PLANT CELL WALL_STRUCTURE AND COMPOSITION

PLANT CELL WALL
DR. L. JOELRI MICHAEL RAJ
ASSISTANT PROFESSOR
DEPT. OF BOTANY
ST. XAVIER’S COLLEGE (AUTONOMOUS)
PALAYAMKOTTAI
Dr. L. Joelri Michael Raj SXC

WHAT IS CELL WALL?
• It is the outer rigid protective supportive and semi transparent
covering of plant cells, fungi and some protists. Cell wall was first
seen in cork cells by Hooke in 1665.
• Its thickness varies in different types of cells from 0.1 µm to 10 µm.
• Cell wall is a non-living extracellular secretion or matrix of the cell
which is closely appressed to it. It is, however, metabolically active
and is capable of growth.

FUNCTIONS OF CELL WALL:
(i) Protects the protoplasm against mechanical injury,
(ii) Protects the cell from attack of pathogens,
(iii) Provides rigidity and shape to the cell,
(iv) Counteracts osmotic pressure.
(v) Gives strength to the land plants to withstand gravitational forces,
(vi) By its growth the wall helps in cell expansion,
(vii) Pits present in the wall help produce a protoplasmic continuum or
simplest amongst cells,

FUNCTIONS OF CELL WALL:
(viii) Walls prevent bursting of plant cells by inhibiting excessive
endosmosis.
(ix) Wall has some enzymatic activity connected with metabolism,
(x) In many cases, wall takes part in offence and defense,
(xi) Cutin and suberin of the cell wall reduce the loss of water through
transpiration,
(xii) Walls of sieve tubes, tracheids and vessels are specialised for long
distance transport,
(xiii) Some seeds store food in the form of hemicellulose in cell wall.

CHEMICAL COMPOSITION OF CELL
WALL:
1. Matrix
• Water— 60%. Hemicellulose— 5- 15% Pectic Substances-2-8%.
Lipids-0.5-3.0%. Proteins— 1-2%
2. Micro fibrils:
• Cellulose/fungus cellulose— 10-15%.
3. Other Ingredients:
• Lignin, cutin, suberin, silica (silicon dioxide), minerals (e.g., iron,
calcium, carbonate), waxes, tannins, resins, gum— variable.

STRUCTURE OF CELL WALL
• A cell wall can have upto three parts— middle lamella, primary wall
and secondary wall.
Middle Lamella:
• It is a thin, amorphous and cementing layer between two adjacent
cells. Middle lamella is the first layer which is deposited at the time
of cytokinesis. It is just like brick work of the common wall between
two adjacent rooms.
• Middle lamella is absent on the outer side of surface cells. It is made
up of calcium and magnesium pectates. The softening of ripe fruits is
caused by partial solubilisation of pectic compounds to produce
jelly-like consistency.

MIDDLE LAMELLA

PRIMARY WALL
• It is the first formed wall of the cell which is produced inner to the
middle lamella. The primary wall is commonly thin (0.1-3.0 µm) and
capable of extension.
• It grows by intussusceptions or addition of materials within the
existing wall. Some cells possess only primary wall, e.g., leaf cells,
fruit cells, of cortex and pith.

PRIMARY WALL
• Primary wall consists of a number of micro fibrils embedded in the
amorphous gel like matrix or ground substance. In the majority of
plants, the micro fibrils are formed of cellulose.
• They are synthesized at plasma membrane by particle rosettes
(terminal complexes) having cellulose synthetize enzyme.

PRIMARY WALL
• Micro fibrils are oriented variously according to the shape and
thickening of the wall. Usually they are arranged in a loose network
due to incomplete cross-linking.
• The matrix of the wall consists of water, pectin, hemicelluloses and
glycoproteins. Pectin is the filler substance of the matrix. Proteins
are structural and enzymatic.
• Protein expansin (Me Queen-Mason et al, 1992) is involved in
loosening and expansion of cell wall through incorporation of more
cellulose. Hemicellulose binds micro fibrils with matrix.

