The secondary cell wall is a structure found in many
plant cell
Plant cells are the cells present in green plants, photosynthetic eukaryotes of the kingdom Plantae. Their distinctive features include primary cell walls containing cellulose, hemicelluloses and pectin, the presence of plastids with the capab ...
s, located between the
primary cell wall and the
plasma membrane
The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment (t ...
. The cell starts producing the secondary cell wall after the primary cell wall is complete and the cell has stopped expanding.
[Buchanan, Gruissem, Jones, ''Biochemistry & molecular biology of plants'', 1st edition, American Society of Plant Physiology, 2000]
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 rigidity a ...
makes the secondary cell wall less flexible and less permeable to water than the primary cell wall.
[Raven, P. H., R. F. Evert, et al. (1999). Biology of plants. New York, W.H. Freeman : Worth Publishers.] 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 formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell wa ...
, along with other
polysaccharides,
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 rigidity a ...
, and
glycoprotein
Glycoproteins are proteins which contain oligosaccharide chains covalently attached to amino acid side-chains. The carbohydrate is attached to the protein in a cotranslational or posttranslational modification. This process is known as glyco ...
. It sometimes consists of three distinct layers - S
1, S
2 and S
3 - where the direction of the
cellulose microfibrils differs between the layers.
The direction of the
microfibrils A microfibril is a very fine fibril, or fiber-like strand, consisting of glycoproteins and cellulose. It is usually, but not always, used as a general term in describing the structure of protein fiber, e.g. hair and sperm tail. Its most frequently o ...
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.
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.
[Taiz, L. and E. Zeiger (2006). Plant physiology. Sunderland, Mass., Sinauer Associates.] Pectin
Pectin ( grc, πηκτικός ': "congealed" and "curdled") is a heteropolysaccharide, a structural acid contained in the primary lamella, in the middle lamella, and in the cell walls of terrestrial plants. The principal, chemical component o ...
s may also be absent from the secondary wall, and unlike primary walls, no
structural proteins or
enzymes
Enzymes () are proteins that act as biological catalysts 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 products. ...
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 porous and fibrous structural tissue found in the stems and roots of trees and other woody plants. It is an organic materiala natural composite of cellulose fibers that are strong in tension and embedded in a matrix of lignin ...
consists mostly of secondary cell wall, and holds the plant up against gravity.
[Campbell, Reece, ''Biology'', 7th edition, Pearson/Benjamin Cummings, 2005]
Some secondary cell walls store nutrients, such as those in the
cotyledon
A cotyledon (; ; ; , gen. (), ) is a significant part of the embryo within the seed of a plant, and is defined as "the embryonic leaf in seed-bearing plants, one or more of which are the first to appear from a germinating seed." The numb ...
s 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 an ...
. These contain little cellulose, and mostly other
polysaccharides.
References
Botany
Cell anatomy