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Cytology
Osmosis and Water
relations
Diffusion is the tendency of gaseous or aqueous
particles to spread from a more concentrated
region to a less concentrated region

Diffusion

Is the tendency for molecules of any
substance to spread out evenly into the
available space
(a) Diffusion of one solute. The membrane
has pores large enough for molecules
of dye to pass through. Random
movement of dye molecules will cause
some to pass through the pores; this
will happen more often on the side
with more molecules. The dye diffuses
from where it is more concentrated
to where it is less concentrated
(called diffusing down a concentration
gradient). This leads to a dynamic
equilibrium: The solute molecules
continue to cross the membrane,
but at equal rates in both directions.
Molecules of dye
Membrane (cross section)
Net diffusion
Net diffusion
Equilibrium

Substances diffuse down their
concentration gradient, the difference in
concentration of a substance from one
area to another
(b) Diffusion of two solutes. Solutions of
two different dyes are separated by a
membrane that is permeable to both.
Each dye diffuses down its own concentration gradient. There will be a net
diffusion of the purple dye toward the
left, even though the total solute
concentration was initially greater on
the left side.
Net diffusion
Net diffusion
Figure 7.11 B
Net diffusion
Net diffusion
Equilibrium
Equilibrium
Osmosis is the a special form of diffusion in which of water
molecules diffuse through a selectively permeable
membrane.
Net water movement occurs from a dilute region (higher
water potential) to a more concentrated region (lower water
potential).
The two conditions for osmosis
a selectively permeable membrane
 a concentration gradient / water
potential gradient

 Why
osmosis is relevant to living cell?
 Why
osmosis is relevant to living cell?

a selectively
permeable membrane

a concentration
gradient / water
potential gradient
Water Balance of Cells Without Walls

Tonicity /concentration strength of a solution:
Is the ability of a solution to cause a cell to gain
or lose water

If a solution is isotonic
 The concentration of solutes is the same as it is
inside the cell



There will be no
net movement of water
If a solution is hypertonic
 The concentration of solutes is greater than it is
inside the cell
 The cell will lose water
If a solution is hypotonic
 The concentration of solutes is less than it is
inside the cell
 The cell will gain water
ψ
Water potential
A measure of the capacity of a solution to
give out water molecules
 Water moves from a region of higher water
potential to a region of lower water
potential
 Pure water has the highest water potential.
The water potential of pure water at
standard laboratory condition is assigned

as
ψ =0
Components of Water potential
Osmotic potential:
Presence of solute lowers water
potential
Solute particles decrease the
‘free energy’ of water molecules
Components of Water potential
Osmotic potential:
Solutes lowers water potential
A more concentrated solution has a
________ water potential
The osmotic potential of pure water is 0
All aqueous solutions have a ________
osmotic potential
What produces the osmotic
strength of cells ?
cytoplasm is a relatively concentrated
solution of : sugar, mineral salts, amino
acids, protein, ………and many other
dissolved substances
Water Balance of Cells without Walls
Animals and other organisms without rigid
cell walls living in hypertonic or hypotonic
environments

Must have special adaptations for
osmoregulation
Paramecium has
contractile vacuoles to
remove excess water
Plant cell_effect of a rigid cell wall
A pressure component
The water column will stop rising at a
certain level, why?
Water movement and water
potential
Components of Water potential
Components of Water potential
Components of Water potential
Pressure potential:
hydrostatic pressure affects water
potential
Components of Water potential
Reverse osmosis:
Obtain pure water from salt water by
applying positive pressure
Water purification with reverse
osmosis
Water Balance of Cells with
a rigid cell wall

Cell walls

Help maintain water balance
A turgid cell: Cell membrane pressing
on the cell wall
Turgor pressure
Expanding cell volume pushes outward on the
cell wall – the pressure pushing on the cell wall
Rigid cell wall pushes back on the cell
membrane and prevent excessive expansion
Turgidity in plant cell
a) Water enters the cell by osmosis
b) Expansion of cell volume forcing the plasma membrane
outwards against the cell wall. A pressure develops called
the turgor pressure
c) The outward pressure is matched by an inward pressure
(wall pressure), equal in magnitude but opposite in
direction
Turgidity and support

These pressures provide mechanical support to the
plant tissue. If a plant experiences a lack of water the
cell becomes plasmolysed, wall pressure is lost and the
plant wilts
Water balance in cells with rigid cell walls
Rhoeo Discolor epidermis
Rhoeo Discolor epidermis
plasmolysed cell

If a plant cell is turgid
It is in a hypotonic environment
 It is very firm, a healthy state in most plants


If a plant cell is flaccid

It is in an isotonic or hypertonic environment
Wilting can be reversed -- if water is
replaced fast enough