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IMBIBITION, DIFFUSION,
OSMOSIS AND PERMEABILITY
Lecture 1 A
Imbibition, Diffusion and Osmosis
• The movement of materials into and out of the cells in
plants takes place in solution or gaseous from.
• Although the exact process of this is not very clear, three
physical processes are usually involved in it.
• They are imbibition diffusion and osmosis.
Imbibition
• Imbibition is the absorption of water by hydrophilic
colloids. Examples of plant material which exhibit
imbibition are dry seeds before germination.
• Different types of organic substances have different
imbibing capacities.
• Proteins have a very high imbibing capacity, starch less
and cellulose least.
• That is why proteinaceous pea seeds swell more on
imbibition than starchy wheat seeds.
Imbibition • Certain substances if placed in a particular liquid absorb it and
swell up.
• For example, when piece of dry wood or dry seeds are placed
in water they absorb the water quickly and swell up
considerably so that their volume is increased.
• These substances are called as imbibants and the
phenomenon as imbibition these exists certain force of
attraction in between the imbibant and the imbibed
substances.
• In plants this is because of the presence of a large number of
hydrophilic colloids both in living as well as dead cells in the
form of proteins, carbohydrates, such as starch, cellulose,
pectic substances etc, which have strong attraction towards
water.
Imbibition
Imbibition
Imbibition plays a very important role in the life of the plants:-
1. The first step in the absorption of water by the roots of higher plants is the imbibition of water by the cell walls of the root hairs.
2. Imbibition of water is very essential for dry seeds before they start germination.
• As a result of imbibition a pressure is developed which is called as imbibition pressure. The magnitude of this pressure is tremendous if the imbibant is confined (i.e., closed) and allowed to imbibe so much so that a rock can be splited if some dry wooden pieces are inserted in a small crack in that rock and then soaked with water.
Diffusion
• Diffusion refers to the process by which molecules
intermingle as a result of their kinetic energy of random
motion. Consider two containers of gas a and b separated
by a partition.
Diffusion
• The molecules of both gases are in constant motion and
make numerous collisions with the partition.
• If the partition is removed, the gases will mix because of
the random velocities of their molecules.
• In time a uniform mixture of a and b molecules will be
produced in the container.
• The tendency toward diffusion is very strong even at room
temperature because of the high molecular velocities
associated with the thermal energy of the particles.
Diffusion
Rate of Diffusion:
• Since the average kinetic energy of different types of molecules
(different masses) which are at thermal equilibrium is the same,
then their average velocities are different.
• Their average diffusion rate is expected to depend upon that
average velocity, which gives a relative diffusion rate :
where the constant K depends upon geometric factors including
the area across which the diffusion is occurring.
Factors affecting rate of diffusion
Factor Effects on diffusion
Distance • Generally, the thicker the wall, the
slower the rate of diffusion.
Concentration
inside and outside
the cell
• The bigger the difference in
concentration between two regions, the
faster the rate of diffusion.
• The higher the concentration gradient,
the faster is the rate of diffusion.
The size of the
molecules or ions
• Generally, the larger the molecule or ion,
the slower the rate of diffusion.
Factors affecting rate of diffusion
Factor Effects on diffusion
Surface area to
volume ratio
• Cells with a larger surface area to volume
ratio have a faster rate of diffusion, and
vice versa.
• This means that rate of diffusion is faster
in smaller cells when compared to larger
cells.
Temperature • The higher the temperature, the faster
the rate.
Pressure • The higher the pressure, the faster the
rate.
Osmosis • If two solutions of different concentration are separated by
a semi-permeable membrane which is permeable to the
smaller solvent molecules but not to the larger solute
molecules, then the solvent will tend to diffuse across the
membrane from the less concentrated to the more
concentrated solution. This process is called osmosis.
Osmosis
• Osmosis is of great importance in biological processes where
the solvent is water.
• The transport of water and other molecules across biological
membranes is essential to many processes in living organisms.
• The energy which drives the process is usually discussed in
terms of osmotic pressure.
Osmotic Pressure :
• Osmosis is a selective diffusion process driven by the internal
energy of the solvent molecules. It is convenient to express the
available energy per unit volume in terms of "osmotic
pressure". It is customary to express this tendency toward
solvent transport in pressure units relative to the pure solvent.
Osmosis
• If a plant cell with water potential –50 is
placed in a solution
Osmosis
• If the osmotic potential of the solution is less
negative than the water potential of the cytoplasm
(the solution is hypotonic), net endosmosis will
occur, i.e. water will move into the cell from the
solution.
• The cell becomes fully turgid.
• Water potential of cytoplasm = -50
• Osmotic potential of solution= -20
Osmosis
• If the solution is hypertonic, net exosmosis occurs
and causes plasmolysis (the cell membrane pulls
away from the cell wall. The cell wall stays intact).
• Water potential of cytoplasm= -50
• Osmotic potential of solution = -80
Osmosis
• If the osmotic potential of the solution is the same
as the water potential of the cytoplasm (the
solution is isotonic),
• The cell is not plasmolysed, but it is not fully
turgid either.
