lecture 7 - plant cell wall Flashcards
What does the plant cell wall enclose?
The protoplast.
What is the major component of the plant cell wall?
Cellulose
What is the structure of cellulose?
Chains of glucose (a polymer), forming ribbons
What are cellulose micro fibrils?
Layers of cellulose ribbons bonded together into strong chains
What are the 2 phases of the cell wall?
Crystalline microfibrillar phase and Non-crystalline matrix
What is the chemical component of the Cell Wall Phase 1?
Cellulose molecules (which form cellulose/fibre microfibrils)
What are the three components of Phase 2 of the plant cell wall?
Pectin and hemicellulose polysaccharides, extensin (a protein)
What is the structure of hemicellulose?
Long chains of 1 type of sugar, with oligosaccharide side chains.
What are the properties of hemicellulose?
Rigid structure, but not as strong as cellulose.
What is the structure and properties of pectin?
Branched negatively charged polysaccharides, which bind water, giving it gel-like properties.
What element of plant cell structure does extensin determine?
Extensibility (expansion)
How does extensin limit extensibility?
When extension fibres crosslink pectin and cellulose, the cell wall become dehydrated, and the cell cannot expand as the wall becomes more rigid.
Where are cellulose microfibrils synthesised?
At rosettes on the cell membrane
Where are cell wall polysaccharides synthesised?
Pectin and hemicellulose are made in the golgi, and transported via vesicles.
Where are cell wall proteins (extensin) synthesised?
In the Rough Endoplasmic Reticulum, then transported by vesicles to the cell wall.
What is the process of cellulose microfibril production?
Rosettes at the plasma membrane move parallel to the underlying cortical microtubules, forming parallel microfibrils on the other side of the plasma membrane.
How are the cell wall components structured in the wall?
Microfibrils of cellulose form layers, each with microfibrils running in different directions. Hemicellulose molecules sit around the larger microfibrils. Pectin sits within the wall as well as forming a gel-like layer between the cell walls of neighbouring cells - csalled the middle lamellae
What are the functions of the plant cell wall? (3)
Influence cell morphology, structural support, prevent excessive water uptake
How does the cell wall influence cell morphology?
The direction of microfibrils in the cell wall determines the direction of expansion when the protoplast pushes against it. Randomly/evenly oriented will allow the cell to expand in all directions into a spherical shape. If the microfibrils are at right angles to the long axis of the cell, it will expand longitudonally.
How does the cell wall provide structural support to a plant cell?
The protoplast (filled with the water filled central vacuole) pushes against the cell wall until it becomes rigid.
What is wiliting, and how does it affect the cell wall?
When water flows out of the protoplast/vacuole, there is less pressure on the cell wall, so it becomes less rigid
How does the cell wall prevent excessive water uptake by the cell?
When the vacuole absorbs large amounts of water via osmosis, the cell wall exerts pressure on the protoplast, preventing it from expanding more.
What is the central vacuole in plant cells?
A membrane bound organelle, which takes up lots of water and molecules, taking up a large amount of the protoplast.
What is osmosis, in terms of the vacuole?
The process in which water flows from a low solute concentration to a high solute concentration, allow it to flow in and out of the vacuole, expanding or diminishing it.
What are the 3 states of the cell wall, which are dependent on the water levels within the cell?
Turgid, flaccid, plasmolyzed
What does turgid mean in terms of a plant cell?
The vacuole is filled with water, so the protoplast pushes out firmly onto the cell way, making it stiff
What does flaccid mean in terms of the plant cell?
There is less water in the vacuole, so the protoplast exerts less force on the cell wall, and it does become as rigid and is flaccid
What is a plasmolyzed plant cell?
A cell which has lost a lot of water, so the cell wall loses a lot of strength.
What is the purpose of secondary cell walls?
To provide thicker, stronger structural support to specfic types of cells.
Do all plant cells have a secondary cell wall?
No
What is the structure of the secondary cell wall?
Three layers of cellulose microfibrils
* Made up of multiple layers
* Microfibrils in each layer have different orientations
* This strengthens the secondary wall
Chemical difference of secondary cell wall
new - lignin
More Cellulose
Less - pectin
What is lignin? And what does it do?
Lignin is a complex polymer
Confers strength, rigidity to the secondary cell wall and acts to exclude water
What are plasmodesmata?
Intercellular connections - gaps in the cell wall that allow molecules to pass to neighbouring cells so they can communicate.
Are the plasmodesmata small enough to prevent organelles from getting through?
Yes, except some pieces of ER slip through in parts.
What are the bits of ER passing through plasmodemata called?
Demosomes
How is the plasma membrane continuous at the plasmodesmata?
It lines the cell wall on either side and coats the sides of the plasmodesmata gaps.
What cell wall components does extensin cross link?
cellulose and pectin
