Cell Adhesion
Tissue can be defined as a
cooperative assembly of cells and
extracellular matrix woven together to
form a multicellular unit with a
distinctive function.
The extracellular matrix is a complex
network of polysaccharides (cellulose
or glycosaminoglycans) and proteins
(collagen) secreted by cells.
For small flimsy cells to join together to
make large stable structures they must
have ways to transmit physical
stresses. There exists two strategies:
Extracellular matrix (ECM). Used
by plants and animals.
Cytoskeleton cell-cell adhesions
that connect the cytoskeletons of
neighbouring cells. Used only by
animals.
Cell Adhesion 1
, Plant Cells
In plant tissues the supportive matrix is called the cell wall.
Cellulose
The primary cell wall only is relatively
thin offering flexibility which can
expand as cells grow. The secondary
cell wall forms when cell growth stops
and the additional polymer lignin is
added making the cell wall thicker,
rigid and no longer expandable.
Cellulose is the most abundant
macromolecule on Earth made up of
long unbranched chains of linked
glucose subunits with hundred of
subunits in each cellulose molecule. 16
cellulose molecules assemble to form
a microfibril through H bonds giving
the wall its tensile strength. The tensile
strength of cellulose is comparable to
steel. It predominates in secondary cell
walls.
The plant cell walls main structural
elements are polysaccharides made
from CO2 and H2O. It contains some
proteins involved in remodelling during
growth in contrast to animal cells
which use lots of protein.
Pectin
Pectin is a structural component of the cell wall which provides resistance to
compressive forces and cross links to form a matrix with cellulose strengthening.
It is made up of long complex polysaccharide chains rich in galacturonic acid. It
Cell Adhesion 2
, can bind to cations due to its high hydration creating a space filling effect similar
to glycosaminoglycans in animal ECM. The middle lamella (rich in pectin) cements
cell walls together.
Orientation of Cellulose Microfibrils in Plant Cell Walls Through
Turgor Pressure
The driving pressure for cell growth is turgor pressure which develops as a result
of osmotic imbalance between the interior of the plant and its surroundings.
Turgor pressure is the pressure water molecules exert against the cell wall which
in a plant cell can be up to 10 atmospheres. The cell wall helps sustain this internal
pressure causing cellulose microfibrils to resist stretching. The orientation of
cellulose microfibrils influences the direction in which the cell elongates as they
form a mesh like structure that can restrict or permit growth in specific directions.
It is important during growth as it influences the tissue shape.
Microfibrils which are orientated perpendicularly to the direction of turgor
pressure resists expansion while those aligned parallel to the pressure
direction allow it. This results in cells expanding in different directions
depending on the microfibril arrangement.
Turgor pressure can influence microtubule alignment indirectly, as cells under
high turgor pressure might adjust microtubule organisation to accommodate
the directional needs for expansion, thus influencing future cellulose
microfibril orientation.
Cell Adhesion 3
Tissue can be defined as a
cooperative assembly of cells and
extracellular matrix woven together to
form a multicellular unit with a
distinctive function.
The extracellular matrix is a complex
network of polysaccharides (cellulose
or glycosaminoglycans) and proteins
(collagen) secreted by cells.
For small flimsy cells to join together to
make large stable structures they must
have ways to transmit physical
stresses. There exists two strategies:
Extracellular matrix (ECM). Used
by plants and animals.
Cytoskeleton cell-cell adhesions
that connect the cytoskeletons of
neighbouring cells. Used only by
animals.
Cell Adhesion 1
, Plant Cells
In plant tissues the supportive matrix is called the cell wall.
Cellulose
The primary cell wall only is relatively
thin offering flexibility which can
expand as cells grow. The secondary
cell wall forms when cell growth stops
and the additional polymer lignin is
added making the cell wall thicker,
rigid and no longer expandable.
Cellulose is the most abundant
macromolecule on Earth made up of
long unbranched chains of linked
glucose subunits with hundred of
subunits in each cellulose molecule. 16
cellulose molecules assemble to form
a microfibril through H bonds giving
the wall its tensile strength. The tensile
strength of cellulose is comparable to
steel. It predominates in secondary cell
walls.
The plant cell walls main structural
elements are polysaccharides made
from CO2 and H2O. It contains some
proteins involved in remodelling during
growth in contrast to animal cells
which use lots of protein.
Pectin
Pectin is a structural component of the cell wall which provides resistance to
compressive forces and cross links to form a matrix with cellulose strengthening.
It is made up of long complex polysaccharide chains rich in galacturonic acid. It
Cell Adhesion 2
, can bind to cations due to its high hydration creating a space filling effect similar
to glycosaminoglycans in animal ECM. The middle lamella (rich in pectin) cements
cell walls together.
Orientation of Cellulose Microfibrils in Plant Cell Walls Through
Turgor Pressure
The driving pressure for cell growth is turgor pressure which develops as a result
of osmotic imbalance between the interior of the plant and its surroundings.
Turgor pressure is the pressure water molecules exert against the cell wall which
in a plant cell can be up to 10 atmospheres. The cell wall helps sustain this internal
pressure causing cellulose microfibrils to resist stretching. The orientation of
cellulose microfibrils influences the direction in which the cell elongates as they
form a mesh like structure that can restrict or permit growth in specific directions.
It is important during growth as it influences the tissue shape.
Microfibrils which are orientated perpendicularly to the direction of turgor
pressure resists expansion while those aligned parallel to the pressure
direction allow it. This results in cells expanding in different directions
depending on the microfibril arrangement.
Turgor pressure can influence microtubule alignment indirectly, as cells under
high turgor pressure might adjust microtubule organisation to accommodate
the directional needs for expansion, thus influencing future cellulose
microfibril orientation.
Cell Adhesion 3
