cell wall structure, composition and function

1. CELL CYCLE
PLANT VIRUSES1. Formation
2. Nature of cell wall
3. Structure (Gross and fine structure)
4. Thickening of cell wall
5. Theories of primary cell wall formation
6. Special structure of cell wall
7. Sculpturing of secondary cell wall

3. 1. Cellulose (carbohydrate)
2. Hemicellulose(carbohydrate)
3. Pectic substances (polymer of uronic acid which are a class of sugar acids with
both carbonyl and carboxylic acid functional groups)
4. Lignin (complex polymer comprised of aromatic alcohols.It is the most abundant
natural polymer typically found in plant cell walls.
5. Cutin, subrin and waxes
6. Gum and mucilages (are carbohydrate polymers of high molecular weight
obtained from plants)

7. 1. Basic unit of SECONDAY cell wall is micellae or microfibril (cellulosic)
2. Each microfibril consist of elementary fibril (40 cellulose
molecule)
3. Elementary fibril composed of glucose units

8. I. Annular: These are ring like thickening. In this case, ring of lignin are
deposited one above the other.
Scalariform: These are ladder like lignin thickenings.
Spiral: In this case, thickenings are arranged in spiral bands.
Reticulate: In this case, lignin thickenings form networks.
Pitted: The entire cell wall is thickened. It has only few tin-thickened
areas called pits.

10.  Mosaic growth:
 1. fibrillary texture loosened at some areas due to turgor pressure
 New microfibrills are deposited in gaps created as a result of strain created by turgor
pressure.
 growth occurs by intussusception or apposition, where new microfibrils are laid down
between the existing microfibrils of expanding wall in an interwoven manner.
 Process is regulated by proteins and harmones.
Multinet growth
1. new microfibrils are deposited over the existing one like the pages of a book.
2. In apposition, the deposition of microfibrils is usually centripetal, i.e. it occurs from
periphery towards the lumen (cavity) of cell.
3. Centrifugal growth, i.e., deposition of wall materials from cell lumen to periphery is
also observed during apposition.

11. Intuscespention apposition

12. 1. Orienation of microfibrils in secondary wall differe in different plants, in different
organs, in different layers of same cell
2. In outer and inner layers almost horizontal or low spiral
3. Central layer parallel to longitudinal axis of the cell

14. Thickenings in the cell wall
1. Perforations at end walls
2. Thickening on inner surface of cell walls
3. Casparian strips
Four different thickenings are also present in the cell wall. These are: ,
Crassulae: These are linear or crescent shaped thickenings of primal) all and middle lamellae. These are
present between the bordered pits. These are ell des eloped in the tracheids.
Trabeculae: These are rod shaped thickenings of sall. These are present in radial walls.
Cystoliths: These are stalk like outgrowths of the wall. These projects into the cell lumen. Cs stoliths are
composed of cellulose and calcium carbonate. These sometimes, completely fill the cell. Cystoliths are present
in certain parenchymatous cells and epidermis.
Warts : These are present on the inner surface of secondary all of conifer tracheids and vessels.

16. 1. Cavities or thin areas within the thick secondary wall are called pits. Pits are left during
process of secondary wall formation. Such cavities or perforations are also found in the primary
wall. These are called Primary pit fields or primary pits or primordial pits
2. Plasmodesmata (cytoplasmic strands which connect neighbouring cells) is the characteristic
feature of primary cell wall its in living cell.
3. Plasmodesmata distribution: usually in groups (in primary pits) or sometime evenly
distributed over entire wall in secondary walls, large groups of plasmodesmata traverse the pit
membrane.
4. Structure of plasmodesmta: At the centre of plasmodesmata, there occurs a tube of membrane,
termed desmotubule.
5. Desmotubule contain axial central rod.
6. A space is present in between the plasmalemma and desmotubule termed cytoplasmic annulus.

17. plasmodesmata

18. 1. From cytoplasm
2. By accumulation of cellulose microfibrill and pectic substances in the wall,
cytoplasmic connections gradually become narrow and form plasmodesmata.
Function:
1. Transport of materials
2. Relay of stimuli

19. 1. Cavities or thin areas within the thick secondary wall are called pits. Pits are left during process of
secondary all formation. Such cavities or perforations are also found in the primary wall. These are
called primary pit fields.
2. Each pit has complementary pit exactly opposite in the wall of neighbouring cell, called as pit pair
3. Each pit has cavity called as pit cavity
4. Pit cavities of pit pair are separated by pit membrane
5. Opening of pit on inner side of cell wall,on the side facing lumen of the cell is called pit aperture

20. Figure: Structure of pits

21. 1. Simple pit: The diameter of the pit cavity remains uniform in simple pit. Pit cavity opens in the
lumen of the cell. Therefore, the pits of the two sides of the common wall form a simple pit.
2. Bordered pit: secondary wall arches over the cavity of the pits. Thus the pit opening become
narrow than the pit diameter. Bordered pits of the opposite cells form bordered pit pair. The cavity
in the thick secondary wall is called pit chamber. The bordered pit opens in the cell lumen by pit
aperture. The pit aperture is circular and much narrower than the diameter of pit chamber. In
certain cases, the pit apertures are lenticular or linear. pit membrane of bordered pit develops an
oval thickening in the middle. It is called torus. Pit membrane is flexible. Pit membrane pushes
the torus. Thus the torus closes the bordered

22.  On the basis of arrangement pits are of following types
 1. Simple pit pair
 Bordered pit pair
 Half bordered pit pair
 Blind pit
 Unilateral compound pitting

23. Simple Pits Bordered Pits
1 Simple pits occurs mainly in parenchymatous cells and rarely in sclerenchymatous cells Occurs in sclerenchymatous cells only. Absent in parenchymatous cells
2
Found in medullary rays, extra-xylary
fibres, companion cells and in
tracheids of some angiosperms
Abundantly found
in vessels of
angiosperms and
tracheids of
gymnosperms and
ferns
3
Pit cavity remains in the same
diameter throughout
Size and shape of
pit cavity varies
4
Closing membrane of pit remain in
same diameter throughout
Closing
membrane of pit
varies in their
diameter during
development
5 Simple organization of pit membrane
Pit membrane
with complex
organization
6 Pit border absent
Pit border
present, formed
by the overarching

25. 2. They may appear as porous or net like, thus termed as
sieve pits
3. Found in tracheary elements (xylem)of dicot
4. Found in more developed xylem so considered to be more
advanced form of pits.
5. The pit aperture may be elliptic, round or linear, due to
wall thickening, pit chamber become small and pit canal
between inner and outer aperture become longer

26.  Arrangements of pits
 Different cells have different arrangements of pits:
 Scalariform: In this case, pits are elongated or linear and arranged in transverse
rows or ladder like.
 Opposite: In this cases pits are one line and in pairs.
 Alternate: In this case. pits of two linear rows alternate w.ith each other.