The secondary cell wall is a structure found in many

plant cell Plant cells are the cells present in Viridiplantae, green plants, photosynthetic eukaryotes of the kingdom Plantae. Their distinctive features include primary cell walls containing cellulose, hemicelluloses and pectin, the presence of plastids ...

s, located between the primary cell wall and the

plasma membrane The cell membrane (also known as the plasma membrane or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of a cell from the outside environment (the extr ...

. The cell starts producing the secondary cell wall after the primary cell wall is complete and the cell has stopped expanding. It is most prevalent in the Ground tissue found in vascular plants, with Collenchyma having little to no lignin, and Sclerenchyma having lignified secondary cells walls.

Structure and function

Secondary cell walls provide additional protection to cells and rigidity and strength to the larger plant. These walls are constructed of layered sheaths of cellulose microfibrils, wherein the fibers are in parallel within each layer. The inclusion of

lignin Lignin is a class of complex organic polymers that form key structural materials in the support tissues of most plants. Lignins are particularly important in the formation of cell walls, especially in wood and bark, because they lend rigidit ...

makes the secondary cell wall less flexible and less permeable to water than the primary cell wall. In addition to making the walls more resistant to degradation, the hydrophobic nature of lignin within these tissues is essential for containing water within the vascular tissues that carry it throughout the plant. The secondary cell wall consists primarily of

cellulose Cellulose is an organic compound with the chemical formula, formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of glycosidic bond, β(1→4) linked glucose, D-glucose units. Cellulose is an important s ...

, along with other

polysaccharides Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrates found in food. They are long-chain polymeric carbohydrates composed of monosaccharide units bound together by glycosidic linkages. This carbohydrate can react with wat ...

, and glycoprotein. It sometimes consists of three distinct layers - S₁, S₂ and S₃ - where the direction of the cellulose microfibrils differs between the layers. The direction of the microfibrils is called microfibril angle (MFA). In the secondary cell wall of fibres of trees a low microfibril angle is found in the S2-layer, while S1 and S3-layers show a higher MFA . However, the MFA can also change depending on the loads on the tissue. It has been shown that in reaction wood the MFA in S2-layer can vary. Tension wood has a low MFA, meaning that the microfibril is oriented parallel to the axis of the fibre. In compression wood the MFA is high and reaches up to 45°. These variations influence the mechanical properties of the cell wall. Plant cell showing primary and secondary wall by CarolineDahl

Plant cell showing primary and secondary wall by CarolineDahl

The secondary cell wall has different ratios of constituents compared to the primary wall. An example of this is that secondary wall in wood contains polysaccharides called xylan, whereas the primary wall contains the polysaccharide xyloglucan. The cells fraction in secondary walls is also higher. Pectins may also be absent from the secondary wall, and unlike primary walls, no structural proteins or

enzymes An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as pro ...

have been identified. Because of the low permeability through the secondary cell wall, cellular transport is carried out through openings in the wall called pits.

Wood Wood is a structural tissue/material found as xylem in the stems and roots of trees and other woody plants. It is an organic materiala natural composite of cellulosic fibers that are strong in tension and embedded in a matrix of lignin t ...

consists mostly of secondary cell wall, and holds the plant up against gravity. Some secondary cell walls store nutrients, such as those in the cotyledons and the

endosperm The endosperm is a tissue produced inside the seeds of most of the flowering plants following double fertilization. It is triploid (meaning three chromosome sets per nucleus) in most species, which may be auxin-driven. It surrounds the Embryo#Pla ...

. These contain little cellulose, and mostly other

Evolution

The first lignified secondary walls evolved 430 million years ago, creating the structure necessary for vascular plants. The genes used to form the constituents of secondary cells walls have also been found in Physcomitrella patens. This suggests that a duplication of these genes was the driver of secondary cells wall formation.

Pathogen resistance

The secondary cells wall plays an active role in pathogen resistance. It has been shown to accumulate anti-microbial peptides that prevent the bacteria and fungus from entering the cell. Lignin has also been shown to prevent the infection of cells. Plant cells will increase the production of lignin generating enzymes when stressed by some pathogens, further lignifying the secondary cell wall. Increased lignin content is particularly effective at resisting vascular pathogens that use the secondary xylem to spread.

References

{{reflist Botany Cell anatomy