Download Solutions - ISpatula

Flaccid differs from the word wilted ; flaccid means that the plant has a
solute concentration but the cell wall does not exert any pressure on the
protoplast to prevent the loss of water ( not turgid).
First we have an initial flaccid cell
Because there is no pressure exerted by the cell wall so the pressure
potential equals zero.
Solute potential = -.07 MPa
Water potential = pressure potential + solute (osmotic) potential
= - .07 MPa
We took this cell and put it in two different environments.
The 1st one contains solutes (sucrose solution) of .4 Molar.
The other contains pure water.
we notice that the container is opened meaning that there is no pressure.
Solute concentration of .4 Molar equal -.9 MPa
Water potential in 1st environment = o-.9= -.9 MPa “it is normal to have a
negative result since we decreased the amount of the water”.
In the 2nd environment :
We also have no pressure exerted on the surface of water so pressure
potential equals zero, and we have no solutes meaning that solute
potential equals zero.
So water potential = zero.
When we insert the flaccid cell which has a solute potential that = -.7 MPa
in the 1st environment, we see that the solute concentration of it is higher
so that the direction of osmosis will from the inside if the cell to the outside.
“Water moves from the region of higher water potential to the region of
lower water potential”
The water will move out from the vacuole leading to the shrinkage of the
cell`s protoplast, the cell membrane will be away from the cell wall. This
process continues till the cell becomes plasmolysed.
In the plasmolysis there is no pressure on the cell wall so pressure potential
= 0 and the solute potential will be equaled to the solute potential outside
=-.9 MPa so water potential = -.9 MPa.
When water potential inside equals water potential outside, net movement
of water will be zero “water that enters the cell equals the water that
moves out”.
Plasmolysis leads to the death of the cell unless we put it in a good
environment.
In the 2nd environment, pure water has a higher water potential that the
cell meaning that the direction of osmosis will be from the outside to the
inside of the cell. The vacuole will be filled of water that starts pushing the
protoplast, the cell membrane will push the inner wall ( inner site of the
cell wall).
The pressure exerted by the water here on the inner site of the cell wall is
called the osmotic pressure.
The cell wall is partially elastic meaning that at a moment it will start
exerting a pressure opposing the osmotic pressure (turgor pressure).
When the turgor pressure = the osmotic pressure
Pressure potential will = .7 MPa
Osmotic (solute) potential =-.7 MPa
So water potential =0 MPa
The cell in this case becomes turgid (very firm); it has no skeleton but the
existence of water gives it support.
So the healthiest condition for a plant is to be put in a hypertonic solution.
Transport at the tissue or organ level:
We consider it as a short distance transport.
When looking at the plant, we assume that it consists of two compartments
1- The cell wall and the extracellular spaces that are connected
together.
We call this compartment the apoplast “anything existed outside the
living part of the cell, and the living part is the cytoplasm that
contains enzymes and other things”.
The apoplast compartments include the cell wall “that contains
cellulose, polysaccharides and proteins”, extracellular spaces
“spaces between cells and the interior of dead cells such as vessel
elements and tracheids.
There is continuity between the apoplastic compartments.
2- The symplastic compartments “ the symplast”
When we say symplast we are talking about the whole entire mass
“the living cells that form the body of the plant”.
There is continuity between the symplastic compartments;
plasmodesma connects the living cells since it is considered as
cytoplasmic channels.
So the plasmodesma is the other part of the symplast.
When we talk about short distance transport, there are many steps in this
process, there are three routes:
1- Movement of water and solutes along the continuum of cell walls
and extracellular spaces “apoplastic route”.
This is applied on the cells that form the root (cortex region) to move
upward and applied on the cells of the leaf, it moves from a cell to
another in the mesophyll.
The soil solution contains water molecules and dissolved minerals,
the root will absorb it to be moved through the plant.
In the epidermal cells, most of the absorption of water and minerals
occur; many of these cells are differentiated into root hairs, modified
cells that account for much of the absorption of water by roots.
The cellulose in the inner surface of root cortex cell wall is
hydrophilic, so the soil solution in the soil will move through the cell
wall “ because water also is hydrophilic” passing in the spaces into
the root cortex.
2- The symplastic route which is through the living parts “ moving via
cytoplasm from cell to another”, but to deliver the solution in the soil
to the rest of the plant, they have to cross at first the plasma
membrane, the plasma membrane is permeable meaning that not
everything can be moved through it. For example: if we have
sodium and potassium ions, potassium will pass because there are
ion channels for it but sodium will not as there are no ion channels
for it. Do there is selection for minerals that are allowed to enter the
cytoplasm “allowed to join the symplastic route”.
3- The transmembrane route
Solution here joins the symplastic route but some of it is transported
into the protoplast of cells of the epidermis and cortex and then it is
moved inward via the symplast.
In this route we have many crossings of the plasma membrane
(repeated crossings).
Moving from the symplast to the apoplast includes crossing of the
membrane and another crossing when moving from the apoplast to
the symplast.