Lecture 7: Cell Walls & Their Role in Regulating Plant Cell Shape Flashcards
What is the cell wall made out of?
Cellulose
What forms microfibrils?
cellulose
there highly organised structures are strong and form a major component of both primary and secondary cell walls
What are the two phases of the cell wall?
Crystalline Microfibrillar Phase
Noncrystalline Matrix
What is crystalline microfibrillar phase made up of?
cellulose
What is the noncrystalline matrix made up of?
pectin polysaccharides
hemicellulose polysaccharides
plus a network of extensin (a protein)
what is hemicellulose?
heterogeneous group of
polysaccharides. Long chain of
one type of sugar and short side chains form a rigid structure.
What is pectin?
Pectin branched, negatively
charged polysaccharides.
Bind water and have gel-like
properties.
How can the extensibility of a cell wall be controlled?
The extensibility (expansion) of cells can be controlled by extensin cross linking
Extensin cross-linking of pectin and cellulose dehydrates
the cell wall, reduces extensibility and increases strength
What is the order of the co-ordinated synthesis?
- Cellulose microfibrils at
plasma membrane - Polysaccharides (pectin &
hemicellulose) in the Golgi
apparatus are transported
to the wall in vesicles - Cell wall proteins
(extensins) from the rough
ER.
What is exocytosis?
Transports material
out of the cell or
delivers it to the cell
surface
Constitutive
exocytosis releases
extracellular matrix
proteins
Cell Wall functions in Regulating Cell Shape
The cell wall:
- influences cell morphology
- provides structural support
- prevents excessive water uptake
Orientation of the Cellulose microbirils influences cell morphology in what two ways?
a) Randomly oriented.
The cell will expand
equally in all directions
b) Right angles to the ultimate long axis of the cell. The cell will expand longitudinally along that axis.
How does the cell wall provide 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 again the cell wall
Water loss from cells reduces the protoplast volume and the
protoplast does not press on the cell wall
How does the cell wall prevent 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 in this
process because they contain
water and make up such a large portion of the protoplast.
What is a vacuole in a plant cell?
A vacuole is an organelle surrounded by a single membrane
It is highly selective, controlling
much of what enters and leaves the vacuole
Water moves in the vacuoles by osmosis (passive transport)
There is high concentrations 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 cells build up a large internal pressure that contributes to plant
structural support.
