Cellular exoskeleton or extracellular matrix (ECM) of plant cell.
Robert Hooke (1665) discovered cell wall when he observed dead empty cells in a very thin slice of cork under his microscope.
Cell wall is the thick, rigid, non-living, semi-elastic, transparent, specialized form of protective extra-cellular matrix that present outside the plasma lemma of cells.
Found in plant cells, fungal cells, some protists and prokaryotes except a few lower plants, gametes and in animal cells.
The thickness varies from 0.1 to 10/µm and xylem vessels have thickest cell wall, while thinnest cell wall found in meristematic and parenchymatous cells.
In plants, cell wall composed of cellulose micro-fibrils embedded in the matrix. Matrix is the gel-like ground substance which consists of water, hemicellulose, pectin, glycoproteins and lipids. The cell wall may have lignin for hardness, silica for stillness and protection, cut in to prevent water loss and suberin for impermeability. In fungi, cell wall composed of chitin or fungal cellulose, a polymer of NAG. In bacteria, cell wall composed of peptidoglycan which consists of polymers of NAG (N-acetyl glucosamine) and NAM (N-acetyl muramic acid) cross-linked by short peptides.
The structure of cell wall determines the architecture and function of plant cell.
A typical cell wall composed of 3-4 layers that are formed sequentially from outside to inwards are as follows: Middle lamella, Primary wall, Secondary wall & occasionally tertiary wall is present (Fig. 3.1).
(a) Middle Lamella.
It is a cementing layer present between adjacent cells but absent on the free surface of plant cells and in plasmadesmata region. Chemically it is composed of pectin (calcium and magnesium-pectate). Softening of fruits and fiber retting involves the dissolution of middle lamella.
(b) Primary Wall:
It is the first formed wall of the cell which is deposited inner to the middle lamella. Primary wall is usually thin (0.1-3.0/µm) and capable of extension. Its thickness increases with the growth of the plant cell. It grows by intussusceptions (internal growth) i.e. wall materials deposited into the existing primary wall. Cells engaged active division, photosynthesis, respiration and secretion have only primary walls.
(c) Secondary wall:
It is deposited inner to primary wall only in mature and non-dividing cells. It is generally seen in parenchyma, collenchyma, sclerenchyma, tracheids and vessels. The secondary wall grows by accretion i.e. deposited in layers. It is about 3 –l0/µm thick and consists of usually three layers, designated as S1, S2 (thickest) and S3, sometimes even more as in latex tube of Euphorbia milli. During the formation of secondary cell wall in tracheids and vessels of xylem, its constituents are deposited unevenly inner to primary wall. As a result, various patterns of secondary thickening develop such as annular, spiral, scalariform, reticulate and pitted.
(d) Tertiary wall:
It is a xylan-rich layer deposited inner to the secondary wall of some plant cells, e.g., tension wood of gymnosperms.
The primary wall and the secondary wall have the same basic structure. In both cases cellulose macro-fibrils are found embedded in the gel-like matrix. Cellulose is the chief constituent of plant cell wall. Each cellulose chain (1 -5/µm long) consists of about 2000-25000 glucose units. Nearly 100 cellulose chains arranged parallel to form minute bundle called crystalline domain or micelle (1.0 nm thick). Micelle is the smallest structural unit of cell wall. About 20-40 micelles assemble in the matrix to form a micro fibril (2.6 nm thick).
Microfibrils are synthesized on the plasma membrane by protein complexes called particle rosettes. Nearly 250 micro fibrils aggregate in bigger bundles called macro fibrils (~ 0.5 µm in diameter, may reach, 4/µm in length). A cotton fibre has 1500 macro fibrils. In primary wall micro fibrils are short, wavy and loosely scattered. In secondary wall micro fibrils are long, straight, close and parallel arranged (Fig. 3.2)
Micro fibrils are synthesized on the plasma membrane by protein complexes called particle rosettes. Matrix contains a glycoprotein called expansin which causes the loosening and expansion of cell wall by the addition of cellulose molecules to the micro fibrils.
These are the depressions in the secondary wall of plant cells.
A pit consists of:
(i) Pit chamber, the actual hole within the secondary wall;
(ii) Pit membrane, composed of middle lamella and primary walls between two adjacent pits; and
(iii) Pit aperture, an opening that communicate pit chamber with the interior of the cell. Pit membrane is permeable like primary wall. Pits of adjacent cells usually occur opposite and form a pit pair. A pit present on the free surface of cell without its corresponding partner is called blind pit (Fig. 3.3).
On the basis of the shape of pit chamber, pits are either simple or bordered. In simple pit the pit chamber has uniform width and appears one ringed in surface view. In bordered pit the pit chamber is flask-shaped with narrow pit aperture and appears bordered on surface view. Sometimes the pit membrane bears a disc-like swelling called torus, and such pit is called aspirated pit. While the cell is growing in size, the primary wall develops holes at places due to stretching are called as primary pit fields through which plasmodes-mata communicate the adjacent cells.
Plasmodesmata (Singl. Plasmodesma):
These are the fine protoplasmic channels (20-40 nm in diameter) that connect the protoplasts of adjacent plant cells (Fig. 3.4). Plasmodesmata were discovered by Tangle (1879) and studied in details by Strasburger (1901). It is roughly cylindrical and contains a fine simple or branched desmotubule which connect ER of 2 adjacent cells. Around desmotubule a cytosolic annulus is present. It is formed around SER that become trapped during cytokinesis within the cell plate. Their number is abundant in the cell walls leading towards site of intense secretion.
(i) Help in transfer of nutrients, stimuli and other material between adjacent cells,
(ii) Produce a protoplasmic continuum called symplast.
(iii) Plant virus like TMV synthesize a protein P30 that nullifies the normal regulatory mechanism of plasmodesmata.
(iv) TMV enlarges plasmodesmata in order to use this route to pass from cell to cell.
1. It provides definite shape and rigidity to cell.
2. It protects the cell from mechanical injury and attacks of pathogens.
3. It prevents the osmotic bursting of the cell.
4. The system of adjacent cell walls throughout the plant body constitue the apoplast.
5. Plasmodesmata in cell wall form a system of interconnected protoplasts called the symplast.
6. Cutin and Suberin deposits check water loss.
7. Phycocolloids (water holding substances) are extracted from the cell wall of marine algae, e.g., algin (brown algae), agar (red algae) and carrageen (red algae) are used commercially.