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Plant cell walls
The Earth’s biomass is largely plant cell walls
Plants have a very
distinctive mode of
growth
Energy from sunlight &
nutrients from soil
Constantly producing
new organs &
extending their
surfaces
in air and soil
Elaboration of surfaces – also applies at
the cellular level and each plant cell has
a thick cell wall
Depicted as in a box
But this wall/box is…..
dynamic and metabolically complex
Plant cell walls
• The presence of a wall at the surface of
plant cells has great significance for plant
growth & development
• No cell movement
• Specific mechanisms of cytokinesis, cell
expansion and cell adhesion/separation
• Cell walls are a major feature of plant cell
differentiation
First plant cell component to be imaged
– least understood today
Robert Hooke's image of cork
published in Micrographia in 1665
ABUNDANT
• Over 70% of assimilated carbon from
photosynthesis ends up in cell walls
• Most abundant renewable resource on
earth ~ 100 billion tonnes of cell walls
produced annually
Wood – very
tough cell walls
0.1 mm
BIOFUELS
Cell Wall Biomass
(lignocellulose) is a
renewable resource
• Burn efficiently
• Saccharification –
enzymology of cell wall
degradation
• Fast growing perennial
grasses e.g.Miscanthus
TS Arabidopsis
flowering stem
Plant organs are
composites of two
types of cell walls
Primary
all cells,
tensile properties
generate/resist internal
turgor pressure,
prevent cell bursting
EXTENDABLE
~ cell expansion
Secondary
certain cells only,
resist compressive forces
prevent cell collapse
All plant cells have a
primary cell wall
In addition, certain cells
have secondary walls
(e.g. xylem vessels &
sclerenchyma fibres)
The images above show the same section of an Arabidopsis
stem labelled with a fluorescent probe for cellulose (blue) and a
monoclonal antibody (with fluorescent tag) to pectin (green).
The thick secondary walls at the top of the image
(sclerified parenchyma) do not contain pectin
Plant cell walls
What is the molecular nature of cell walls?
Polymers of sugar molecules
Polysaccharides – including cellulose and
pectins - used as industrial polymers,
materials and food additives
CARBON HYDROGEN OXYGEN
No nitrogen
Great complexity
More ways of linking than
amino acids in proteins
Eukaryotic cell walls are
fibrous composites:
– fibres embedded in a matrix
– components are largely
polysaccharides (glycans)
• Fibres provide tensile strength & rigidity
• Matrix components connect and spaces
fibres and control cell wall porosity
Cellulose fibre
3 nm
Polymerized glucose
Glucose
Glucuronic acid
Xylose
Galactose
Mannose
Galacturonic acid
Rhamnose
Fucose
Arabinose
• cellulose
•
•
•
•
•
•
•
•
•
heteroxylans
xyloglucans
galactoglucomannans
1,3-glucans
1,3-1,4-glucans
galacturonans
rhamnogalacturonans
xylogalacturonans
galactans
Non-cellulosic,
non-pectic
matrix glycans
pectins
GENERIC
CELL WALL
STRUCTURES
Matrix glycans
Somerville et al. (2004) Science 306, 2206
Primary cell walls
• Outside the plasma membrane
• Rigid  cell shape
• Tensile strength turgor pressure
• Extendable  plant growth
• Function in cell adhesion
Removal of cell wall with wall-degrading
enzymes - releases protoplasts
Plasma membrane
Primary cell wall
CELL
PROTOPLAST
CELL EXPANSION
Cell elongation
is a major
feature of plant
growth
Credits: John Innes Centre
During plant cell expansion:
• Cellulose-matrix glycan network is stretched
and microfibrils slide apart
• Extensibility of cell wall determined by
cross-links between microfibrils
• New cell wall material is added and the
primary cell wall is maintained at a even
thickness
• The direction of cell expansion is
determined by the orientation of the
cellulose microfibrils
– elongation at right angles to microfibrils
The orientation of the cellulose determines
how a plant cell will expand
Transversely oriented cellulose microfibrils leads to cells
expanding as cylinders, rather than spheres
Other CW factors control extent of cell expansion
Two mechanisms of turgor-driven cell growth
Diffuse growth (adhered cells, in
multicellular organs)
Tip growth (polarised growth of single, unadhered
cells)
Differential cell expansion of cells in
an organ is the basis of tropisms
Greater cell
expansion
Cytokinesis
• When a plant cell divides it does not
contract in the middle during cytokinesis
• A new wall is constructed as a partition
across a cell to produce two daughter cells
• This is the cell plate
• Primary cell wall material is deposited on
both sides & it becomes the middle lamella
• The middle lamella is the junction between
adjacent plant cells
Animal cell
Plant cell
Microtubules of the plant cytoskeleton
are involved in determining the:
– orientation of cellulose microfibrils
(interphase cortical array)
– separation of chromosomes at mitosis
(mitotic array)
– orientation of the cell plate before mitosis
(pre-prophase band)
– construction of the cell plate
(phragmoplast)
Formation of a phragmoplast
Figure 17-57 Molecular Biology of the Cell (© Garland Science 2008)
Cell plate formation
Plasma membrane
Primary cell-wall
At cytokinesis the cell
plate ‘grows’ from the
centre of the cell
The cell plate partitions
the parent cell producing
two daughter cells
As the cells grow primary wall
is deposited on both sides of
the cell plate, which becomes
the middle lamella
Intercellular space formation
New middle lamella
Cell 1
Cell 2
Middle lamella of parent cell
Cell 3
Region of primary cell wall
that is dismantled to
allow…...
…old and new middle lamellae
to link up to form intercellular
space between three cells
Cell adhesion/separation
• In plants, cell adhesion is a consequence of
the mechanism of cytokinesis
• Plant cells have fixed neighbours for life
• Cell separation is a more active & highly
regulated process in all tissues
– intercellular space
– leaf and fruit abscission
– pod dehiscence to release seed
Plasmodesmata (plasmodesma)
• Plasma membrane-lined
pores across cell walls
connecting adjacent plant
cells.
• Also contain
ER membrane
Plasmodesmata:
membrane-lined channels linking plant cells across cell walls
Plasmodesmata (plasmodesma)
• PD often grouped together in thinner
regions of cell walls called pit fields
• Transport small molecules up to 1000 mw
• Viruses exploit PD to move form cell to cell
• Evidence also for specific movement
through PD of transcription
factors/nucleic acids that can regulate
gene expression & influence meristem
development
• Plant tissue & organs can be viewed as
consisting of two continuums - separated by
the plasma membrane
• Apoplast: cell wall & intercellular space
outside the protoplast
• Symplast: inside of protoplasts cytoplasm, ER etc. linked by PD
• Phloem transport symplastic
• Xylem transport apoplastic
Secondary cell walls
• Only in certain cells: e.g. xylem, sclerenchyma fibres
• Inside primary cell walls
• Thick: three layers with differing orientation of microfibrils
Secondary cell walls
• Often lignified
– LIGNIN: a phenolic macromolecule built
up from monomers such as coniferyl
alcohol
• Hydrophobic
• Resist compression & cell collapse
Polyphenolic lignin structure - extensively cross-linked,
tough, hydrophobic polymer of secondary cell walls/wood
Secondary Cell Walls
A sclereid from pear fruit - a ‘stone cell’
• cannot extend
(deposited after
growth completed)
• Protoplast must
contract during
development
• Cells with SCWs often
dead at maturity
Cotton fibres – non-lignified SCW