The following points highlight the three physical processes involved in the movement of materials in plant cells: 1. Diffusion 2. Osmosis and 3. Imbibition.
Contents
Physical Process # 1. Diffusion:
If a small bottle filled with some gas or vapours is opened at a certain place in the room, very soon its molecules become evenly distributed throughout the available space in that room. Similarly, if a solute is placed in its solvent, it is dissolved and its particles move so that they are evenly distributed throughout the container. This movement of particles or molecules from a region of higher concentration to a region of lower concentration is called as diffusion. The rate of diffusion of gases is faster than liquids or solutes.
The diffusing particles have a certain pressure called as the diffusion pressure which is directly proportional to the number or concentration of the diffusing particles. Therefore, the diffusion takes place always from a region of higher diffusion pressure to a region of lower diffusion pressure i.e., along a diffusion pressure gradient.
The rate of diffusion increases if:
(i) The diffusion pressure gradient is steeper
(ii) The temperature is increased
(iii) The density of the diffusing particles is lesser
(iv) The medium through which diffusion occurs is less concentrated.
Diffusion of more than one substance at the same time and place may be at different rates and in different directions but is independent of each other. A very common example of this is the gaseous exchange in plants.
Beside osmotic diffusion (see osmosis) the above-mentioned simple diffusion also plays a very important role in the life of the plants:
i. It is an essential step in the exchange of gases during respiration and photosynthesis.
ii. During passive salt uptake, the ions are absorbed by simple process of diffusion.
iii. Last step in stomatal transpiration is the diffusion of water vapours from the intercellular spaces into the outer atmosphere through open stomata.
Physical Process # 2. Osmosis:
If a solution and its pure solvent are separated by a semipermeable membrane (which allows only solvent and not the solute to pass through it) the solvent molecules diffuse into the solution. This diffusion of solvent molecules into the solution through a semipermeable membrane is called as osmosis (sometimes called as osmotic diffusion).
In case, there are two solutions of different concentrations separated by the semi-permeable membrane, the diffusion of solvent will take place from the less concentrated solution into the more concentrated solution till both the solutions attain equal concentration.
The phenomenon of osmosis can be demonstrated by the following simple experiment:
Mouth of a thistle funnel is tied with goat bladder (it acts as semi-permeable membrane) and concentrated sugar solution is filled in it whose level is marked on its narrow neck. It is now placed in a beaker of water. After sometime the level of the sugar solution in the thistle funnel rises (Fig. 3.1).
Osmotic Pressure:
As a result of the separation of solution from its solvent or the two solutions by the semi-permeable membrane, a pressure is developed in solution due to the presence of dissolved solutes in it. This is called as osmotic pressure (O.P.).
Osmotic pressure is measured in terms of atmospheres.
Osmotic pressure is directly proportional to the concentration of dissolved solutes in the solution. More conc. solution has higher osmotic pressure.
Osmotic pressure of solution is always higher than its pure solvent.
Osmotic pressure does not increase by the addition of insoluble solute in the solution.
Thus, during osmosis the movement of solvent molecules takes place from the solution whose osmotic pressure is lower (i.e., less concentrated or hypotonic) into the solution whose osmotic pressure is higher (i.e., more concentrated or hypertonic).
Osmotic diffusion of solvent molecules will not take place if the two solutions separated by the semi-permeable membrane are of equal concentrations having equal osmotic pressures (i.e., they are isotonic).
Plant Cells as Osmotic Systems:
Living cells in plants form osmotic systems due to the presence of semi-permeable plasma membrane and the cell sap having a certain osmotic pressure. Plasma-membrane actually is not truly semi-permeable as it allows certain solutes to pass through it and hence, it is known as selectively permeable or differentially permeable membrane. The tonoplast or the vacuolar membrane also possesses the same nature. The solvent in case of plants is always water. The cell wall is permeable.
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 enters into the cell sap by osmosis. This process is called as end- osmosis. As a result of entry of the water into the cell sap, a pressure is developed which presses the protoplasm against the cell wall and the cell becomes turgid. This pressure is called as turgor pressure. Consequence of the turgor pressure is the wall pressure which is exerted by the elastic cell wall against the expanding protoplasm. At a given time turgor pressure (T.P.) equals the wall pressure (W.P.).
