CHAPTER: 5
Diffusion
& Osmosis
Solutions:
• Solutions are made of solute and a solvent
• Solvent - the liquid into which the solute is poured
and dissolved. Usually we will use water as solvent.
• Solute - substance that is dissolved or put into the
solvent. Salt, sugar and sucrose are solutes.
Significance
of Diffusion:
Importance
of Diffusion to plant life in general:
- It
is an essential process in exchange of gases (02 and CO2) during
respiration and photosynthesis.
- Uptake
of minerals is also affected by the process of diffusion.
- It helps in removal of excess water by
the process of transpiration.
- Translocation
of organic solutes also takes place by diffusion means.
- Fragrance of flowers or sweet scent emitted by flowers spread in air by diffusion means attracts insects to materialize pollination.
Factor
Affecting the Rate of Diffusion:
Ø Particle
size, temperature, concentration difference and diffusion distance affect the
rate of diffusion.
1.
Particle Size:
• The
particle size highly influences the rate of diffusion. Since the heat of the
environment is the energy source for diffusion, a smaller particle at a given
temperature moves faster than a larger particle. The rate of diffusion shares
an inversely proportional relationship with the particle size.
2.
Temperature:
• Temperature
and the rate of diffusion have a directly proportional relationship with one
another. The rate of diffusion increases as the temperature increases. At
higher temperatures, particles move faster because more energy is available to
diffuse them.
3.
Concentration difference:
• The
rate of diffusion increases as the concentration difference increases. A
substance diffusing between two areas exhibits a concentration difference as
the particles diffuse from one side of the wall to the other side. For example,
if a semipermeable bag of plain water is placed in salt water, the rate of
diffusion increases because the salt water has a higher concentration of
particles than the plain water.
4.
Density of Diffusing Substance:
The rate of diffusion is inversely proportional
to the square root of their relative density of their diffusing substance. The
larger the molecule, slower is the rate of its diffusion.
Diffusion
Pressure (DP):
• The
pressure exerted due to the tendency of the particles of a substance to diffuse
is called its diffusion pressure.
• The
DP is directly proportional to the concentration of the number of diffusing particles.
• The
greater is the concentration of particles, the greater is their diffusion
pressure.
Diffusion
Pressure Deficit (DPD) or Suction Pressure:
• Diffusion
pressure of a solution is always lower than its pure solvent.
• The
difference between the diffusion pressure of the solution and its solvent at a
particular temperature and atmospheric
condition called as diffusion pressure deficit (DPD).
• It
is increased by the addition of solutes, lowering temperature and pressure.
• If
the solution is more concentrated, its DPD increases, but it decreases with the
dilution of the solution.
• DPD
is directly proportional to the concentration of the solution.
• The
DPD of the cell sap or the cells is a measure of the ability of the cells to
absorb water and hence it is often called as suction pressure (SP).
• It
is related with osmotic pressure (OP) and turgor pressure (TP) of cell sap and
also the wall pressure (WP) as,
DPD (or, SP) = OP – WP
WP = TP
DPD = OP –
TP
In fully turgid cell
(endo-osmosis)
OP = TP , therefore
DPD = 0
In fully palsmolysed
cell (Exo-osmosis)
TP = 0 ,
Therefore, DPD = OP
• Osmosis
is the movement of WATER across a semi-permeable membrane
• At
first the concentration of solute is very high on the left.
But over time, the water moves across the
semi-permeable membrane and dilutes the particles.
- Osmosis is a special case of
diffusion
- Osmosis involves the diffusion of water through a membrane
- The membrane may be artificial and
non-living e.g. Cellophane
- In biology, the important membrane is the cell membrane
Osmotic
Pressure:
As
a result of separation of solution from its solvent or the two
solution by the semi-permeable membrane, a pressure is developed
in solution due to the presence of dissolved solute in it. This is called as osmotic
pressure(O.P.)
Osmotic
pressure is directly proportional to the concentration of dissolved solute in
the solution.
*
More concentration solution has higher osmotic pressure
Hypotonic
– The solution on one side of a membrane where the solute concentration is less
than on the other side. Hypotonic Solutions contain a low
concentration of solute relative to another solution.
Hypertonic – The solution on one side of a
membrane where the solute concentration is greater than on the other side. Hypertonic
Solutions contain a high concentration of solute relative to
another solution.
Osmosis
system & Plant cell:
• In
plant cell semi-permeable membrane is plasma membrane & cell sap
• The
solvent in case of plant is always water
• If
a living plant cell or tissue is placed in water or hypotonic solution water
enters into the cell sap by osmosis/end osmosis
Endosmosis
& Exosmosis:
• If
a living plant cell or tissue is placed in water or hypotonic
solution(whose O.P. is lower than that of cell sap) water enter into the cell
sap by osmosis. This process is called as endosmosis
• Entry
of water into cell sap – pressure developed and presses the protoplasm against the cell wall – turgid. The pressure
is called turgid pressure.
• If
the plant cell or the tissue placed in hypertonic solution (whose O.P.
is higher than that of cell sap) the water comes out of the cell sap into the
outer solution and the cell become Flaccid. The process is known as exosomosis.
Plant
cell in High water potential:
1. Cell
vacuole has lower water potential compared to solutions outside cell
2. Water
enters cell by osmosis.
3. Vacuole
increases in size, pushes against cell wall
4. Cell
wall exerts opposing pressure (against turgor
pressure)
5. Plant
cell expands and become turgid (cell
does not bursts) .
Plant
cell in Low water potential:
- Vacuole
has higher water potential compared to solution outside cell.
- Water
leave cells by osmosis
- Vacuole
decreases in size
- Cytoplasm shrinks away from cell wall ( Plasmolysis.)
Significant
of osmosis in plant:
• Large
quantities of water are absorbed by roots from the soil by osmosis
• Cell
to cell movement of water and other substance dissolved in it involves this
process
• Opening
and closing of stomata depend upon the turgid pressure of the guard cell
• The
resistance of plants to drought and frost increases with increases in osmotic
pressure of their cell.
• Turgidity
of cells of the young seedling allows them to come out of the soil
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