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 .