Cell Walls And Their Role In Regulating Plant Cell Shape Flashcards
A plant cell =
Cell wall + protoplastr
Cellulose in the cell wall key points
- the most abundant organic macromolecule on earth
- glucose polymer
- highly ordered
- long, ribbon-like structures
Cellulose forms
Microfibrils
Microfibrils - what are they / what do they do
Highly ordered structures that are strong and form a major component of primary of both primary and secondary cell walls
Two phases of cell wall structure
Phase 1: Crystalline Microbibrillar phase
- cellulose
Phase 2: Noncrystalline Matrix
- Pectin polysacchaarides
- Hemicellulose polysaccharides
Plus a network of Extensin (a protein)
Hemicellulose
A heterogenous group of polysaccarides.
- Long chain of one type of sugar and short side chains form rigid structure.
Pectin
Branches, negatively charged polysaccharides
- bind water and have gel like properties
- not as strong as hemicellulose - more like jam
- negatively charged so can bind water
The potein extension
- the extensibility (expansion) of cells can be controlled by extensin cross linking
- extension cross-linking of pectin and cellulose dehydrates the cell wall, reduces extensibility and increases strength
The protein extensin while growing / not growing
- when cell still growing, extensions are random, cell takes up water and expands
- when stopped growing, the extension cross-linking dehydrates and strengthens, much more rigid. Cant get bigger but much stronger
How the primary wall is synthesised
Co-ordinated synthesis and delivery of:
- cellulose microfibrils are synthesised at plasma membrane
- polysaccharides (pectin and hemicellulose) in the Golgi apparatus are transported to the wall in the vesicles
- cell wall proteins (extensions) from the rER
The vesicles fuse to the plasma membrane
What is rosette?
A big protein complex (enzyme) that spans the plasma membrane and miles through it leaving begins a cellulose trail that is excluded to the outside of the membrane as a cellulose
- it makes cellulose by joining together glucose molecules, creating layers of microfibrils.
Exocytosis - what is it and what type of exocytosis is needed for the making of the cell wall
- Transports material out of the cell or delivers it to the cell surface
- Constitutive exocytosis releases extracellular matrix proteins (brings pectin and stuff to the cell surface to make cell wall)
Different parts of the cytoskeleton are responsible for making different parts of the wall
The cellulose-producing rosettes moves ———- to the cortical Microtubules
Parallel
Where are cortical tubules located and how do they interact with rosettes to determine shape
- they are right up against plasma membrane
- as the rosette moves through and makes cellulose, it runs around the Microtubules
- position of Microtubules therefore determine where fibril is put down thus determining shape
Middle lemalla
- found at the other side of the primary wall to the plasma membrane
- it is a sticky substance that sticks cells together
- made of mainly pectin
3 key functions of the cell wall:
- influences cell morphology: ( cell shape - different shapes of cell performing different functions thus must be different structures)
- provides structural support: (support comes from primary but mainly secondary)
- prevents excessive water uptake
How the two different orientations of the cellulose microfibrils influence cell morphology
Randomly orientated:
- the cell will expand equally in all directions (sphare)
Right angles to the ultimate long axis of the cell
- the cell will expand longitudinally along that axis
How the cell wall provides structual support:
- protoplast expands and pushes against cell wall, as water enters the cell by osmosis.
- cells become rigid and this maintains the plant structure
- wilting occurs when the protoplast is not pushing against cell wall
- water loss from cells reduces protoplast volume and the protoplast does not press on the cell wall.
What limits the volume of water that can be taken up
Pressure from the cell wall
(Vacuoles are important in this process because they contain water and make up such a large portion of the protoplast)
Vacuole definition
An organelle surrounded by a single membrane
- highly selective, controlling much of what enters and leaves the vacuole
- water moves in the vacuoles by osmosis (passive transport)
How many vacuoles does a typical mature plant have
One
Osmosis
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
Vacuole function in regulating cell shape
- high conc 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 large internal pressure that contributes to plant structural suppport
The secondary cell wall
- produced only after growth has stopped
- thicker and stringer then primary walls
- provides more structural support then primary cell wall
(Not all plant cells have a secondary cell wall)
Secondary cell wall: structure
- made up of Multiple layers (3)
- microfibrils in each layer have different orientations
- this strengthens the secondary wall
Chemical characteristics of secondary cell wall
- more cellulose
- less pectin (hense why its stronger)
- Lignin
Lignin definition and function
- a complex polymer and is the second most abundant organic macromolecule
- confers strength, rigidity to the secondary cell wall and acts to exclude water
(Provides structural support for specific cell types such as transporting water cells, and for the whole plant)
Plasmodesmata definition
- intercellular connections, that enable cell to cell communication
How the plasmodesmata work:
- the plasma membrane is continuous
- small enough to prevent organelle movements - although endoplasmic reticulum is connected through plasmadesmata
- ALLOWS THE FREE EXCHANGE OF FREE MOLECULES
