lesson 5 bio101 (c)dr. evangelista
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The cell wall Functions • Support the plant against force of gravity
• Protection from desiccation
• Maintains shape of the cell
• Prevents excessive uptake of water
• Communication between cells
Classification of wall layers
1. Primary wall
2. Secondary wall
Middle lamella
Composed of Ca pectate
Commonly lignified in woody tissues
Primary wall
Contains cellulose, hemicelluloses and some pectin, may become lignified
Passes through a period of growth in surface area and possibly increase in thickness
Associated with living protoplast
Secondary wall Consists mainly of cellulose and hemicelluloses,
and lignin
Generally laid down after the 10 wall ceases to increase in surface area
Surface growth is not a characteristic of the 20 wall
Supplementary wall whose principal function is support
Associated with dead cells or highly specialized cells
Pits/primary pit fields
Primary pit fields Found in primary walls Primary wall is relatively thin but is
continuous across the pit field area Show concentrations of plasmodesmata
Pits Found in cells with 20 walls 20 wall layers are completely interrupted
at the pit
2 types of pit a. Simple pit B. Bordered pit
Pits/primary pit fields
simple pit
consists of the pit cavity and the pit membrane
may coalesce as wall thickens forming a ramiform pit
bordered pit With pit chamber and pit aperture
in gymnosperms, with torus and margo
aspirated pit-pair – the displacement of pit membranes
vestured pits
found in some dicots
outgrowths develop on the secondary walls of pits giving it a sieve-like appearance
Arrangement of bordered pits
Scalariform pitting- pits elongated or linear and form ladder-like series
Opposite pitting- pits arranged in horizontal rows; crowded pits appear rectangular in face view
Alternate pitting- pits in diagonal rows; crowding gives the borders hexagonal outlines in face view
Types of pit-pair simple pit-pair bordered pit-pair half-bordered pit-pair – a simple pit
complemented with a bordered pit blind pit- a pit without a complementary
structure /occurs opposite an intercellular space
unilaterally compound pitting- 2 or more pits opposite one pit in the adjacent cell
Plasmodesmata cytoplasmic strands interconnecting the living protoplasts
of the plant body concerned with material transport and conduction of
stimuli arise during cell division because of the persistence of ER
tubules in the organizing cell plate; the desmotubule appears solid through the plasmodesmata
plasmodesmata multiply by splitting; during growth of the wall in surface area the plasmodesmata are stretched laterally and then split by interposition of wall substance
Chemical composition of walls
cellulose hemicelluloses pectin mucilages Gums lignin (impregnation of the wall)
carbohydrate constituents
silica calcium carbonate tannins resins
Chemical composition of walls
mineral substances
organic compounds
cutin suberin fatty compounds waxes water- found in microcapillaries or
associated with hydrophilic substances
Chemical composition of walls
Microscopic and submicroscopic structure of the wall
Structural elements Cellulose – amicroscopic component Elementary microfibril- contains 100
cellulose molecules in a transection; Micelle – crystalline aggregates of cellulose
separated longitudinally by amorphous regions or regions of less perfect molecular order
Microfibril- contains 2000 cellulose molecules in transection; the basic structural unit of the cell wall
Macrofibril – contains 500,000 cellulose molecules in transection
Secondary wall of a fiber- contains 2,000,000,000 cellulose molecules
Microscopic and submicroscopic structure of the wall
Orientation of microfibrils Primary wall- when first formed shows a
predominantly transverse orientation of microfibrils but the orientation becomes more disperse as the wall increases in surface area during cell enlargement; primary wall shows an increasing degree of parallelization of microfibrils in the centripetal direction
Secondary walls have parallel texture
PRIMARY WALL
• Thin
• Orientation of microfibrils is random
SECONDARY WALL
• Parallel texture of microfibrils
Properties of the walls *Cellulose would have a major influence
upon the properties of the wall because of their abundance
1. Plasticity- property of becoming permanently deformed when subjected to changes in shape or size
e.g. permanent extension in certain stages of growth of cells in volume)
2. Elasticity- property of recovery of the original size and shape after deformation
(illustrated by the reversible changes in volume in response to changes in turgor pressure)
Properties of the walls
Properties of the walls
3. Tensile strength one of the remarkable features of the
cellulose
lignin increases resistance of walls to pressure and protects the cellulose fibrils from becoming creased
Formation of walls Cell plate – is the first evident partition between
new protoplasts; it arises in the equatorial plane of a fibrous spindle, the phragmoplast
In highly vacuolated cell The nucleus comes to occupy the region
formerly occupied by the vacuole and is surrounded by dense cytoplasm (cytoplasmic plate- phragmosome)
The phragmosome forms a living medium in which the phragmoplast and cell plate develop
Growth of walls In thickness By apposition- successive deposition of wall
material, layer upon layer
usually centripetal; centrifugal in pollen grains
By intussusception- intercalation of new particles among those existing in the wall
Structure of cell wall
• Secondary wall
-synthesized after most cell growth has ceased
-microfibrils arranged in parallel within wall lamellae
-composed of:
S1- shallow helix of microfibrils
S2- thickest layer; steep helix of microfibrils
S3- shallow helix of microfibrils
In surface area By mosaic growth – synthesis of wall material
occurs in localized regions scattered over the wall, in which the cytoplasm pushes apart the existing microfibrils and weaves in new ones.
By multinet growth- apposition of successive layers of microfibrils, with the earlier becoming modified in microfibril orientation by wall extension during cell enlargement
Growth of walls
Formation of intercellular spaces
Schizogeny- intercellular spaces result from separation of cell walls from each other
intercellular substance partly dissolved but does not disappear; lines the intercellular space
In water plants the big aerenchyma develops similarly but divides perpendicularly to the circumference of the air space; resin ducts and secretory ducts of Compositae
Lysigeny –intercellular space arises by dissolution of the cells themselves so that the breakdown products are released in the resulting cavity
e.g. secretory cavities of Eucalyptus, Citrus
Formation of intercellular spaces
rhexigenous- result from tearing or breaking of cells
Formation of intercellular spaces