LEC 4: THE CELL WALL Flashcards
What scientist was the first to report what a cell looked like?
Robert Hooke
How did scientists FIRST differentiate plant and animal cells?
Plant cells had a distinctive cell wall surrounding the cell content while animal cells did not.
What are the 4 main functions of the cell wall
- provide rigidity/structure to the cell AND the plant
- limit the expansion of the cell
- create turgor pressure in vacuoles and in the cell
- are a physical and live barrier for metabolic activity
What is a plant cell that does not have a cell wall called?
Protoplast
True or False: Cell walls are degraded while the nuclear genome of the plant replicates and divides
False, the cell wall stays in tact as chromosomes separate
What ancestral organisms had/have cell walls?
- Bacterial ancestors
- Green algae
- Land plants
- Fungi
What is the cell wall of bacteria made of?
Peptidoglycan
Do green algae and land plants have the same cell wall composition?
No, although green algae and land plants have a cell wall made of cellulose land plants have more complex proteins and molecules that accompany the wall
What is the cell wall of fungi made of?
Chitin-glucan
What is the primary cell wall of plant cells made of?
cellulose, hemicellulose, pectin
What is cellulose?
A long string of polymeric compounds that make microfibrils
How are cellulose and hemicellulose alike, but different from pectin?
Cellulose and hemicellulose are both hydrophobic while pectin is hydrophilic
What function does hemicellulose hold in the construction of the cell wall?
Hemicellulose tethers itself to cellulose to limit the extensibility of the cell wall and regulate cell expansion.
How are pectin and ripe fruits related?
Pectin structures change as the plant ripens, causing the pectin to break down making the fruit easier to eat (more elastic)
What function does pectin serve in the cell wall?
Its hydrophilic properties allow the cell to be elastic and mobile, but simultaneously regulates how elastic the cell can be
What is the Middle Lamella’s function in plants?
It acts like a glue between adjacent cells
What is the middle lamella made of?
Pectin
What use does a secondary cell wall serve?
It adds rigidity to cells that need to be structurally withstanding (xylem, woody tissues etc)
What other molecules does the secondary cell wall contain?
Lignin, suberin and cutin
What are two components of the primary cell wall that are essential to the plant?
- Primary cell walls have enzymes that complete metabolic functions
- Primary cell walls have chemicals that can be toxins used to defend the plant from invaders
What is the most abundant macromolecule in the world?
Cellulose
What is the basic unit of cellulose? What linkage binds these units?
D-glucose units are bound by a Beta 1-4 linkage
What term is given to units of glucose that are strung together and aligned to form threads?
Microfibrils
What happens when glucose units align perfectly in the cell wall?
Crystalline structures are formed
What are the crystalline structures formed by glucose units called?
Micelles
What allows plants to withstand the strength of turgor pressure induced by vacuoles?
The rigidity of the cell wall
What differentiates hemicellulose from cellulose?
Cellulose can perfectly align to create a straight chain of glucose units.
Hemicellulose has hydrogen bonds between units that create kinks
What creates limited extendibility in the cell wall?
The zig-zagged arrangement of hemicellulose around cellulose
What makes pectin such a dynamic component of the cell wall?
It creates elasticity for the cell wall but also limits that plasticity
How does pectin encourage plasticity?
Hydrophilic properties of pectin allow the cell to elongate and become elastic
How does pectin discourage plasticity?
Calcium cross linkages provided by pectin stop the cell from stretching any further than necessary
A cell is able to pass material to another cell through what structure?
Plasmodesmata
What connects two adjacent cells together?
The pit membrane along the middle lamella
A shared endoplasmic reticulum between two cells is called what?
Desmotubule
What must occur (mostly) before a secondary cell wall forms?
The cells must stop growing or die
What is the secondary cell wall made of?
Cellulose, hemicellulose
What does the primary cell wall have that the secondary cell wall does not have? Why?
Pectin, proteins, plasmodesmata, pit membrane.
Because cells are not alive, they no longer need to be elastic or complete metabolic functions. Since they do not need to exchange material and no longer need to be connected there is no plasmodesmata or pit membrane
What creates rigidity in primary and secondary cell walls?
The orientation of cellulose microfibrils
If cellulose microfibrils are arranged amorphously, what shape will the cell take?
Amorphous = no direction. Cell will be spherical
If cellulose microfibrils are arranged in a horizontal direction, what shape will the cell take?
The cell will grow vertically upward
What molecule allows the cell wall to loosen for elongation?
Expansins
What do expansins control
They control dynamic growth/elongation of the cell
What enzyme is used to generate cellulose cell wall?
Cellulose synthase
What substrate is used by cellulose synthase?
UDP glucose, or Uridine Disphosphate Glucose
What is the product of cellulose synthase and UDP glucose enzyme-substrate?
A polymeric cellulose molecule made of glucose linked with beta 1-4 linkages
What are the main substances present in the matrix of a plant cell?
Hemicellulose, pectin and glycoproteins
How are other matrix components delivered?
Secretory vesicles produced in the Golgi Apparatus
Increased pectin concentrations in the matrix of a cell creates what conditions?
More elastic conditions
Increased hemicellulose concentrations in the matrix of a cell creates what conditions?
More rigid conditions
What determines cell type/differentiation
The cells location
What is true about lignin deposition in the cell wall of plants over time?
As the plant develops more lignin is deposited to increase structural rigidity
What is an example of a plant species and toxin that holds chemicals in their cell walls?
Sanguinarine is held in the cell wall of California Poppys
What does it mean if a substance is cytotoxic?
It is toxic to the cell that produces it and to those that come in contact with it
When scientists tested sanguinarine presence in California Poppys, what did they do to prove the toxin was only present in the cell wall?
They plasmolyzed the cell - or created a hypertonic solution so the plasma membrane and vacuoles collapsed on themselves.
Since the cell was essentially empty, they could see that sanguinarine was only present in the cell wall