PRIMARY CELL WALL
https://www.youtube.com/watch?v=zp2WW2TYcng

SECONDARY WALL:
• It is produced in some mature cells when the latter have stopped
growth, e.g., tracheids, vessel elements, fibres, collenchyma’s.
• Secondary wall is laid inner to the primary wall by accretion or
deposition of materials over the surface of existing structure.
• It is thick (3—10 µm) and made up of at least three layers, sometimes
more (e.g., latex tube of Euphorbia milli). They are named as S1, S2,
S3, Sx, etc.

SECONDARY WALL:
• The innermost layer of the secondary wall is sometimes distinct both
chemically as well as in staining properties due to the presence of
xylans. It is then called tertiary wall, e.g., tension wood in
gymnosperms.
• Secondary wall may be absent, irregularly deposited or formed
uniformly in the cells.
• This results in differentiation of cells— parenchyma, collenchyma,
sclerenchyma, tracheids and vessels.

CELL WALL STRUCTURE

SECONDARY WALL
• The composition of secondary wall is basically similar to the primary
wall in having cellulose micro-fibrils embedded in a matrix of pectin
and hemicelluloses.
• Cellulose micro-fibrils of the secondary wall lie close, parallel and at
an angle to the longitudinal axis of the cell.
• Their orientation is different in the different layers of the secondary
wall. A number of different materials may be deposited in the wall.

DIFFERENT MATERIALS DEPOSITED IN THE
SCONDARY CELL WALL
The important ones are:
(a) Lignin:
It reduces the water content of the wall matrix and increases its
hardness. However, water permeability is not affected. The
characteristic of lignification’s (and cutinisation) has evolved with the
evolution of land plants,
(b) Suberin:
The wall of cork and endodermal cells contains a special fatty
substance called suberin. Suberin makes the walls impermeable,

DIFFERENT MATERIALS DEPOSITED IN THE
SCONDARY CELL WALL
(c) Cutin:
The epidermal cells possess another fatty substance called cutin. Cutin
is also laid as a distinct layer on the outside of the epidermal cell walls.
It is known as cuticle. Cutin reduces the rate of epidermal or surface
transpiration. Other substances which can be deposited in the cell wall
are silica (e.g., grasses), minerals, waxes, tannins, resins, gums, etc.

PLASMODESMATA
• Plasmodesmata (singular- plasmodesma) are cytoplasmic bridges
between adjacent plant cells which develop in the minute pores of
their walls. They form a protoplasmic continuum called symplast.
• Cell wall and intercellular spaces form a non-living component of
the plant body called apoplasm.
• A plasmoderma is 40—50 nm in diameter. It may be simple or
branched

PLASMODESMATA
• Plasmodesma is lined by plasma membrane. It encloses tubular
extension of endoplasmic reticulum called desmotubule.
• The space between desmotubule and plasma membrane contains 8-
10 microchannels.
• Plasmodesmata form channels for controlled passage of small sized
particles between adjacent cells as well as transfer of some specific
signals.

DIFFERENCE BETWEEN THE PRIMARY AND
SECONDARY CELL WALL

PITS
• Pits are un-thickened areas in the secondary walls of plant cells.
They, therefore, appear as depressions. Pits generally occur in pairs
on the wall of two adjacent cells. A pit has a cavity or pit chamber
and a pit membrane.
• The pit membrane consists of primary wall and middle lamella. Pits
are of two types, simple and bordered.

PITS
• Simple pit has uniform width of the pit chamber.
• In bordered pit, the pit chamber is flask-shaped because the
secondary wall overarches its mouth.
• Pit membrane is permeable. It may have minute sub-microscopic
pores. Therefore, pits help in rapid translocation between two
adjacent cells.

PITS

DIFFERENCE BETWEEN CELLULOSE AND AMYLOSE

Cellulose is derived from D-glucose units,
which condense through β(1→4)-glycosidic bonds.
This linkage motif contrasts with that for α(1→4)-glycosidic
bonds present in starch and glycogen. Cellulose is a
straight chain polymer.

CELL WALL 3D
https://www.youtube.com/watch?v=3P6Ycqf9bqI&t=190s

PLANT CELL WALL_STRUCTURE AND COMPOSITION

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