T.P. = W.P.
If on the other hand, the plant cell or the tissue is 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 becomes flaccid. This process is known as ex-osmosis. Cell or tissue will remain as such in isotonic solution.
Significance of Osmosis in Plants:
(1) Large quantities of water are absorbed by roots from the soil by osmosis.
(2) Cell to cell movement of water and other substances dissolved in it involves this process.
(3) Opening and closing of stomata depend upon the turgor pressure of the guard cells.
(4) Due to osmosis the turgidity of the cells and hence the shape or form of their organs is maintained.
(5) The resistance of plants to drought and frost increases with increase in osmotic pressure of their cells.
(6) Turgidity of the cells of the young seedlings allows them to come out of the soil.
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 ex-osmosis and the protoplasm begins to contract from the cell wall. This is called as incipient plasmolysis.
If the outer hypertonic solution is very much concentrated in comparison to the cell sap, the process of ex-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 cell or the tissue is said to be plasmolysed. Because of the permeable cell wall the space in between the cell wall and plasma-membrane in plasmolysed cells is filled with outer hypertonic solution (Fig. 3.2).
If a plasmolysed cell or tissue is placed in water, process of end-osmosis 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 deplasmolysis.
Advantages of Plasmolysis:
1. It indicates the semi-permeable nature of the plasma-membrane.
2. This phenomenon is utilized in salting of meat and fishes and addition of concentrated sugar solution to jams and jellies to check the growth of fungi and bacteria which become plasmolysed in conc. solution.
3. It is also used in determining the O.P. of the cell sap.
Determination of O.P. of Cell Sap by Plasmolytic Method:
This method consists in placing pieces of the plant tissue (the O.P. of whose cell sap is to be measured) in sugar solutions of varying but known concentrations and finding out the particular sugar solution that causes incipient plasmolysis. The O.P. of the cell sap will be approximately equal to the O.P. of this sugar solution which is then known from any standard table. (In fact, the O.P. of the cell sap measured in this way is slightly higher because the sugar solution causing incipient plasmolysis will not be exactly isotonic but slightly hypertonic).
Diffusion Pressure Deficit (D.P.D.) (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 temp, and atm. conditions is called as Diffusion Pressure Deficit (D.P.D). If the solution is more concentrated its D.P.D. increases but it decreases with the dilution of the solution.
D.P.D. is directly proportional to the concentration of the solution.
In case of plants the cell sap is a watery solution of many inorganic and organic substances; i.e., it’s pure solvent is water. If these cells are placed in pure water the water will enter into the cells due to higher D.P.D. of the cell sap or water deficit.
In other words, the D.P.D. 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 the Suction Pressure (S.P.). It is related with osmotic pressure (O.P.) and turgor pressure (T.P.) of cell sap and also the wall pressure (W.P.) as follows:
D.P.D (S.P.) = O.P. – W.P.
but (W.P.) = T.P.
therefore, D.P.D. (S.P.) = O.P. – T.P.
Due to the entry of the water the osmotic pressure of the cell sap decreases while its turgor pressure is increased so much so that in a fully turgid cell turgor pressure equals the osmotic pressure:
O.P = T.P. (in fully turgid cell)
and hence, D.P.D. (S.P) = O (zero)
On the other hand, the removal of water from the cell sap (ex-osmosis) results in an increase of its O.P. and decrease of the turgor pressure so much so that in fully plasmolysed cells the value of tureor pressure becomes zero.
T.P. = O (in fully plasmolysed cell) and hence, S.P. = O.P.
In case, the cell is placed in a hypotonic solution instead of pure water, the suction pressure of the cell sap will be:
S.P. = (O.P. – O.P.1) – T.P.
where O.P1. is the osmotic pressure of outer hypotonic solution.
