The cell wall is the outer, rigid wall of a cell, dividing the protoplast (the interior, including the cytoplasm and nucleus) from the cell’s external environment. The plant cell wall is both unique to and a major feature of plants, perhaps second only to the plant’s photosynthetic ability.
The primary functions of the cell wall in plant cells include are to provide protection for the enclosed cytoplasm and give mechanical support to the entire plant structure. Plant cell walls are part of the extracellular matrix, a complex mixture of extracellular materials found between cells.
These materials are synthesized by the intracellular contents and transported through the plasma membranes. All plant cell types consist of at least a primary cell wall, and many also produce a secondary cell wall. In addition, certain cells also secrete specialized substances into the extracellular matrix.
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Primary Cell Wall
The primary cell wall is first synthesized by young, actively growing cells. It is thin and is composed of cellulose embedded within a noncellulose matrix. Cellulose is a polysaccharide polymer composed entirely of glucose molecules joined together end on end to form long, unbranched chains.
These chains may consist of up to several thousand glucose molecules. The chemical bond joining the glucose molecules in cellulose is slightly different from that found in starch, another polysaccharide composed entirely of glucose. It is this difference in bonds that makes cellulose a structural polysaccharide.
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| Primary Cell Wall |
Microfibrils are groupings of about 50 to 60 cellulose chains that are parallel to one another and held together by hydrogen bonds. The tensile strength of microfibrils is comparable to steel wire of the same thickness. The cellulose microfibrils make up approximately 25 percent of the primary cell wall and are arranged somewhat randomly (more parallel in fast growing cells) within the noncellulose matrix.
Noncellulose Matrix
The noncellulose matrix of the primary cell well accounts for about 60 percent to 80 percent of the primary cellwall and is composed of hemicelluloses, pectins, and extensins. Hemicelluloses are highly branched polysaccharide structures composed of heterogeneous mixtures of sugars, some of which are chemically modified.
Pectins are also heterogeneous mixtures of sugars but are particularly rich in galacturonic acid. Pectic substances are also a major component of the outermost middle lamella, which can be thought of as the cement that holds adjacent cells together.
Pectins, extracted from unripe fruits, have been used commercially as thickening agents for jellies and jams. The softness of ripe fruits is due to the enzymatic breakdown of pectins within the middle lamella. The hemicelluloses and various pectins are thought to coat and reinforce the cellulose microfibrils.
Extensins are protein components of the matrix. They are glycoproteins that contain a high amount of a modified amino acid called hydroxyproline. Extensins make up about 10 percent of the matrix material, add strength to the cell wall, and are involved in cell growth.
Secondary Cell Wall
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| Secondary Cell Wall |
In some cell types there may even be two or more distinct layers of secondary cell wall deposition. Its composition is somewhat similar to the primary cell wall but also differs significantly. Cellulo semicrofibrils can make up to 45 percent of this wall, while hemicelluloses and pectins can make up to about 20 percent.
However, what most distinguishes the secondary cell wall is the deposition of lignin. Lignin is an extremely tough and durable complex compound characterized by interlocking phenolic groups.
Lignin is stronger than cellulose microfibrils, and together they are responsible for the superior strength of many types of wood. In cells that undergo secondary cell wall formation, lignin may also be deposited in the preexisting middle lamella and primary cell wall areas.
Specialized Cell Wall Substances
Certain cell types, especially the epidermis, which is exposed directly to the outer environment, secrete a variety of highly waterproof and protective substances, such as cutin, suberin, and a variety ofwaxes. These substances are deposited on the outside of the primary cell wall. Cutin and suberin are polymers composed of long-chain fatty acids that are linked or esterified at the acid ends.
Suberin differs in that it contains dicarboxylic fatty acids and various phenolic compounds. For the most part, cutin is associated with and is the main constituent of the cuticle of the above ground epidermis, while suberin is mostly associated with the below ground epidermis.
Suberin is also the major component of the Casparian strip found in the endodermis of the root. This important root tissue forces water and dissolved minerals to move intra cellularly into the vascular tissue. Additionally, suberin constitutes scar tissue that is formed when cells are injured or otherwise wounded.
Waxes are a family of extremely water-proof substances characterized by long-chain alcohols linked with long-chain fatty acids or hydrocarbons. They are typically secreted as droplets on the exterior of the cuticle and crystallize in a variety of geometric patterns.
Plasmodesmata
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| Plasmodesmata |
This ER channel is referred to as the desmotubule. Thus, each plasmodesma contains an inner desmotubule. Some cells contain areas called primary pit fields, in which numerous plasmodesmata are found.
The purpose of plasmodesmata involves cell-to-cell communication via transport of small molecules. Movement is not thought to occur inside the desmotubule, as it is too narrow, but rather through the cytoplasmic channel between the desmotubule and the plasmodesma itself. The presence of plasmodesmata allows for a continuous cytoplasmic connection within plant tissues called the symplast.
In cells which form thick secondary cell walls, particularly the xylemvessels and tracheids,which contain no living protoplast at maturity, numerous pit pairs among adjacent cells are found that allow multiple pathways for the flow of water.