plant phsiology

3. Plant cell wall and membrane
Dr. Baljinder Singh Gill
Panjab University
Chandigarh

5. • The cell wall is a tough, flexible but sometimes fairly rigid layer
that surrounds some types of cells.
• It is located outside the cell membrane and provides these cells
with structural support and protection, in addition to acting as a
filtering mechanism.
cell wall

6. MAJOR FUCTION OF CELL WALL
maintaining/determining cell shape,
mechanical strength
As a natural product, the plant cell wall is used commercially in the
form of paper, textiles, fibers (cotton, flax, hemp, and others), charcoal,
lumber, and other wood products. Another major use of plant cell walls
is in the form of extracted polysaccharides that have been modified to
make plastics, films, coatings, adhesives, gels, and thickeners in a huge
variety of products

7. • A major function of the cell wall is to act as a pressure vessel, preventing
over-expansion when water enters the cell. Cell walls are found in plants
and prokaryotic cells but not in mycoplasmas.
• The material in the cell wall varies between species, and can also differ
depending on cell type and developmental stage.
cell wall

8. • The cell wall serves a similar purpose in organisms that
possess them.
• The wall gives cells rigidity and strength, offering protection
against mechanical stress.
• The cell wall also limits the entry of large molecules that
may be toxic to the cell.
property

9. • The primary cell wall of most plant cells is semi-permeable and permits the
passage of small molecules and small proteins, with size exclusion
estimated to be 30-60 kDa.
• Key nutrients, especially water and carbon dioxide, are distributed
throughout the plant from cell wall to cell wall in apoplastic flow.
• The pH is an important factor governing the transport of molecules through
cell walls.
permeability

10. • The walls of plant cells must have sufficient tensile strength to withstand
internal osmotic pressures of several times atmospheric pressure that result
from the difference in solute concentration between the cell interior and
external water. Plant cell walls vary from 0.1 to 10 µm in thickness
Composition
• xylan, 20-35%, a type of hemicellulose
• lignin, 10-25%, a complex phenolic polymer that penetrates the spaces in
the cell wall between cellulose, hemicellulose and pectin components,
driving out water and strengthening the wall.
plant cell walls

11. Components of Plant Cell Wall
Plant cell wall composed of three layers.
1. The middle lamella
2. The primary cell wall
3. The secondary cell wall

12. The primary wall and
middle lamella
account for most of
the apoplast in
growing tissue. The
symplast is another
unique feature of
plant tissues. This
self-contiguous
phase exists because
tube-like structrues
known as
plasmodesmata
connect the
cytoplasm of
different cells.

14. The cell wall of Arabidopsis
thaliana. Transmission electron
micrograph of WT Arabidopsis
thaliana

15. Composition of Plant Cell Wall
• In the primary plant cell wall, the major
carbohydrates are cellulose, hemicellulose and
pectin.
• The outer part of the primary cell wall of the
plant cell wall is made of wax and cutin and is
known as plant cuticle.
• The secondary cell wall contains cellulose,
xylan, lignin. And one type of hemicellulose.

16. CELL WALL
gatekeeper, shape and rigidity
There are holes all over the cell wall called plasmodesmata. These
are holes that allow for nutrients to enter the cell as well as waste to
exit the cell. These small holes can cause the cell to lose water, and
this is when the plant will start to droop. But as soon as the plant can
get a drink, it will bounce right back up to its proper shape.

17. Primary wall composition and architecture
Composed predominantly of polysaccharides,
lesser amounts of structural glycoproteins, phenolic esters ionically and
covalently bound minerals (e.g. calcium and boron), and enzymes.
The major polysaccharides in the primary wall are
Cellulose - 1,4-linked -β D-glucose
Hemicellulose-xyloglucan-1,4-linked -β D-hexosyl residues
Pectin -1,4-linked -α D-galacturonic acid.

18. The covalently cross-linked model
Peter Albersheim and colleagues in 1973 proposed that the wall matrix
polymers (xyloglucan, pectin, and glycoprotein) are covalently linked to
one another. The binding of xyloglucan to cellulose microfibrils results
in a non-covalently cross-linked cellulose-hemicellulose network that
gives the wall tensile strength. This model has been questioned because
of the lack of evidence for the existence of covalent linkages between
xyloglucan, pectin and glycoprotein.

20. The tether model
Xyloglucan molecules are
hydrogen bonded to and cross-
link cellulose microfibrils. The
cellulose-xyloglucan network is
emeshed in a non-covalently
cross-linked pectic network
The diffuse layer model
Xyloglucan molecules are hydrogen
bonded to the surface of cellulose
microfibrils but do not directly cross
link them. The tightly-bound
xyloglucan is surrounded by a layer of
less-tightly bound polysaccharides. The
cellulose and xyloglucan are embedded
in a pectic matrix.

