Secondary cell wall
Encyclopedia
The secondary cell wall is a structure found in many plant cell
s, located between the primary cell wall and the plasma membrane. The cell starts producing the secondary cell wall after the primary cell wall is complete and the cell has stopped expanding.
The secondary cell wall consists mainly of cellulose
, but also other polysaccharides, lignin
, and glycoprotein
s. It sometimes consists of three distinct layers - S1, S2 and S3 - where the direction of the Cellulose microfibrils
differs between the layers. Apparently there are no Structural proteins or enzymes in the secondary wall.
The secondary cell wall has different ratios of wall constituents compared to the primary wall. An example of this is that wood secondary walls contain xylans, whereas the primary wall contains xyloglucans and the cellulose fraction is higher in the secondary wall. Pectin
s may also be absent from the secondary wall and apparently it contain no Structural proteins or enzymes.
The Cellulose microfibrils
give tensile strength, whereas lignification in addition to making the secondary wall impermeable to water also give a "brittle" texture. Conceptually this give lignified secondary wall properties resembling armored concrete, where the cellulose microfibrils act as the armoring and the lignin as concrete.
Lignification of the secondary wall confer resistance to pathogen
s by two mechanisms. As lignin repel water, hydrolytic enzymes are less likely to attack and successfully penetrate the wall and it lowers the nutritional value of the wall, providing less energy to pathogens.
The secondary wall usually is absent under the regions of the primary wall, which contain pit pairs, giving rise to a pit cavity. (this is somewhat simplified and someone who know more about it than the author of this sentence should expand the section, e.g. describing simple and bordered pit cavities).
Wood
consists mostly of secondary cell wall, and holds the plant up against gravity.
Some secondary cell walls store nutrients, such as those in the cotyledon
s and the endosperm
. These contain little cellulose, and mostly other polysaccharides.
Plant cell
Plant cells are eukaryotic cells that differ in several key respects from the cells of other eukaryotic organisms. Their distinctive features include:...
s, located between the primary cell wall and the plasma membrane. The cell starts producing the secondary cell wall after the primary cell wall is complete and the cell has stopped expanding.
The secondary cell wall consists mainly of cellulose
Cellulose
Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to over ten thousand β linked D-glucose units....
, but also other polysaccharides, lignin
Lignin
Lignin or lignen is a complex chemical compound most commonly derived from wood, and an integral part of the secondary cell walls of plants and some algae. The term was introduced in 1819 by de Candolle and is derived from the Latin word lignum, meaning wood...
, and glycoprotein
Glycoprotein
Glycoproteins are proteins that contain oligosaccharide chains covalently attached to polypeptide side-chains. The carbohydrate is attached to the protein in a cotranslational or posttranslational modification. This process is known as glycosylation. In proteins that have segments extending...
s. It sometimes consists of three distinct layers - S1, S2 and S3 - where the direction of the Cellulose microfibrils
Cellulose
Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to over ten thousand β linked D-glucose units....
differs between the layers. Apparently there are no Structural proteins or enzymes in the secondary wall.
The secondary cell wall has different ratios of wall constituents compared to the primary wall. An example of this is that wood secondary walls contain xylans, whereas the primary wall contains xyloglucans and the cellulose fraction is higher in the secondary wall. Pectin
Pectin
Pectin is a structural heteropolysaccharide contained in the primary cell walls of terrestrial plants. It was first isolated and described in 1825 by Henri Braconnot...
s may also be absent from the secondary wall and apparently it contain no Structural proteins or enzymes.
The Cellulose microfibrils
Cellulose
Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to over ten thousand β linked D-glucose units....
give tensile strength, whereas lignification in addition to making the secondary wall impermeable to water also give a "brittle" texture. Conceptually this give lignified secondary wall properties resembling armored concrete, where the cellulose microfibrils act as the armoring and the lignin as concrete.
Lignification of the secondary wall confer resistance to pathogen
Pathogen
A pathogen gignomai "I give birth to") or infectious agent — colloquially, a germ — is a microbe or microorganism such as a virus, bacterium, prion, or fungus that causes disease in its animal or plant host...
s by two mechanisms. As lignin repel water, hydrolytic enzymes are less likely to attack and successfully penetrate the wall and it lowers the nutritional value of the wall, providing less energy to pathogens.
The secondary wall usually is absent under the regions of the primary wall, which contain pit pairs, giving rise to a pit cavity. (this is somewhat simplified and someone who know more about it than the author of this sentence should expand the section, e.g. describing simple and bordered pit cavities).
Wood
Wood
Wood is a hard, fibrous tissue found in many trees. It has been used for hundreds of thousands of years for both fuel and as a construction material. It is an organic material, a natural composite of cellulose fibers embedded in a matrix of lignin which resists compression...
consists mostly of secondary cell wall, and holds the plant up against gravity.
Some secondary cell walls store nutrients, such as those in the cotyledon
Cotyledon
A cotyledon , is a significant part of the embryo within the seed of a plant. Upon germination, the cotyledon may become the embryonic first leaves of a seedling. The number of cotyledons present is one characteristic used by botanists to classify the flowering plants...
s and the endosperm
Endosperm
Endosperm is the tissue produced inside the seeds of most flowering plants around the time of fertilization. It surrounds the embryo and provides nutrition in the form of starch, though it can also contain oils and protein. This makes endosperm an important source of nutrition in human diet...
. These contain little cellulose, and mostly other polysaccharides.