• Water potential of cytoplasm= -50
• Osmotic potential of solution= -50
Osmosis
Plasmolysis :
• In normal condition the protoplasm is tightly pressed
against the cell wall.
• If this plant cell or tissue is placed in a hypertonic solution
water comes out from the cell sap into the outer solution
due to exo-osmosis and the protoplasm begins to contract
from the cell wall.
• This is called as primary plasmolysis.
Osmosis
Plasmolysis :
• If the outer hypertonic solution is very much concentrated
in comparison to the cell sap the process of exo-osmosis
and contraction or shrinkage of protoplasm continues and
ultimately the protoplasm separates from the cell wall and
assumes a spherical form, this phenomenon is called as
plasmolysis and the tissue is said to be plasmolysed.
Osmosis
Plasmolysis :
• If a plasmolysed cell or tissue is placed in water the
process of endoosmosis takes place.
• Water enters into the cell sap the cell becomes turgid and
the protoplasm again assumes its normal shape and
position. This phenomenon is called as depalsmolysis.
Permeability
Permeability
• It is the ability of a membrane to permit transport through it.
• In living cells, permeability refers to the passage of water
and solutes through the plasma-membranes
(cytomembranes).
• However, it should be noticed that the passage of dissolved
solutes across membranes is independent of the passage
of water and the passage of water and the entrance of
solutes both proceed towards their own equilibrium and is
controlled by the demand of each individual cell.
Permeability
• The structure and composition of plasma
membrane play an essential role in determining
and controlling the rate by which the permeating
substance can pass through the membrane.
• The plasma membrane is essentially a lipid bilayer
modified by proteins i.e. is a mosaic structure of
lipo-proteins.
Permeability
Permeability of plasma membranes to non-
electrolytes:
Permeability
Permeability of plasma membranes to non-electrolytes:
• The permeation of non-electrolytes across membranes is
essentially passively which is a simple diffusion process
follows the laws of simple diffusion.
• The concentration difference of a permeating molecule on
both sides of the membrane acts as the driving force.
• According to the law of diffusion, the permeating molecules
diffuse from side of high concentration to the side of low
concentration until equilibrium established.
Permeability
Permeability of plasma membranes to non-electrolytes:
• The permeability of the plasma membrane to different substances varies greatly with the permeating substance.
• Non-electrolytes cross the membrane mainly through the lipid part and a correlation is found between membrane permeability and membrane lipid composition.
• Thus the degree of permeability of non-electrolyte substance depends on the degree of its solubility and hence its diffusion across the phospholipid part of the plasma membrane.
• Substances with higher degree of solubility in phosphlipids have the higher degree of permeability.
Permeability
Permeability of plasma membranes to non-electrolytes:
• The rate of permeation is found to be affected with
molecular size.
• Smaller molecules permeate more quickly than larger
molecules of the same degree of solubility in phospholipids.
• This is due to the fact that: these smaller molecules can
pass not only through the lipoidal spots of the membrane,
but also through the pores in the plasma membranes
between the lipoidal particles which are suited for
permeation of smaller molecules.
Permeability
Permeability of plasma membranes
to electrolytes:
• The intake of electrolytes by plant cells
occurs, chiefly in the form of ions and the two
ions of an electrolyte may be absorbed by
two different rates.
• Ions of electrolytes cross the membrane
through the protein molecules present in the
membrane structure.
• In studying the effect of ion concentration in
the surrounding medium at low
concentrations on the rate of ion absorption
i.e. ion permeation through membranes, it
appears that the pattern of ion absorption
was similar to the effect of substrate
concentration on the rate of enzyme action.
Permeability
Permeability of plasma membranes to electrolytes:
• In this way it appears that the protein molecules present in
membrane and responsible for the intake of ions are
enzymatic in nature.
• Thus, these protein molecules carrying ions from outside to
insides across the membrane are called carrier molecules.
• These carrier molecules are found to be specific for ions i.e.
each carrier molecule is specific or selective for a specific
ion.
• For this reason the plasma membranes are known as
selective permeable membrane.
Permeability Selective permeability
• It means that plasma membrane can permit for specific ion to permeate
across it at very high rate and another to permeate at very low rate at
equal concentration of both ions in the medium.
• Due to this selectivity in ion absorption, some ions may be absorbed
and accumulated inside the cell at concentration much greater than that
in the external medium which means that these ions are absorbed
against their concentration gradient, i.e. from the side of low to the side
of high concentration.
Permeability
Selective permeability
• On the other hand, some other ions may present at relatively
high concentrations in the medium and permeated at very
low rate and the cells maintain very low concentration of
these ions inside the cell.
• For example, in the marine alga Ulva lactuca, it is found that
the concentration of K+ was 80 mM and that of Na+ 10 mM
inside the cell whereas their concentration in the seawater
are 10 and 500 mM respectively.
Permeability
Selective permeability
• The absorption of same ions i.e. K+ against its concentration
gradient indicates that the absorption of such ions is an
active process.
• The active absorption of ions mediated by the carrier
molecules derived its energy from the metabolic energy of
the plant cell.
• Thus selective permeability of the cellular membranes is
dependent on the metabolic activities if living cells and it will
be lost if the plant cell is treated with metabolic inhibitors or
killed.
