Cell Walls Flashcards
What is the major component of the cell wall
Cellulose
What is Cellulose
The most abundant organic macromolecule on Earth
Glucose polymer
Highly ordered
Long, ribbon like structures
What does cellulose form
Microfibrils
Outline the two phases of the cell wall structure
Phase 1: Crystalline microfibrillar phase
Phase 2: Noncreystalline matrix
What is the crystalline microfibrillar phase
A highly ordered cellulose microfibrillar structure
What is the non crystalline matrix composed of
Pectin polysaccharides
Hemicellulose polysaccharides
Plus a network of extensin
What is Hemicellulose
A heterogeneous group of polysaccharides. Long chain of one sugar type and branched chains of another. Form a rigid structure
What is pectin
Branched, negatively charged polysaccharides. Bind water and have gel like properties
What is extensin
A protein that controls the extensibility of (expansion) of cells
How does extensin control extensibility
Extensin cross-linking with pectin and cellulose (in the cell wall) dehydrates the cell wall, reduces extensibility and increases strength
How does cellulose get to the cell wall
It is synthesised at the plasma membrane by cellulose synthase rosette that travels along cortical microtubules
How do pectin and hemicellulose (polysaccharides) reach the cell wall
They are transported from the Golgi apparatus to the cell wall in vesicles
How do cell wall proteins (extensins) get to the cell wall
Made in the rough endoplasmic reticulum and then transported to the cell wall
How is cellulose synthesised to make the cell wall
Cellulose synthase rosette move parallel following cortical microtubules (under plasma membrane) and synthesise cellulose on the exterior of the cell thus synthesising the cell wall
What is the middle lamella
A layer that cements together the primary cell walls of two adjacent plant cells. It is pectin rich
What are the 3 functions of the cell wall in regulating cell shape
Influence cell morphology
Provides structural support
Prevents excess water uptake
How does orientation o cellulose microfibrils influence cell morphology
Randomly orientated cellulose microfibrils allow cell to expand equally in all directions (equal cell wall pressure)
Ordered cellulose microfibrils will allow cell to expand perpendicular to the cellulose microfibrils (cell wall pressure prevents expansion along microfibril plane)
How does the cell wall provide structural support
Protoplasm pushes against cell wall. Cell becomes rigid and thus maintains plant structure. Many rigid cells result in rigid plant
What is wilting
Wilting occurs when the protoplast is not pushing against the cell wall (low turgor pressure) thus plant looses structure
How does cell wall prevent excessive water uptake
As water enters cell, protoplast expands and pushes against cell wall.
Pressure from cell wall limits volume of water that can be taken up.
Where is water stored in a plant cell
Central vacuole
Outline the 3 characteristics of the central vacuole
Organelle surrounded by a single membrane
Is highly selective, controlling much of what enters and leaves
Water crosses membrane via osmosis
How does the central vacuole contribute to cell shape/overall plant structure
High concentration of solutes in vacuole
Results in water uptake via osmosis
Plant cell wall limits cell uptake of water (and prevents bursting)
Plant cells build up large internal pressure that contributes to cell shape, and overall plant structure
What are the characteristics of the secondary cell wall
Not all plants have a secondary cell wall
Produced only after cell growth has stopped
Thicker and stronger than primary cell wall
Provides more structural support than primary cell wall
Outline the structure of the secondary cell wall
Made up of multiple layers
Microfibrils in each layer have different orientations
This strengthens the secondary wall
What are the chemical characteristics of the secondary cell wall
More cellulose (stronger)
Less pectin (less gel like substance)
Lignin
What is lignin
The second most abundant organic macromolecule
A complex polymer
Confers strength, rigidity to the secondary cell wall and acts to exclude water
What does the secondary cell wall provide structural support for
Specific cell types (such as water transporting cells, and for the whole plant)
How do cells with a cell wall communicate
Plasmodesmata
What are plasmodesmata
Intercellular connections that enable cell to cell communication
Outline the characteristics of plasmodesmata
The plasma membrane is continuous from cell to cell
Small enough to prevent organelle movements, although endoplasmic reticulum is connected through plasmodesmata
Allows the free exchange of small molecules