Thus it is quite obvious that the D.P.D. or S.P. in case of plant cells is not directly proportional to their osmotic pressure or the concentration of the cell sap but depends both on O.P. and T.P. Higher osmotic pressure of the cell sap is usually accompanied by lower turgor pressure so that its D.P.D. or S.P. is greater and water enters into it (Fig. 3.3 A).
But, sometimes it is possible that two cells are in contact with each other one having higher osmotic pressure and also higher turgor pressure than the other cell (Fig. 3.3 B), and still its does not draw water. It is because of its lower D.P.D or suction pressure (S.P)., no matter its O.P. is higher.
Determination of D.P.D. Or Suction Pressure:
A number of cylinders of tissues (2 or 3 cm. long) are cut from a large sized potato tuber or beet root etc. with the help of the cork-borer after the skin of the potato tuber etc. has been removed. The cylinders of tissue are weighed and placed in different test tubes containing sugar solution of varying but known concentrations (e.g. 0.50 M, 0.45 M, 0.40 M and so on). The test tubes (each of which contains one cylinder of tissue) are plugged.
After about 24 hours, the cylinders of tissue are taken out of the sugar solutions from the test tubes, dried with filter paper, and again weighed. The sugar solution in which the weight of the cylinder of tissue does not change is noted. D.P.D. or the Suction Pressure of the tissue will be equal to the osmotic pressure of this sugar solution which can be known from any standard table by using vant’s Hoff equation:
O.P. = C i R T
where,
C = Concentration of solution expressed as molality (mols per kg of water).
i = The activity coefficient (for non-electrolytes such as sugars it is 1; for electrolytes such as NaCl it varies with their concentration).
R = gas constant.
T = absolute temperature (K) = °C + 273.
Concept of Water Potential and Osmotic Relations of Plant Cells:
According to thermodynamic laws every component of a system possesses free energy capable of doing work under constant temperature conditions. Osmotic movement of water involves certain work done and in-fact the main driving force behind this movement is the difference between free energies of water on two sides of the semi-permeable membrane. For non electrolytes, free energy/mol. is known as chemical potential (denoted by Greek letter psi, Ψ). With reference to water this is called as water potential (Ψw).
Like other substances, the absolute value of water potential cannot be determined and measured; instead this value for pure water is arbitrarily fixed as zero at one atmosphere and a particular temperature. However, deviations from this value can be ascertained. Although water potential can be expressed in energy terms (ergs/mol.), it is usually expressed in pressure units such as bars or atmospheres (1 bar = 106 dynes/sq. cm. or 0.987 atmospheres).
In more recent literature, water potential is expressed in SI unit of pressure called pascals (Pa). One pascal is equal to the force of one newton per square meter (Nm-2). One Mega Pascal (MPa) is equal to 10 bars or 9.87 atmospheres (Mega = one million i.e., 106)
Water potential is lowered by the addition of solutes and because water potential value is zero for pure water, all other water potential values will be negative. In other words, the movement of water will take place in osmotic or other systems from a region of higher water potential (less negative) to a region of lower water potential (more negative).
Osmotic pressure (O.P.) in a solution results due to the presence of solutes and the latter lower the water potential (as mentioned earlier). Therefore, osmotic pressure is a quantitative index of the lowering of water potential in a solution and using thermodynamic terminology is called as osmotic potential (Ψs). Osmotic pressure and osmotic potential values are numerically equal but while the former has positive sign, the latter carries a negative sign (If OP = 20 atms. then, Ψs = — 20 atms).
In an open osmotic system, the water potential and the osmotic potential values are numerically similar and also have same sign i.e., negative (similar will be the case in plasmolysed cells). On the other hand, in a closed osmotic system e.g., in plant cells a pressure is imposed on water which increases the water potential.
In plants this pressure is called as turgor pressure. This is the actual pressure with positive sign and ranges between zero and numerical osmotic potential value. The potential created by such pressures is called as hydrostatic pressure or pressure potential (Ψp). Thus in such cases water potential is equal to osmotic potential plus pressure potential.
(Ψw = Ψs + Ψp)
Summarizing, it is more appropriate to say that osmotic entry of water into a cell depends, on its lower water potential than the outer solution or other cell, instead of saying that it is due to its higher D.P.D. (diffusion pressure deficit). Water potential values of plant cells under different osmotic conditions are as follows, (see also Fig. 3.4).
