Lecture 7 (cell walls) Flashcards
Why is there a difference between plants and animals on a cellular and sub cellular level?
Plants and animals have different life strategies e.g. plants aren’t mobile compared to animals and are effectively ‘stuck’ in the environment that they are in
Plant cell components
nucleus Golgi apparatus Central vacuole Mitochondria Cell wall Chloroplasts Plasmodesmata
Protoplast
Plasma membrane and everything that is inside it excluding the cell wall
Cell wall structure
cellulose is a major component of the cell wall. It is the most abundant organic macromolecule on earth. It is a glucose polymer, highly ordered, and it forms long ribbon like structures
Cellulose forms microfibrils. These highly organised structures are strong and form a major component of both primary and secondary cell walls
In cellulose there is a 1,4 linkage/bonding between each glucose molecule and because they are so ordered you get bonding between the glucose molecules in the different chains. These then go together to make up a bigger structure known as a microfibril. Cellulose microfibrils are made of glucose bonded in such a way that it makes it very strong
Two phases in cell wall structures
Crystalline microfibrillar phase - cellulose (this is a very ordered phase)
Non-crystalline matrix - Pectic polysaccharides and hemicellulosic polysaccharides plus a network of extensin (a protein)
The plant cell wall can be separated into two distinct phases; a highly crystalline phase consisting of the microfibrils and an ‘amorphous’ phase which is the matrix in which the microfibrils are embedded.
Non-crystalline matrix - hemicellulose and pectin
Hemicellulose - a heterogeneous group of polysaccharides. Long chain of one type of sugar and short side chains (of different sugars) form a rigid structure (not as rigid as cellulose as it is not as ordered as cellulose)
Pectin - Branched polysaccharides, found in plant cell walls, where they form a matrix in which cellulose microfibrils are added. Pectin is branched, negatively charged polysaccharides. Bond water and have gel like properties.
Hemicellulose
Heterogeneous group of polysaccharides (mixture of polysaccharides) deposited along with cellulose in the cell walls of plants and fungi to provide added strength.
Pectin
Branched polysaccharides, found in plant cell walls, where they form a matrix in which cellulose microfibrils are added.
Protein - extensin
Extensin cross-linking of pectin and cellulose dehydrates the cell call, reduces extensibility and increases strength. It controls the expansion of cells.When it loses water it becomes stronger and more rigid
The extensin protein can cross link between the cellulose and the pectin and this can dehydrate the cell wall and change the properties of the cell wall so that the cell wall becomes even more rigid
Primary cell wall
In plants, a relatively thin and flexible layer that surrounds the plasma membrane of a young cell.
Synthesis of primary cell wall
Coordinated (sense and respond to environment) synthesis and delivery of (all the components to the cell wall) :
1) Cellulose microfibrils at plasma membrane (the glucose polymer cellulose is assembled into long microfibrils. The rigidity and orientation of these microfibrils controls cell expansion) (this layer becomes the cell wall)
2) Polysaccharides (pectin and hemicellulose) in the Golgi apparatus are transported to the wall in vesicles (these vesicles are filled with polysaccharides and are extruded out to where the cellulose microfibrils are)
3) Cell wall proteins (extensins) from the rough ER (in vesicles, they travel through the cell to the plasma membrane, fuse with the plasma membrane and are extruded outside)
The vesicles fuse to the plasma membrane. (exocytosis) (now cellulose microfibrils, hemicellulose, pectin and extensins are all outside.) (cell wall can now be considered the ECM)
Exocytosis
Transports material out of the cell or delivers it to the cell surface
Constitutive exocytosis releases ECM proteins, we can think of the cell wall as the ECM. In plant cells, instead of ECM proteins, the hemicellulose, pectin and extensin proteins that make up the cell wall are released
Cytoplasmic streaming
Cytoplasmic streaming involves active movement of the cytoplasmic contents due to the action of microfilaments. Microfilaments make up part of the cell’s cytoskeleton.
Chloroplasts are the site of photosynthesis therefore cytoplasmic streaming is important in gaining the maximum amount of sunlight to use in this process
Synthesis of primary cell wall - rosettes and microtubules
The cellulose-producing rosettes move parallel to the cortical microtubules
This is a highly regulated process with lots of components that give us the cell wall
Cellulose producing rosettes
produce cellulose and they sit across the cell membrane and they extrude the cellulose microfibrils to the outside (they sit inside the membrane)
Also associated with microtubules on the inside of the cell so they run down the length of the microtubule and push the plasma membrane out of it way as it runs down the length of the microtubule, it is extruding cellulose microfibrils outside of the cell
Position of microtubules determines the location of where the cellulose is laid down
Cortical microtubules
The position of microtubules determines the location of where the cellulose is laid down
Inside the cell
Middle lamella
The middle lamella is a pectin layer which cements the cell of walls of two adjoining plant cells together.
