THE CELL WALL

•The plant cell wall is a remarkable structure. It

provides the most significant difference between

plant cells and other eukaryotic cells.

•The cell wall is rigid (up to many micrometers in

thickness) and gives plant cells a very defined

shape.

•While most cells have a outer membrane, none is

comparable in strength to the plant cell wall. The

cell wall is the reason for the difference between

plant and animal cell functions. Because the plant

has evolved this rigid structure.

INTRODUCTION

•On the basis of chemical composition of cell wall there are

three types of cell wall:

1) Green Plant Cell Wall : which is made up of Cellulose.

2) Cell Wall of Fungi: made up of Chitin.

3) Bacteria Cell Wall: made up of Mucopeptide and Muramic

Acid.

•The cell wall is composed of Cellulose, fibres,

polysaccharides and proteins i.e Living Protoplast.

•It consist of the following:

Middle lamella

Primary Cell Wall

Secondary Cell Wall

Tertiary Cell Wall

•It is present between two adjacent cells.

•It is situated outside primary cell wall and is made up

of calcium and magnesium pectate.

•It acts as cement which holds the adjacent cells

together.

2. PRIMARY CELL WALL

1. MIDDLE LAMELLA

•It is present beneath Middle lamella.

•It is made up of Cellulose, Hemi-cellulose, pectic

substances, lipids, proteins, minerals, elements

and water.

3. SECONDARY CELL WALL•It is made up of Cellulose, Hemi-cellulose

and polysaccharides.

•Secondary Cell wall is deposited Lignin. It is

present beneath Primary Cell wall.

4. TERTIARY CELL WALL•Tertiary Cell wall is deposited in few cells.

•It is considered to be dry residue of

protoplast.

•Besides Cellulose and Hemi-cellulose, Xylan

is also present.

•On the whole, each cell's cell wall

interacts with its neighbours to

form a tightly bound plant

structure. Despite the rigidity of

the cell wall, chemical signals and

cellular excretions are allowed to

pass between cells.

The primary wall of cells are capable of expansion. The middle

lamella is formed during cell division and grows coordinately during

cell expansion. Contact between certain cells is maintained by the

middle lamella , and the cell corners are often filled with pectin rich

polysaccharides. In older cells the material in the cell corners is

sometimes degraded and an air space forms.

Primary (1°)

Cell walls

Secondary (2°)

Cell walls

1° walls are extensible. 2° walls are not extensible

(they are found in support

tissues, water-conducting

cells).

They have little (or no) lignin,

they may contain simple

phenolic cross-links.

They contain lignin.

They are produced by cells to

the inside of the 1°cell wall,

just outside of the cell

membrane.

COMPARISION BETWEEN CELL WALLS

Cell Wall components

1.Cellulose

2.Other carbohydrates

3.Lignin (other polyphenolics)

4.Proteins

The main components of Cell wall are as follows:

Carbohydrates

•Classified by solubilities

•Pectins – complex carbohydrates extracted in

water using Calcium chelators

oPolyuronic acids

oArabinans

oGlactans

•Hemicelluloses – soluble in 4M KOH

oXylans - common

oMannans – abundant in conifers

oArabinoglactans

•Microfibrillar components

oCellulose

oBeta 1,4 mannans – algae,Beta 1,3 xylans - algae

Non - carbohydrate

•Lignin – built of phenylpropane units that are

highly cross-linked

•Proteins –

oEnzymes of the cell wall

oStructural proteins – extensin being a very

abundant protein

The Cell wall is made of two or three structurally

independent networks

These networks interlace.

•Cellulose and cross-linking glycans

•Pectic polysaccharides

•Structural proteins and/or phenylpropanoid

network

.

One model of how the orientation of newly deposited cellulose microfibrils

might be determined by the orientation of cortical microtubules.

The large cellulose synthase complexes are integral membrane proteins that continuously synthesize cellulose microfibrils on the outer face of the plasma membrane. The distal ends of the stiff microfibrilsbecome integrated into the texture of the wall, and their elongation at the proximal end pushes the synthase complex along in the plane of the membrane. Because the cortical array of microtubules is attached to the plasma membrane in a way that confines this complex to defined membrane channels, the microtubule orientation determines the axis along which the microfibrils are laid down.

This electron micrograph of a shadowed replica from a rapidly frozen and

deep-etched cell wall shows the largely parallel arrangements of cellulose

microfibrils, oriented perpendicular to the axis of cell elongation. The microfibrils

are cross-linked by, and interwoven with, a complex web of matrix molecules.

(Brian Wells and Keith Roberts.)

w

The orientation of cellulose microfibrils in the primary cell

wall.

•Cutin – polymeric network of oxygenated

C16 and C18 fatty acids

•Inelastic and hydrophobic but NOT a significant

barrier to water loss – pathogen defense – yes

Suberin – similar but longer fatty acids, less oxygenated

And linked to phenolics – more hydrophobic than cutin

Aerial surfaces covered with waxes – extremely long

Chain fatty acids – prevents water loss

Lignin- very resistant to solubilization

40% HCl used

Made of linked phenylpropane units.

An example of

a “lignan”

component

•Is an important source of strength (rigidity) for plant cells

and, as such, supports the shape of plant cells, tissues and

organs. It is also an important barrier to pathogens and

insects.

•They generally try to breach that barrier by producing and

secreting cell wall-degrading enzymes.

•However, because the barrier is made of sugars and amino

acids, the wall itself is also food for insects and pathogens.

•Because of its composition, the wall is also a potentially

important feedstock for production of biofuels

…particularly if we can learn to operate like insects and

pathogens and take it apart efficiently.

Functions of plant cell wall

Cell walls

1.Plant cell walls provide protection against

abrasion, osmotic stress, and pathogens.

2.Microfibrils of cellulose form the fibrous

component of the cell wall.

C. The matrix of cell wall contains hemicellulose,

pectins, and hydroxyproline-rich, proline-rich, and

glycine-rich structural proteins.