Cell walls and their role in regulating plant cell shape Flashcards
Lecture 8
Major component in the cell wall
Cellulose
Describe cellulose
- The most abundant organic macromolecule on Earth
-Glucose polymer
-Highly ordered
Long, ribbon like structures
Cellulose forms
Microfibrils
Microfibrils
Highly organised structures that are strong and form a major component of both primary and secondary cell walls
Two phases of the cell wall structure
Microfibrils and matrix
What consists of microfibrils
Cellulose
What consists of matrix
- Pectin polysaccharides
- Hemicellulose polysaccharides plus a network of Extensin
Hemicellulose
Heterogeneous group of polysaccharides. Long chain of one type of sugar and short side chains that form a rigid structure
Pectin
Branched, negatively charged polysaccharides. Bing water and have gel like properties.
The extensibility (expansion) of cells can be controlled by
extensin cross linking
Effect of extensin cross linking of pectin and cellulose
It dehydrates the cell wall, reducing extensibility and increasing strength. The cell cannot expand in size
The synthesis of the primary cell wall consists of the o-ordinated synthesis and delivery of: A
Cellulose microfibrils at plasma membrane
The synthesis of the primary cell wall consists of the o-ordinated synthesis and delivery of: B
Polysaccharide (pectin and hemicellulose) in the golgi apparatus are transported to the wall in vesicles
The vesicles fuse to
the plasma membrane
The synthesis of the primary cell wall consists of the o-ordinated synthesis and delivery of: C
Cell wall proteins (extensins) from the rough endoplasmic reticulum
Exocytosis
Transports material out of the cell or delivers it to the cell surface
Constitutive exocytosis releases
extracellular matrix proteins
Cytoskeleton
A network of microtubules, microfilaments and intermediate filaments that extend throughout the cytoplasm
Functions of the cytoskeleton
Maintaining the position of organelles and providing structural support
The cellulose producing rosettes move
parallel to the cortical microtubules
Cellulose producing rosettes are
protein complexes (enzymes) that span the plasma membrane
Orientation of the cellulose microfibrils influences..
cell morphology
Cell wall functions in regulating cell shape
- Influences cell morphology
- Provides structural support
- Prevents excessive water uptake
If the cellulose microfibrils are orientated where the right angles to the ultimate long axis to the cell,
the cell will expand longitudinally along that axis
if the cellulose microfibrils are randomly oriented,
the cell will expand equally in all directions
The cell wall provides..
structural support
The protoplast pushes against the cell wall.
The cells become rigid and this maintains the plant structure
Wilting occurs when..
the protoplast is not pushing against the cell wall.
Water loss from cells reduces the protoplast volume ..
and the protoplast does not press on the cell wall
The cell wall prevents ..
excessive water uptake
As water enters the cell by osmosis,
the protoplast expands and pushes against the cell wall.
Pressure from the cell wall limits..
the volume of water that can be taken up
Vacuoles are important because..
they contain water and make up such a large portion of the protoplast.
A vacuole is..
an organelle surrounded by a single membrane
Vacuole is highly selective,
controlling much of what enters and leaves the vacuole
Water moves in the vacuoles by..
osmosis (passive transport)
Typical mature plant cell has..
a single large vacuole
Osmosis is..
the diffusion of water across a selectively permeable membrane
Movement in osmosis occurs from
a high water (low solute) concentration to a low water (high solute) concentration
There is high concentration of solutes in the vacuole. This results in…
water uptake into the vacuole by osmosis
The plant cell wall limits…
water uptake and prevents the cell bursting.
Plant cell builds up a large internal pressure that contributes to..
plant structural support
Not all plants cells have..
a secondary cell wall
The secondary cell wall is produced only after…
cell growth has stopped
The secondary cell wall is ….. than primary cell walls
thicker and stronger
The secondary cell wall provides more..
structural support than primary cell wall
The secondary cell wall structure
- Made up of multiple layers
- Microfibrils in each layer have different orientations
-This strengthens the secondary wall
Chemical characteristics of the secondary cell wall
- more cellulose
-less pectin
-lignin
Lignin is the second most
abundant organic macromolecule
Lignin is a complex
polymer
Lignin confers…
strength and rigidity to the secondary cell wall and acts to exclude water
The secondary cell wall provides structural support for:
specific cell types such as water transporting cells, and for the whole plant
Plasmodesmata
are intercellular connections, that enable cell to cell communication
Is the plasma membrane continuous
Yes
The plasmodesmata is small enough to prevent organelle movements,,
although endoplasmic reticulum is connected through plasmodesmata.
Plasmodesmata allows
the free exchange of small molecules