21. The stratified layer model
Xyloglucan molecules are
hydrogen bonded to and
cross-link cellulose
microfibrils. The cellulose-
xyloglucan lamellae are
separated by strata of pectic
polysaccharides.

22. Cellulose microfibrils are coated with hemicelluloses (such as xyloglucan), which may also cross-link the
microfibrils to one another. Pectins form an interlocking matrix gel, perhaps interacting with
structuralproteins. (From Brett and Waldron 1996.)

23. Cellulose Microfibrils Are Synthesized at the Plasma Membrane

24. From: Biochemistry and Molecular Biology of plants

26. The middle lamella - It is first layer formed during cell division.
This layer is rich in pectin. It is the outermost layer, joins together
adjacent plant cells and holds them together
The primary cell wall - It is formed after the middle lamella. It is
composed of pectin compounds, hemicellulose and glycoproteins.
The secondary cell wall - It is a thick layer formed inside the
primary cell wall. It is extremely rigid and provides strength. It is
composed of cellulose, hemicellulose and lignin.

27. Plant cell wall
Varied architecture- stained tissue
appearance and composition vary in
different cell e.g cell wall of
parenchyma of pith and cortex thin.
And epidermal cells, collenchyma,
phloem fibers, xylem tracheary
elements, and other forms of
sclerenchyma have thicker, multilayered
walls
Individual cell vary e.g stomata pores

28. Primary cell walls from onion parenchyma. (A) This surface
view of cell wall fragments was taken through the use of
Nomarski optics
Secondary walls are the cell walls that form after cell growth
(enlargement) has ceased. Xylem cells strengthened by
Lignin.

29. The much thicker and stronger secondary wall which accounts for
most of the carbohydrate in biomass, is deposited once the cell has
ceased to grow. The secondary walls of xylem fibers, tracheids, and
sclereids are further strengthened by the incorporation of lignin.
The secondary walls of woody tissue and grasses are composed
predominantly of cellulose, lignin, and hemicellulose
The middle lamella (plural lamellae), high in pectin and contains
different proteins compared with the bulk of the wall.

32. Cellulose Microfibrils Are Synthesized at the Plasma Membrane

33. Matrix Polymers Are Synthesized in the Golgi and
Secreted in Vesicles
The enzymes responsible for synthesis are sugar-nucleotide
polysaccharide glycosyltransferases transfer monosaccharides from
sugar nucleotides to the growing end of the polysaccharide chain.
Unlike cellulose, which forms a crystalline microfibril, the matrix
polysaccharides are much less ordered and are often described as
amorphous

35. new wall polymers are continuously synthesized and secreted at the
same time that the preexisting wall is expanding. Wall expansion may
be highly localized (as in the case of tip growth) or evenly distributed
over the wall surface (diffuse growth).
tip growth is characteristic of root hairs and pollen tubes
fibers, some sclereids, and trichomes intermediate between diffuse
growth and tip growth

39. Plasma membrane
• Contact between cell and environment
• Keeps useful materials inside and
harmful stuff outside
• Allows transport, communication in
both directions

40. Plasma membrane components
 Phospholipid bilayer
 Cholesterol
 Proteins
 Glycocalyx

41. polar
head
nonpolar
tails
P –
Phospholipid bilayer
hydrophobic molecules hydrophilic molecules
cytosol

42. THE PLASMA MEMBRANE
phospholipids cholesterol
cytoskeleton
peripheral
protein
integral
protein
Cholesterol blocks some small molecules, adds fluidity

45. • Membrane Proteins
– span entire membrane or lie on
either side
– Purposes
• Structural Support
• Recognition
• Communication
• Transport

46. • Glycocalyx
– Composed of sugars protruding from
lipids and proteins
– Functions
• Binding sites for proteins
• Lubricate cells.
• Stick cells down.

47. • selectively permeable
• hold teams of enzymes
Membranes organize the chemical
activities of cells
Cytoplasm
→
→

48. • Many membrane proteins are enzymes
• Some proteins function as receptors for
chemical messages from other cells
– The binding of a messenger to a receptor may
trigger signal transduction
Enzyme activity Signal transduction
Messenger molecule
Receptor
Activated
molecule

49. Surfaces allow exchange of signals and molecules.
• Plant cells connect by plasmodesmata
Cell surfaces protect, support, and join cells

50. Vacuole
Layers of one
plant cell wall
Walls of
two
adjacent
plant cells
PLASMODESMATA
Cytoplasm
Plasma membrane

51. Pure phospholipid bilayers are permeable to gases, such as O2
and CO2, but they are barely permeable to water and nearly
impermeable to inorganic ions and other hydrophilic solutes,
such as sucrose and amino acids. Proteins are required to
transport protons, inorganic ions, and organic solutes across the
plasma membrane and the tonoplast at rates sufficient to meet
the needs of the cell.