Бурение грунтовых зондов, установка энергетических колодцев

Plant cells are different from animal cells, and one important fea­ture of plant cells is the presence of a cell wall. The relative rigidity of plant cell wall renders the strength to plants and plant materi­als like wood. The plant cell wall serves a variety of functions like giving strength and acting like a membrane. The thickness as well as the composition and organization of cell walls can vary signifi­cantly from one plant to another. Many plant cells have both a pri­mary and secondary cell wall. The primary cell wall accommodates the cell as it grows, and a secondary cell wall develops inside the primary wall after the cell has stopped growing.

A representative plant cell drawing is shown in Figure 4.3., illus­trating the rigid partitions between cells. Middle lamella is the first layer formed during cell division. It makes up the outer wall of the cell and is shared by adjacent cells. In the course of cell growth the dimensions of the cell wall vary according to the type of macro­molecule of which it is composed. The first wall deposited after cell division is called the middle lamella and is essentially composed of pectic material and proteins. As the cell ages and differentiates, it

secretes new materials which form a mixture with the constituents of the primary cell wall thereby leading to the formation of a sec­ondary cell wall as well. The other wall polymer lignins are incor­porated into the spaces around the polysaccharide fibrillar elements to form lignin-polysaccharides. They contribute to the mechanical strength of the plant cell wall and confer resistance to pathogens.

The basic structural unit of the cell wall cellulose is slender aggre­gates of cellobiose chains called microfibrils or elementary fibrils. The structural arrangement of these microfibrils is shown in Figure 4.4., and depending on the plant, these microfibrils are 2-20 nm in diameter. The lengths of microfibrils can also vary widely, typically in the 0.1-100 цm range [1]. These cellulose microfibrils consist of overlapping parallel arrays of cellulose chains forming into bun­dles, and on average there are about 40 cellulose chains, aggregated together to make a microfibril.

In the cell wall, cellulose microfibrils are set in a complex structural arrangement with two other major components; another polysaccha­ride hemicellulose and aromatic ring containing polymer lignin, as shown in Figure 4.4. The carbohydrate polymers cellulose and hemi — celluloses are tightly bound to the lignin by hydrogen and covalent bonds. Besides the linear polysaccharide cellulose, the cell wall con­tains two groups of branched polysaccharides, the hemicelluloses and pectins. These polysaccharides are organized into a network

with the cellulose microfibrils, crosslinking glycans increasing the tensile strength of the cellulose. The extensive network of pectins provides the cell wall with the ability to resist compression. In addi­tion to these networks, a small amount of protein can also be found in all plant primary cell walls. In fact, cellulose and hemicellulose are the main structural components of the green plants, many forms of algae and the oomycetes. The complex molecular architecture in the cell wall provides the strength and the resistance to attack, acting as nature’s protection feature which is essential for the life of the cell.

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