Permeability
Selective permeability
• Absorption of the two ions of an electrolyte like KNO3 will
occur in the form of K+ and NO3- however an electric balance
must be maintained at both sides of the plasma membrane.
• Thus, when the cation of an electrolyte is absorbed an anion
carrying the same amount of the opposite charge will be
absorbed simultaneously.
• For example if the electrolyte is KNO3, it will be ionize into K+
and NO3- and both ions will be simultaneously absorbed at
their respective carrier molecules.
Factor affecting the permeability of the
cytomembranes
Temperature Hydrogen ion concentration
Oxygen tension
Dissolved electrolytes in
the medium and ion interaction
Light
Factor affecting the permeability of the
cytomembranes
1- Temperature:
• In general, an increase in temperature results in an increase
in the permeability of the plant cells, provided the
temperature is not increased after a maximum which will
cause disorganization of the protoplasm.
• The influence of temperature must be due to the decrease
in the viscosity of protoplasm and to the increase in the
kinetic activity of the permeating molecules in case of non-
electrolytes.
Factor affecting the permeability of the
cytomembranes
2- Hydrogen ion concentration (PH) :
• The ionization of electrolytes or valence number of some
ions and hence the availability of ions in the soil solution is
affected with the changes in hydrogen ion concentration of
the soil solution.
• For example the monovalent phosphate ion (H2PO4-) is the
form of phosphorus most readily taken up by plants.
• However , as the medium approaches a more alkaline PH ,
production of first the bivalent phosphate ion (HPO4 --) and
then the trivalent (PO4---) ion is favored.
Factor affecting the permeability of the
cytomembranes
2- Hydrogen ion concentration (PH) :
• The bivalent ion is only sparingly available to plants, while
the trivalent ion is not available at all.
• Since the monovalent ion is absorbed very readily than the
bivalent one, absorption of phosphate is accelerated at an
acid PH.
• However, PH values outside the physiological range, will
cause damage to plant tissues, cell membranes and carriers
and will inhibit, salt absorption.
Factor affecting the permeability of the
cytomembranes
3- Light:
• Light usually increases the cell permeability.
• The effects of light on opening and closing of stomata and
on photosynthesis will indirectly affected salt absorption.
• Opened stomata increase the mass flow of water in the
transpiration stream and thus may indirectly influence salt
absorption.
• The energy derived from the photosynthetic process
provides energy for salt absorption and the oxygen given off
improves conditions for the active absorption of ions.
Factor affecting the permeability of the
cytomembranes
4- Oxygen tension :
• The active portion of ion absorption is inhibited by the
absence of oxygen , this support the early observations that
oxygen is important for active absorption of ions.
• On the other hand , the complete absence of oxygen causes
an increase in the membrane permeability and the
membrane becomes leaky and loses its selective
permeability due to losing the cell viability.
Factor affecting the permeability of the
cytomembranes
5-dissloved electrolytes in the medium and ion interaction:
• It was found that certain marine plants were killed if they were kept
in solutions of NaCl isotonic to that of seawater than in ditilled
water.
• Also, these plants are not killed in isotonic NaCl solution to which
small amount of CaCl2 have been added.
• Under these condition, the reduction of toxicity of the individual
salts when mixed together in appropriate proportion is attributed to
each salt being antagonize the injurious effects of the other.
• A solution containing various salts such proportion as to show no
toxicity to plant cells is known as a balanced solution.
Factor affecting the permeability of the
cytomembranes
5-Dissloved electrolytes in the medium and ion
interaction:
• It is well known now that the absorption of one ion may be
influenced by the presence of another ion. In most cases the
absorption of one ion is retarded by the presence of another,
this phenomenon is known as ion antagonism.
• On the other hand and in very rare ,cases ,the absorption of
one ion is accelerated by the presence of another , a
phenomenon which is called ion synergy.
Factor affecting the permeability of the
cytomembranes
5-dissloved electrolytes in the medium and ion interaction:
• Ion antagonism could be demonstrated between monovalent and diavalent cations of simple salts.
• When discs of the red beet root are placed in distilled water, the anthocyanin pigment, contained in the vacuole, permeates outwards at such a slow rate as to be hardly detected.
• If, however the discs are pleased in a dilute solution of NaCl, a rapid outward diffusion, of pigment occurs.
• If now the tissue is transferred to a solutioncontaining CaCl2 in addition to NaCl, the permeability is again decreased and the loss of pigment slows down and ceases.
• The Ca++ ions have antagonised the permeability increasing effect of Na+ ions.
Factor affecting the permeability of the
cytomembranes
5-dissloved electrolytes in the medium and ion
Interaction:
• Ion antagonism which occurs between similar ions like that
between Ca2+ and Mg2+ and that between K+ and Na+ has
been described as being due to competition between these
similar ions for the binding sites on the carrier molecule since
ions of similar electric charged are supposed to be absorbed
via the same carrier molecules.
• On the other hand ion antagonism demonstrated between the
divalent ions of Ca2+ and the monovalent ions of K+ and Na+
could be attributed to the effect of Ca2+ ions on the
permeability of the plasma membrane .