Molecular Biology of the Cell. 4th edition.
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Madison, Wiscosin Google Scholar. Monties B Les polimeres vegetaux. Gauthier-Villars, Paris Google Scholar. Elsevier, New York Google Scholar. Unwin Hyman, London Google Scholar. Chemistry and biochemistry, vol IIA. Stoddart RW The biosynthesis of polysaccharides. Croom Held, Sidney Google Scholar. Biogenesis and biodegradation. Wiley, Canada Google Scholar. B Surface view more Although the cell walls of higher plants vary in both composition and organization, they are all constructed, like animal extracellular matrices, using a structural principle common to all fiber-composites, including fibreglass and reinforced concrete.
One component provides tensile strength, while another, in which the first is embedded, provides resistance to compression. While the principle is the same in plants and animals, the chemistry is different. Unlike the animal extracellular matrix , which is rich in protein and other nitrogen-containing polymers, the plant cell wall is made almost entirely of polymers that contain no nitrogen, including cellulose and lignin. Trees make a huge investment in the cellulose and lignin that comprise the bulk of their biomass. In the cell walls of higher plants, the tensile fibers are made from the polysaccharide cellulose , the most abundant organic macromolecule on Earth, tightly linked into a network by cross-linking glycans.
In primary cell walls, the matrix in which the cellulose network is embedded is composed of pectin , a highly hydrated network of polysaccharides rich in galacturonic acid. Secondary cell walls contain additional components, such as lignin, which is hard and occupies the interstices between the other components, making the walls rigid and permanent. All of these molecules are held together by a combination of covalent and noncovalent bonds to form a highly complex structure, whose composition, thickness and architecture depends on the cell type.
We focus here on the primary cell wall and the molecular architecture that underlies its remarkable combination of strength, resilience, and plasticity, as seen in the growing parts of a plant.
Plant cell walls: the skeleton of the plant world
The aqueous extracellular environment of a plant cell consists of the fluid contained in the walls that surround the cell. Although the fluid in the plant cell wall contains more solutes than does the water in the plant's external milieu for example, soil , it is still hypotonic in comparison with the cell interior. This osmotic imbalance causes the cell to develop a large internal hydrostatic pressure, or turgor pressure , that pushes outward on the cell wall, just as an inner tube pushes outward on a tire.
The turgor pressure increases just to the point where the cell is in osmotic equilibrium , with no net influx of water despite the salt imbalance see Panel , pp. This pressure is vital to plants because it is the main driving force for cell expansion during growth, and it provides much of the mechanical rigidity of living plant tissues. Compare the wilted leaf of a dehydrated plant, for example, with the turgid leaf of a well-watered one. It is the mechanical strength of the cell wall that allows plant cells to sustain this internal pressure.
The cellulose molecules provide tensile strength to the primary cell wall. Each molecule consists of a linear chain of at least glucose residues that are covalently linked to one another to form a ribbonlike structure, which is stabilized by hydrogen bonds within the chain Figure In addition, intermolecular hydrogen bonds between adjacent cellulose molecules cause them to adhere strongly to one another in overlapping parallel arrays, forming a bundle of about 40 cellulose chains, all of which have the same polarity.
These highly ordered crystalline aggregates, many micrometers long, are called cellulose microfibrils , and they have a tensile strength comparable to steel. Sets of microfibrils are arranged in layers, or lamellae, with each microfibril about 20—40 nm from its neighbors and connected to them by long cross-linking glycan molecules that are bound by hydrogen bonds to the surface of the microfibrils.
The primary cell wall consists of several such lamellae arranged in a plywoodlike network Figure Each glucose is inverted with respect to its neighbors, and the resulting disacchride repeat occurs hundreds of times in a single cellulose molecule. Scale model of a portion of a primary cell wall showing the two major polysaccharide networks. The orthogonally arranged layers of cellulose microfibrils green are tied into a network by cross-linking glycans red that form hydrogen bonds with the more The cross-linking glycans are a heterogeneous group of branched polysaccharides that bind tightly to the surface of each cellulose microfibril and thereby help to cross-link microfibrils into a complex network.
Their function is analogous to that of the fibril-associated collagens discussed earlier see Figure There are many classes of cross-linking glycans, but they all have a long linear backbone composed of one type of sugar glucose , xylose, or mannose from which short side chains of other sugars protrude. It is the backbone sugar molecules that form hydrogen bonds with the surface of cellulose microfibrils, cross-linking them in the process.
