to investigate the water potential of potato tuber cells.
Aim:
The aim of this practical is to investigate the water potential of potato tuber cells.
Introduction:
Water potential (Yw, psi), which is a measure of the energy state of water is affected by dissolved solutes, pressure and matrix particles. The contribution to water potential by dissolved solutes, termed osmotic potential (Ys), is always negative in sign. In other words, solutes decrease the water potential. The contribution of pressure (Yp) may be positive, negative or zero, but is generally positive since most plant cells are turgid (turgor pressure). The contribution due to the binding of water to colloidal particles (matrix) and surfaces, termed matrix potential (Ym), also lowers the water potential. Although it is often small enough to be ignored, matrix potential is important when talking about soil water relations. Thus, the water potential of a plant system can be arithmetically represented by the equation:
Yw = Ys + Yp + Ym
Prediction:
I predict that the higher the water concentration of sucrose solution the lower the water potential, and the lower the concentration of the solution the higher the water potential. Therefore, when the highest concentration of sucrose solution is immersed with a potato cylinder it has a low water potential and as a result the potato cylinder will lose water and also mass. However, when a potato cylinder is added to a solution of low concentration it has higher water potential and therefore, the potato cylinder gains water and mass.
Thus, it could be stated that water potential is indirectly proportional to concentration. This means that when the concentration increases the water potential decreases and vice versa.
Potato cylinder that is placed in the highest concentration of sucrose solution will have the lowest water potential, and as a result the potato cylinder will have the greatest loss in water and therefore, when weighed, you would observe there is a loss in mass compared to the other potato cylinders. As the concentration decreases less water and mass is lost. Except for when one of the potato cylinders make neither a loss or gain in mass, which means the water potential of one potato cylinder is equal to that of the solution (it is in a state of equilibrium). As the concentration becomes lower after this point is met, the potato cylinders will gain water and therefore, gain in mass and will have the highest value.
Theory:
There are many factors, which determine the flow of water between cells, and over a semi-permeable membrane, these are listed below:
Water Potential.
The cytoplasm of the plant cell, with its enclosed vacuole, is contained within a membrane that is more permeable to water than to most solutes. The water potential of a cell relative to that of the surrounding solution determines whether water will move into or out of the cell. Water potential can be described mathematically as the sum of the osmotic potential and the pressure potential.
The osmotic potential (OP) is a function of the dissolved solute concentration (see equation below), and it tends to pull water into the cell via osmosis. In opposition to this force is the pressure potential (PP), which equals the pressure of the cell wall and membrane on the cell contents. While the osmotic potential is always negative, the pressure potential may be positive (pressure) or negative (tension), but is usually positive.
R = OP+PP
If a cell is placed in a solution, which has a R that is higher than that of the cell, there will be a net movement of water into the cell. However, if the surrounding solution has a lower R than in the cell, there will be a net movement of water out of the cell. If this latter situation continues, the plasma membrane and cytoplasm will pull away from the cell wall, a condition known as plasmolysis.
At incipient plasmolysis, there is no longer a pressure potential exerted by the wall (i.e. PP=0), and therefore, under that condition, R=OP. It should also be noted that for solutions, R = OP. A solution which just causes incipient plasmolysis thus, has a water potential and by inference, osmotic potential that is similar to the water potential (and osmotic potential) of the cell cytoplasm.
Matrix Potential
Water molecules can form hydrogen bonds with the surface of soil minerals (adsorption) as well as with other water molecules (cohesion). In soil (i.e., the soil water content is less than the porosity), adsorptive forces develop between the soil mineral surfaces and the soil water. These forces exert a pull on the soil water. This pull between the soil and the water molecules close to the particle surface is distributed throughout the soil water by the cohesive forces between water molecules. The process is much like the forces that develop within a chain of people holding hands with the person on each end of the chain holding onto a fixed structure like a building. If any member of the chain tries to move, they are restrained by the pull of their neighbor, which can be traced, to the pull between the end-person and the building.
As external forces attempt to remove water from the soil, water is restrained or held in the soil by these adhesive and cohesive forces. This places the soil water under tension, this tension or pull on the soil water causes the potential energy of the water to decrease relative to free water (i.e., water not held under tension). Therefore, water in soil can be held under tension because of the adsorption of water to the soil particles. Water held under tension has less potential energy per unit quantity of water than reference water (free water), therefore, has a ...
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As external forces attempt to remove water from the soil, water is restrained or held in the soil by these adhesive and cohesive forces. This places the soil water under tension, this tension or pull on the soil water causes the potential energy of the water to decrease relative to free water (i.e., water not held under tension). Therefore, water in soil can be held under tension because of the adsorption of water to the soil particles. Water held under tension has less potential energy per unit quantity of water than reference water (free water), therefore, has a lower water potential.