Ψw = Ψs (as Ψp = nil)…………………………………………………………. in plasmolysed or flaccid cell. (lowest)
Ψw = Ψs + Ψp ………………………………………………………………….. in partially turgid cell. (higher)
Ψw (highest) = zero (as Ψp numerically equals but both having opposite signs)……………………… in fully turgid cell. equal Ѱs but both having opposite signs).
If for example, there are 2 cells A and B in contact with each other. The cell A has a pressure potential (turgor pressure) of 4 bars and contains sap with an osmotic potential of – 12 bars. Cell B has pressure potential of 2 bars and contains sap with osmotic potential of – 5 bars. Then,
Ψw of cell A = Ψs + Ψp
= – 12 + (+ 4) = – 8 bars
Ψw of cell B = Ψs + Ψp
= -5+ (+2) = – 3 bars
Hence, water will move from cell B to cell A (i.e., towards lower or more negative water potential) with a force of (- 8 – (- 3) = – 5 bars.
Besides osmotic potential (Ψs) and pressure potential (Ψp), there is another component i.e., gravity component (Ψg) which contributes to cell water potential. Accordingly,
Ψw = Ψs + Ψp + Ψg
The gravity component (Ψg) depends on the height (h) of the water above the reference- state water, the density of water (pw) and the acceleration due to gravity (g):
Ψg = pwgh
Although Ψg may have significant effect on water potential if vertical distance during water transport in plant is substantially higher. But, at the cell level, Ψg is insignificant in comparison to Ψs and Ψp and is therefore, omitted and the equation for Ψw simplifies to Ψw = Ψs + Ψp as discussed earlier.
Determination of Water Potential (Ψw):
One of the common methods of measuring the water potential (Ψw) or osmotic potential (Ψs) of a plant tissue is by tissue-weight change method which is identical to the one already described earlier to determine the suction pressure or D.P.D. of a plant tissue. The main difference is in terminology only.
Tissue-weight change method is based on equilibrating pre-weighed samples of tissues in solutions of known osmotic potential (Ψs) and finding out the solution which has its osmotic potential Ψs equal to the water potential (Ψw) of the plant tissue. If, osmotic potential of the solution is more negative than water potential of the tissue, water will come out the tissue into the bathing solution and the tissue will lose weight.
On the contrary, if osmotic potential of the solution is less negative than water potential of the tissue, water will enter into the tissue and the latter will gain weight. The osmotic potential Ψs of the solution in which the tissue weight remains unchanged is deemed to be the water potential Ψw of the tissue.
(The solution which is used to prepare the solution should be one that does not pass easily through membranes or harm the tissue such as sucrose, sorbitol, mannitol or polyethylene glycol (PEG). The last one gives best results).
A number of equal sized cylinders of tissues (about 2 or 3 cm. long) are cut from a large un-skinned potato tuber with the help of a cork-borer and are weighed. These are now placed in beakers containing solution of varying but known concentrations (molality) e.g., 0.1 M, 0.2 M, 0.3 M, 0.4 M etc.
After about 24 hours, the cylinders of tissues are taken out of the solution from beakers, dried with filter paper to remove excess solution and are reweighed. The solution in which weight of cylinders of tissue does not change is noted. Water potential (Ψw) of the plant tissue will be equal to the osmotic potential (Ψs) of this solution which can easily be calculated by vant’s Hoff equation:
Ψs = – C i R T
where,
C = concentration of solution expressed as molality (i.e., mols of solute per kg of H2O).
i = the activity coefficient. For non-electrolytes such as sucrose, sorbitol, mannitol or PEG it is 1; but for electrolytes such as NaCl it varies with their concentration and is invariably less than one.
R = gas constant (0.00831 kg. MPa mol-1 K-1 or 0.0831 kg. bars mol-1 K-1 or 0.080205 kg. atm mol-1 k-1).
