aluminum face sheets and close-celled aluminum foam cores are numerically simulated by loading the beams at mid-span. Three types of aluminum foam core having identical areal density are considered: homogeneous core, low-high gradient-density core, and high-low gradient-density core. Two-dimensional (2D) finite element (FE) models are created from tomographic images of the foam, which represent the cell shape and geometric distribution of real foams fabricated using the powder metallurgy foaming technique with blowing agents. To determine the mechanical properties of cell wall material, the numerically predicted uniaxial stress versus strain curve for homogeneous aluminum foam is fitted to that measured experimentally. The shock impulse is simulated using a uniformly distributed pressure versus time history. Depending upon the initial impulse applied, it is demonstrated that the deformation and deflection of the sandwich beam can be either bend or stretch dominated. Within the bend dominated domain, the foam-cored sandwich beams outperform monolithic solid beams of equal mass. However, the benefits diminish as the response of the sandwich becomes stretch dominated. Whilst the sandwich beam with homogeneous foam core shows the smallest mid-span deflection of its back facesheet, that with low-high gradient-density core achieves the largest average compressive strain and absorbs the most internal energy.

Non-extractable Procyanidins and Lignin are Important Factors in the Bile Acid Binding and Radical Scavenging Properties of Cell Wall Material in some Fruits

The cell wall components and the food functions of alcohol-insoluble solids (AIS) of Chinese quince, quince, hawthorn, apple, pear and blueberry fruits were analyzed. Chinese quince contained characteristically high contents of cellulose, lignin, and non-extractable procyanidins (NEPCs). On the other hand, the quince AIS contained the highest proportion of NEPCs, the highest mean degree of polymerization (mDP), the strongest radical scavenging activity, and strong bile acid binding activity. In fruit AIS, the lignin and NEPC contents both showed positive correlations with the bile acid binding and radical scavenging activities. The value for mDP × NEPC content was a good index for the radical scavenging activity. The results suggest that highly polymerized NEPCs and lignin are important factors of cell wall components of fruits to having a high functionality, and Chinese quince and quince are interesting fruits from this view point.

Physico-chemical properties of cell wall materials obtained from ten grape varieties and their byproducts: grape pomaces and stems

The availability and the potential of wine by-products, grape pomaces and stems, obtained from ten different grape ( Vitis vinifera L.) varieties (six red and four white) as raw materials for the production of dietary fibre (DF) concentrates were evaluated. The overall chemical composition, carbohydrate analysis and functional properties of DF from fresh grapes, grape pomaces and stems from those grape varieties were analysed. Both winemaking by-products presented considerable quantities of DF, ranging from 60% to 90% of total dry matter. The cell wall polysaccharides (CWP) composition of grape pomaces and fresh grapes was rather similar, pectic substances being the main component of the cell walls (40–54% total CWP). In contrast, cellulose was the predominant cell wall polymer for the stems (40–49% total CWP). In addition, the pectin content of grape pomaces exhibited significant differences between red and white grape varieties. The degree of methyl-esterification of uronic acids from wine by-products ranged from 21 to 39%; thus, pectic polysaccharides from the different samples could be classified as low methyl-esterified pectins. Klason lignin accounted for around 20–25% of DF in both grape pomaces and stems. With regard to the potential incorporation of DF concentrates into the food chain, functional properties were also determined. Swelling, water and fat retention capacity results showed great variability depending on the grape variety analysed.