Both the backbone and the side-chain sugars vary according to the plant species and its stage of development. Coextensive with this network of cellulose microfibrils and cross-linking glycans is another cross-linked polysaccharide network based on pectins see Figure Pectins are a heterogeneous group of branched polysaccharides that contain many negatively charged galacturonic acid units. Because of their negative charge, pectins are highly hydrated and associated with a cloud of cations, resembling the glycosaminoglycans of animal cells in the large amount of space they occupy see Figure Although covalent bonds also play a part in linking the components together, very little is known about their nature.
Regulated separation of cells at the middle lamella underlies such processes as the ripening of tomatoes and the abscission detachment of leaves in the fall.
Plant Cells, Chloroplasts, Cell Walls | Learn Science at Scitable
Many of these proteins are enzymes, responsible for wall turnover and remodelling, particularly during growth. Another class of wall proteins contains high levels of hydroxyproline, as in collagen. These proteins are thought to strengthen the wall, and they are produced in greatly increased amounts as a local response to attack by pathogens. From the genome sequence of Arabidopsis, it has been estimated that more than genes are required to synthesize, assemble, and remodel the plant cell wall. Some of the main polymers found in the primary and secondary cell wall are listed in Table For a plant cell to grow or change its shape, the cell wall has to stretch or deform.
Because of their crystalline structure, however, individual cellulose microfibrils are unable to stretch. Thus, stretching or deformation of the cell wall must involve either the sliding of microfibrils past one another, the separation of adjacent microfibrils, or both.
As we discuss next, the direction in which the growing cell enlarges depends in part on the orientation of the cellulose microfibrils in the primary wall, which in turn depends on the orientation of microtubules in the underlying cell cortex at the time the wall was deposited.
Composition of plant cell walls
The final shape of a growing plant cell, and hence the final form of the plant, is determined by controlled cell expansion. Expansion occurs in response to turgor pressure in a direction that depends in part on the arrangement of the cellulose microfibrils in the wall. Cells, therefore, anticipate their future morphology by controlling the orientation of microfibrils that they deposit in the wall. Unlike most other matrix macromolecules, which are made in the endoplasmic reticulum and Golgi apparatus and are secreted, cellulose, like hyaluronan, is spun out from the surface of the cell by a plasma- membrane -bound enzyme complex cellulose synthase , which uses as its substrate the sugar nucleotide UDP- glucose supplied from the cytosol.
As they are being synthesized, the nascent cellulose chains assemble spontaneously into microfibrils that form on the extracellular surface of the plasma membrane —forming a layer, or lamella, in which all the microfibrils have more or less the same alignment see Figure Each new lamella forms internally to the previous one, so that the wall consists of concentrically arranged lamellae, with the oldest on the outside. The most recently deposited microfibrils in elongating cells commonly lie perpendicular to the axis of cell elongation Figure Although the orientation of the microfibrils in the outer lamellae that were laid down earlier may be different, it is the orientation of these inner lamellae that is thought to have a dominant influence on the direction of cell expansion Figure The orientation of cellulose microfibrils in the primary cell wall of an elongating carrot cell.
This electron micrograph of a shadowed replica from a rapidly frozen and deep-etched cell wall shows the largely parallel arrangements of cellulose microfibrils, more How the orientation of cellulose microfibrils within the cell wall influences the direction in which the cell elongates.
The cells in A and B start off with identical shapes shown here as cubes but with different orientations of cellulose microfibrils more An important clue to the mechanism that dictates this orientation came from observations of the microtubules in plant cells. These are arranged in the cortical cytoplasm with the same orientation as the cellulose microfibrils that are currently being deposited in the cell wall in that region.
These cortical microtubules form a cortical array close to the cytosolic face of the plasma membrane , held there by poorly characterized proteins Figure The congruent orientation of the cortical array of microtubules lying just inside the plasma membrane and cellulose microfibrils lying just outside is seen in many types and shapes of plant cells and is present during both primary and secondary cell-wall deposition, suggesting a causal relationship.
The cortical array of microtubules in a plant cell. A A grazing section of a root-tip cell from Timothy grass, showing a cortical array of microtubules lying just below the plasma membrane.