Plant cell wall: composition, summary and how it is formed. Details on the median lamella, hemicellulose, cellulose and specializations (lignin, waxes, suberin).
Plant cell wall, summary and insights. On this page we will see: how it is formed, composition, formation of the median lamella, primary and secondary wall. Synthesis of cellulose, formation of the median lamella, chemical composition and notes on mitosis.
Plant cell wall, summary
Let's start with a brief introduction that will do fromsummaryto this topic. Below we will deepen the composition and formation of theplant cell wallstarting from cell plate. Go with the summary.
Thereplant cell wallis made up of three sectors:
- median lamella
More external and is in communion between two contiguous cells. It is almost completely composed of pectic substances. It can have very low percentages of cellulose and lignin.
- Primary wall
Composed of hemicellulose, pectic substances, cellulose and 1 - 8% of structural proteins.
- Secondary wall
Composed almost exclusively of cellulose and sometimes lignin, suberin (in the case of cork) or waxes.
When the secondary wall is present, the median lamella tends to shrink or disappear completely.
Green wall, functions
Theregreen wallperforms various functions. First of all it contains the phenomenon of cell turgor while maintaining the shape of the cell (containment function).
It provides mechanical strength and forms a physical barrier to protect the cell from pathogens.
Plant cell wall: composition
Let's explore the structure of thegreen walldwelling on the various molecular structures that compose it. We continue to see how the plant cell is made.
Among the constituents of the vegetal wall we point out thehemicellulosewhich are highly branched compounds closely associated with cellulose. The composition sees the link between different sugar residues (xylose, glucose, mannose, arabinose, fucose…). Unlike cellulose, hemicellulose is digested by our digestive system.
Therecelluloseis a polymer ofbeta-D-glucosewhich binds with a bond between the carbon in position 1 and the carbon in position 4 of the subsequent residue. This structure forms very long filaments that associate with each other to form cellulose fibrils. Cellulose fibrils join together to form a microfibril, more cellulose microfibrils form a macrofibril… more macrofibrils form a cellulose fiber!
It follows that thecelluloseit is given by a highly ordered structure. These are fibrils placed parallel to each other. Intercalated between the parallel fibrils, there are fibrils placed perpendicularly which give more strength to the structure.
This highly ordered structure is linked to the microtubules that act in the synthesis phase. The synthesis of cellulose is operated by an enzymatic complex located at the level of thecell membrane. This enzyme compound (cellulose - synthase) provides a structure that "fishes" in the cytosol the substrates it transforms and polymerizes. As the beta-D-glucose chains are formed and the fibrils polymerize, they protrude outside thecell membraneand form the vegetal wall.
Pectins or pectic substances
Pectin is the polymer of galacturanic acid. Galacturanic acid is formed by the oxidation of the CH2OH group of carbon 6 of galactose. Galactose is nothing more than an isomer of glucose (isomer on carbon 4).
How the median lamella is formed? Very simple. Its synthesis involves the endoplasmic reticulum and the Golgi. We can start with a parallelism: if in theanimal cell, at the time of mitotic division a mitotic spindle is formed with consequent constriction of the mother cell, intense endomembranous activity is recorded in the plant cell.
From the Golgi, vesicles and a set of microtubules are formed right at the level of the cell division septum, this system is called fragmoplast and represents the ancestral form of the median lamella. In the last phase of mitosis, when the two daughter cells are formed, the median lamella is already present.
Unlike themedian lamella which is formed in the final stages of mitosis, theprimary wallit is formed only in the already developed cell. How is it formed? The cellulose-synthase enzymatic complex produces and releases it close to themedian lamella.
Theresecondary wall instead it is found only in mature cells.
Thereligninis more characteristic of the secondary wallbut it can also be present in the primary wall and in certain circumstances (and low concentrations) also in the median lamella. It is a polymer containing phenyl propane with a highly substituted aromatic ring so it is impossible to describe it by referring to a single molecular structure as is possible forcelluloseand thepectin.
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