Mostly contains pectin and it is a jelly-like substance that joins two cells together
First of all, the cell excretes pectin (the middle lamella is the furthest away therefore it must be made first) to make the middle lamella and then you get laying down of the primary cell wall
Cell wall functions in regulating cell shape
influences cell morphology (which is determined by how the cell wall is put down)
Provides structural support
Prevents excessive water uptake
Cell wall function - cell morphology
Orientation of cellulose microfibrils influences cell morphology (the position of the cytoskeleton (microtubules) determines where the cellulose microfibrils are laid down and that determines cell structure)
Randomly orientated - the cell will expand equally in all directions (circle)
Right angles to the ultimate long axis to the cell - the cell will expand longitudinally along that axis (rectangle)
Therefore this means that plant cells can come in many different shapes as it is determined by the laying down of the cell wall which is determined by the microtubules
Cell wall function - provides structural support
The protoplast pushes against the cell wall. The cells become rigid and this maintains plant structure
Wilting occurs when the protoplast is not pushing against the cell wall (there is a lack of cellular pressure)
Water loss from cells reduces the protoplast volume and the protoplast does not press on the cell wall
Cell wall function - prevents excessive water uptake
Water is constantly moving around a plant in a passive process
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
Vacuole
A large membrane-enclosed organelle present in many plant cells; helps maintain turgor pressure of the plant cell, plays a limited storage role, and is also capable of a lysosome-like function in intracellular digestion.
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)
Typical mature plant cell has a single large vacuole
Osmosis
Osmosis is the movement of a solvent across a semipermeable membrane toward a higher concentration of solute (lower concentration of solvent)
A type of facilitated diffusion
Movement from a high water (low solute) concentration to a low water (high solute) concentration
Vacuoles - function in regulation of cell shape
There is high concentration 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
Uptake of water without or with a cell wall
Normal cell - Cells osmoregulate to prevent swelling or shrinking under varying conditions. Not enough water, the cell with shrink or if there is too much water, the cell could burst. In both of these cases, the cells can die
Animal cells are much more vulnerable than plant cells as they have no cell wall
Plant in hypotonic, are turgid/normal. Plant in isotonic is flaccid. Plant in hypertonic is plasmolyzed (protoplast shrinks) (a plant cell can bounce back much better from being plasmolyzed than the equivalent in an animal cell)
Secondary cell wall
In plant cells, a strong and durable matrix that is often deposited in several laminated layers around the plasma membrane and provides protection and support.
Not all plants cells have a secondary cell wall
Produced only after cell growth has stopped
Thicker and stronger than primary cell walls
Provides more structural support than primary cell wall
Secondary cell wall structure and chemical characteristics
Made up of multiple layers (three layered secondary wall) - layers are laid down at different times and with different orientations which gives strength
Microfibrils in each layer have different orientations
This strengthens the secondary wall
Chemical characteristics - more cellu.ose, less pectin, and lignin
Lignin
A strong polymer embedded in the cellulose matrix of the secondary cell wall
Second most abundant organic macromolecule with the first being cellulose
Lignin is a complex polymer
Confers strength, rigidity to the secondary cell wall and acts to exclude water (lignin presents a very technical problem as if you can’t get water in it is very hard to break down. This is good for plants as it provides a strong inert wall meaning very few things can get to the glucose in that cell wall
Sits where the pectin would sit in a primary cell wall
Secondary cell wall is very energy rich but hard to break down which is in part due to the structure of cellulose and in part due to lignin which excludes water
Secondary cell wall function
Structural support - the secondary cell wall provides structural support for specific cell types such as water transporting cells, and for the whole plant
What happens is that the secondary cell wall is laid down and then the c ell dies and then they are a hollow tube for the conduction of water and these cells will run throughout the plant (veins on a leaf) and these give strength to the leaf
How do cells with a cell wall communicate?
Plasmodesmata
Plasmodesmata
Open channels through the cell wall connecting the cytoplasm and plasma membrane of adjacent plant cells, allowing water, small molecules and some larger molecules to pass between cells.
Plasmodesmata are intercellular connections (equivalent to gap suctions in animal cells), that enable cell to cell communication
The plasma membrane is continuous
Small enough to prevent organelle movements - although ER is connected through plasmodesmata
Allows the free exchange of small molecules
What is the number of plasmodesmata affected by?
An active cell will have lots of plasmodesmata (if It is involved with lots of transport of molecules)
