Many plant cells have walls that are strong enough to withstand the osmotic pressure from the difference in solute concentration between the cell interior and distilled water. Plant cell walls vary from 1/10 to several µm thick. The cell wall gives a definite shape and protects the protoplasm. It is non-living and is permeable. Adjacent cells in the plant body remain inter-connected by plasmodesmata. Cell wall composed chiefly of cellulose.
Cell Wall Layers
Cell wall consists of 3 parts
1. Middle lamella:
- It chiefly consists of pectic acid in the form of Ca and Mg salts
- Pectic acid is a large polygalacturonic acid compound in which α-D-galacturonic acid units are joined together by glycosidic linkages in (1:4).
- It is hydrophilic.
2. Primary wall:
- Chiefly consists of cellulose, a long straight chain polysaccharide in which β-D-Glucose units are joined together by glycosidic linkages in (1:4).
- An association of about 100 cellulose chains is termed as a Micelle, 20 micelles constitute a microfibril while aggregation of 250 microfibrils is called fibril.
- Cellulose is strongly hydrophilic.
- Besides cellulose, the primary wall also contains lignin, hemicellulose, some pectic substances and proteins which form the amorphous matrix.
3. Secondary Wall:
- It is more pronounced in dead cells such as tracheids and sclerenchyma.
- If chiefly consists of cellulose and lignin.
- Three distinct layers have been observed in the secondary wall, each having a different but definite orientation of the cellulose strands.
Comparison of some characteristics of Primary and Secondary Wall
|S.No.||Primary Wall||Secondary Wall|
|1||Extensible layer||Non-Extensible layer|
|2||The dispersed texture of microfibril||Parallel texture of microfibrils (to the long axis)|
|3||Smallest cellulosic unit micelle or microfibril||Smallest Cellulose unit microfibril or fibril|
|4||Cellulose (5.2%)||Cellulose (50 to 94%)|
|5||Lipid content (5 to 10%)||Normally no lipid|
|6||Proteins (5%)||Proteins deficient|
|7||Hemicellulose (50%)||Hemicellulose (25%)|
The composition of Cell Wall
In the primary (growing) plant cell wall, the significant carbohydrates are cellulose, hemicellulose, and pectin. The cellulose microfibrils are linked via hemicellulosic tethers to form the cellulose-hemicellulose network, which is embedded in the pectin matrix. The most common hemicellulose in the primary cell wall is xyloglucan. In grass cell walls, xyloglucan and pectin are reduced in abundance and partially replaced by glucuronoarabinoxylan, hemicellulose. Primary cell walls characteristically extend (grow) by a mechanism called acid growth, which involves the turgor-driven movement of the strong cellulose microfibrils within the weaker hemicellulose/pectin matrix, catalysed by expansin proteins. The outer part of the primary cell wall of the plant epidermis is usually impregnated with cutin and wax, forming a permeability barrier known as the plant cuticle.
Secondary cell walls contain a wide range of additional compounds that modify their mechanical properties and permeability. The major polymers that makeup wood (largely secondary cell walls) include:
a type of hemicelluloses
Lignin, 10-25%, a complex phenolic polymer that penetrates the spaces in the cell wall between cellulose, hemicellulose and pectin components, driving out water and strengthening the wall.
Additionally, structural proteins (1-5%) are found in most plant cell walls; they are classified as hydroxyproline-rich glycoproteins (HRGP), arabinogalactan proteins (AGP), glycine-rich proteins (GRPs), and proline-rich proteins (PRPs). Each class of glycoprotein is defined by a characteristic, highly repetitive protein sequence. Most are glycosylated, contain hydroxyproline (Hyp) and become cross-linked in the cell wall. These proteins are often concentrated in specialised cells and cell corners. Cell walls of the epidermis and endodermis may also contain suberin or cutin, two polyester-like polymers that protect the cell from herbivores. The relative composition of carbohydrates, secondary compounds, and protein varies between plants and between the cell type and age.
Plant cells walls also contain numerous enzymes, such as hydrolases, esterases, peroxidases, and transglycosylases, that cut, trim and cross-link wall polymers.
The walls of cork cells in the bark of trees are impregnated with suberin, and suberin also forms the permeability barrier in primary roots known as the Casparian strip. Secondary walls – especially in grasses – may also contain microscopic silica crystals, which may strengthen the wall and protect it from herbivores.
Cell walls in some plant tissues also function as storage depots for carbohydrates that can be broken down and resorbed to supply the metabolic and growth needs of the plant. For example, endosperm cell walls in the seeds of cereal grasses, nasturtium, and other species, are rich in glucans and other polysaccharides that are readily digested by enzymes during seed germination to form simple sugars that nourish the growing embryo. Cellulose microfibrils are not readily digested by plants, however.
Formation of Cell Wall
The middle lamella is laid down first, formed from the cell plate during cytokinesis, and the primary cell wall is then deposited inside the middle lamella. The actual structure of the cell wall is not clearly defined, and several models exist – the covalently linked cross model, the tether model, the diffuse layer model and the stratified layer model. However, the primary cell wall can be defined as composed of cellulose microfibrils aligned at all angles. Microfibrils are held together by hydrogen bonds to provide high tensile strength. The cells are maintained along and share the gelatinous membrane called the middle lamella, which contains magnesium and calcium pectates (salts of pectic acid). Cells interact though plasmodesma(ta), which are inter-connecting channels of cytoplasm that connect to the protoplasts of adjacent cells across the cell wall.
In some plants and cell types, after a maximum size or point in development has been reached, a secondary wall is constructed between the plasma membrane and primary wall. Unlike the primary wall, the microfibrils are aligned mostly in the same direction, and with each additional layer, the orientation changes slightly. Cells with secondary cell walls are rigid. Cell to cell communication is possible through pits in the secondary cell wall that allow plasmodesma to connect cells through the secondary cell walls.