OSMOSIS

DEFINITION

Osmosis is the passive movement of water (solvent
molecules) from a region of higher concentration of
water molecules across a partially permeable
membrane to a region of lower concentration of water
molecules

A concentrated solution has a lower concentration of
water molecules.
The movement of water is also restricted due to the
attraction between solutes and water molecules.
There are fewer water molecules with a high kinetic
energy to move across the membrane
The concentrated solution has a low water potential.
Water moves down a water potential gradient

Osmosis





WATER POTENTIAL

Water potential is the tendency of a solution to
lose water
The symbol used for water potential is Ψ the Greek
letter psi, and is usually measured in kilopascal (kPa).

water moves from a solution with high water potential
to one with low water potential.

Water potential is decreased by addition of solute.

Water potential can be increased by the application of
pressure.

In pure water or a dilute solution with very few solute
molecules, the water molecules have a high free
kinetic energy and has the tendency to move very
freely.

The dilute solution has a high water potential.

The water potential of pure water Ψ w at atmospheric
pressure is arbitrarily given the value 0 kPa.

The addition of solutes lowers the water potential.
The water potential of solutions therefore, is lower
than pure water and has a negative value.

The greater the concentration of solutes, the more
negative is the water potential.

SOLUTE POTENTIAL

Solute potential is the potential or force of attraction
towards water molecules caused by dissolved
substances (solutes) inside the solution.
The attraction between solute molecules and water
molecules reduces the random movement of water
molecules.
The addition of more solute molecules lowers the
water potential of a solution.

PRESSURE POTENTIAL

Pressure potential is the pressure exerted on a fluid by
its surrounding.
As water diffuses into a plant cell, the cell contents
expand causing a pressure (turgor pressure) on the
cellulose cell wall.
The cell wall develops an inward pressure (pressure
potential) to resist the influx of water.

PRESSURE POTENTIAL

The pressure potential has a positive value when the
plant cell is turgid and 0 kPa when the cell is flaccid
The water potential of an animal cell depends on its
solute potential because the pressure potential
generated by the cell membrane is negligible

WATER POTENTIAL IN A PLANT

The water potential of a plant cell is the sum of its
solute potential and pressure potential.

EXERCISE





TONICITY OF SOLUTION

The term tonicity is a measure of the osmotic
pressure gradient of two solutions separated by a
semi-permeable membrane.

The osmotic pressure gradient is defined by the water
potential of the two solutions.

Tonicity is influenced only by the solutes that cannot
cross the semi-permeable membrane. These solutes
can exert an osmotic pressure.

Solutes that can move freely across the membrane do
not affect tonicity. This is because, these solutes, when
reaching equilibrium, will always be in equal
concentrations on both sides of the membrane.

Hypotonic, hypertonic and
isotonic solutions

The hypotonic solution is the solution with the lower
solute concentration,
The hypertonic solution is the solution with the
higher solute concentration.
An isotonic solution has the same solute
concentration as the other solution.

OSMOSIS IN PLANT CELLS

When a plant cell is placed in a hypotonic solution,
water enters the cell by osmosis.
The vacuole expands and the cell contents press
against the cell wall.
As more water enters, the pressure potential produced
by the cell wall increases until the pressure potential
equals the solute potential.

OSMOSIS IN ANIMAL & PLANT CELLS

There is no net movement of water in either direction.
The plant cell is said to be turgid (Figure 3.12(d)).

Turgid cells give support to herbaceous plants, like
morning glory and balsam plant.

Plant cells do not burst because they are surrounded
by a strong, cellulose cell wall.

OSMOSIS IN ANIMAL & PLANT CELLS

In isotonic solutions, there is no net movement
of water molecules and no change in the volume
of the cell (Figure 3.12(e)).

OSMOSIS IN ANIMAL & PLANT CELLS

The protoplast starts to shrink away from the cell wall.

Plasmolysis of the cell occurs and the cell becomes
flaccid (Figure 3.12(F).

When cells become flaccid, they cause the plant to
wilt.

OSMOSIS IN ANIMAL CELLS

When an animal cell, for example, a red blood cell, is
placed in a hypotonic solution, water enters the cell by
osmosis. There is a net movement of water into the
cell.
The cell expands and the thin plasma membrane
bursts, releasing the cell contents.
A red blood cell is said to be haemolysed (Figure
3.12(a)).

In isotonic solutions, there is no net movement of
water molecules and no change in the shape or volume
of the cell (Figure 3.12(b)).

In hypertonic solutions, there is a net outflow of water
by osmosis from the cell (Figure 3.12(c)).

The cell shrinks and the plasma membrane has a
crinkled appearance.

The cell is said to be crenated.

DEFINITION OF OSMOSIS

Movement of water molecules from
region of high water potential to lower
water potential across a partially
permeable membrane