T = absolute temperature (K) = °C + 273.
i. (For example, osmotic potential for 1.0 molal solution of sorbitol at 20°C is calculated as – (1.0 mol kg-1) (1.0) (0.00831 kg MPa mol-1 °K-1) (293 °K) = -2.43 MPa.
ii. Osmotic potential of the cell sap is in fact the total sum of all osmotic potentials caused by total solutes dissolved in it without regard to molecular species and is called as osmolality.
Physical Process # 3. Imbibition:
Certain substances if placed in a particular liquid absorb it and swell up. For example, when some pieces of gum or a piece or 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.
There exists certain force of attraction in between the imbibant and the imbibed substance. 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 plays a very important role in the life of the plants:
i. 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.
ii. 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 and allowed to imbibe so much so that a rock can be splitted if some dry wooden pieces are inserted in a small crack in that rock and then soaked with water.
Using thermodynamic terminology, water moves by imbibition into a substance only when its water potential exceeds that of the imbibant. The old term imbibition pressure is replaced by the term matric potential (Ψm). The matric potential in an imbibant results primarily from adsorptive forces which bind water molecules to micelles or molecules of the imbibant and is analogous to the osmotic potential of a solution.
In a solution, the greater the amount of water present in proportion to a given amount of solute, the less negative (or higher) is the osmotic potential. Similarly, in an imbibant the greater the amount of water present in proportion to a given amount of imbibant, the less negative is the matric potential. With reference to pure water, the values of matric potentials are always negative (like osmotic potentials values that are negative).
The water potential of an imbibant is equal to its matric potential (always negative) plus any turgor or other pressure (pressure potential) which may be imposed upon the imbibant:
Ψw = Ψm + Ψp
If the imbibant is unconfined, no turgor or such pressure is involved and hence the above equation simplifies to:
Ψw = Ψm
Experimental Work:
Exp. 1. Preparation of a Semi-permeable Membrane:
A cylindrical porous pot containing CuSO4 solution is placed in a beaker containing potassium ferrocyanide solution. CuSO4 solution will diffuse outward into the beaker while the potassium ferrocyanide solution inward into the porous pot. The two solutions will meet within the walls of the porous pot and a membrane of copper ferrocyanide will be formed which acts as semipermeable membrane (Fig. 3.5). The porous pot is now taken out of the beaker and its remaining CuSO4 solution removed. It is carefully washed and is ready for use.
Exp. 2. Effect of Temperature and Alcohol on the Permeability of the Plasma-membrane:
Small equal sized cylinders of beet root tissue are cut with the help of a cork-borer and thoroughly washed with water. Each of these cylinders is placed in a separate test tube containing water at different temperature e.g., 0°C, 10°, 20°, 30°, 40°, 60°, 70°, and 80°C. One of the cylinders is placed in test tube containing alcohol instead of water at ordinary temperature.
After sometime it is observed that the water in those test tubes which were kept at lower temperature, room temp., or slightly higher temp, remain colourless while the water in test tubes kept at higher temperatures (e.g., 60°, 70°, 80°C) becomes red coloured. The intensity of the colour increases with the increase in temperature. The red colour is due to the diffusion of the betacyanin pigments from the cell sap into the water because at higher temperatures the semi-permeability of the plasma-membrane is gradually lost.
The alcohol in test tube containing cylinder of beet root tissue also becomes red coloured because the cells are killed with alcohol (alcohol is a dehydrating agent) and the plasma-membrane is destroyed so that the betacyanin pigments diffuse freely out of the dead cell into alcohol in the test tube to make it coloured.
Exp 3. To Demonstrate Osmosis by Means of Potato Osmoscope:
Skin of a large sized potato tuber is removed and one side of it is cut to make a flat base. A cavity is made in the centre of the potato tuber nearly up to the flat base and is filled with concentrated sugar solution whose level is marked with a pin (Fig. 3.6).
The potato tuber is now placed in a beaker of water on its flat base. After sometime the level of the sugar solution rises in the cavity. It is because of the osmotic diffusion of water into the sugar solution through the tissue of the potato tuber (end-osmosis) which acts as a semi-permeable membrane.