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