The decrease in water potential caused by the adsorption of water to the soil surfaces is called the matrix potential component of the soil water potential. Matrix potential is always negative or zero since the adsorption of water onto soil surfaces can only lower the potential energy relative to reference water.
Osmotic or Solute Potential
The presence of dissolved solutes can decrease the potential energy of water relative to the reference state (pure water). Solutes that reduce the potential energy of water are called osmotically active solutes. Inorganic salts are all osmotically active and many large organic molecules are osmotically active. The reduction in potential energy from dissolved solutes arises partly from the hydration of the solute or the forming of chemical bonds between the solute and water molecules. However, solutes also should lower the potential energy of water in an ideal thermodynamic solution where chemical interactions do not occur.
Soil water contains many dissolved salts as well as organic molecules released by plant roots and micro-organisms. Soil water is not pure water but rather a solution and the presence of osmotically active solutes reduces the soil water potential. The reduction in soil water potential caused by the presence of dissolved solutes is called osmotic or solute potential component of the soil water potential. Osmotic potential is always negative or zero because dissolved solutes can only lower the potential energy of water.
Gravitational Potential
The elevation of the soil water affects soil water potential in the same way as the elevation of any other object. A rock at the top of a hill occupies a higher elevation in the earth's gravitational field that at the bottom of the hill. Therefore, it has a higher potential energy. Soil water located higher in the soil profile therefore, has higher potential energy than water deeper in the soil profile. The same is true for plant water. The increase or decrease in soil water potential caused by changes in elevation is called the gravitational water potential component of the soil water potential.
The movement of water from regions of higher gravitational potential (higher elevation) to regions of lower gravitational potential (lower elevations) is the reason for the common phrase "water runs downhill". In general water moves from regions of higher to lower water potential not just gravitational potential. If the gravitational potential component is all that changes from place to place then water actually does run downhill. However, if other components of the water potential change appropriately water can, in fact, run uphill. For example, when a dry sponge is placed vertically in a dish of water, water will move uphill into the sponge. Water moving out of the soil into the plant is also an example of water moving uphill. To generalisee the "downhill" concept, it is often stated that water moves down a "potential energy" hill.
The reference state for soil water relevant to gravitational forces is an arbitrary but specified elevation. An elevation is chosen randomly where the gravitational potential is defined to be zero. This elevation is usually the soil surface or the water table but it can be any elevation at all.
The sign of the gravitational potential can be negative or positive. Soil (or plant) water located at an elevation above the specified reference elevation will have a positive gravitational potential. Water located below the specified reference elevation will have a negative gravitational potential. Although the choice of reference elevation is random, it must be kept constant during any set of calculations. The difference in gravitational potential from place to place in the soil-plant system is what is important rather than the absolute value of gravitational potential. If the reference elevation is kept constant, then differences in gravitational potential will remain constant regardless of the specific reference elevation chosen.
Pressure Potential
Applying positive pressure to an object increases its potential energy. Applying negative pressure (e.g., suction or tension) to an object decreases its potential energy. This is true of soil water as other objects. The change in water potential caused by the external application of pressure or suction to the soil water is called the pressure potential component of the soil water potential. The constraint that the pressure must be applied externally distinguishes pressure potential from the tension applied in the development of the matric potential.
The pressures exerted on the soil water can come from several sources, but the primary source considered is ponded water or hydrostatic pressure. Water is often ponded on the soil surface during irrigation or heavy rains. This standing water exerts a positive pressure on the water in the soil.
When there is no standing water on a soil, the external pressure applied to the soil is limited to the pressure of the atmosphere. Therefore, the applied pressure is atmospheric pressure. This is the pressure condition specified in the reference state for soil water potential so the without water ponding the pressure potential is zero. When water is ponded on a soil, the applied pressure is increased by the weight of the ponded water. This increase in applied pressure increases the potential energy of the water in the soil so the pressure potential component of water potential is positive.
Pressure can also be applied to soil water by increasing the pressure of the soil air and from the weight of soil laying on top of the location of interest. For example, when barometric pressures change, the pressure of the soil air near the surface is either higher or lower than the new atmospheric pressure until equilibrium can be re-established. This causes temporary changes in the pressure applied to the soil water and resultant changes in soil water potential. These changes are referred to as the pneumatic potential component of water potential.
Pure water has the highest possible water potential. Water molecules will always move from a region of high water potential (pure water) to a region of low water potential via a seem - permeable membrane, until equilibrium is reached. The maximum water potential is given as zero, and all solutions (sucrose solution) have a lower water potential than pure water, this is because pure water has the maximum water potential and so obviously all solutions otherwise must have a lower water potential and therefore, always has a negative water potential value.
Pure water has the maximum water potential; therefore, the potato cylinder, which is placed in distilled water during the practical, has the highest value as it gains water and mass.
It is stated that " if samples of a tissue are immersed in a range of solutions of different concentrations, the cell will gain water, and mass, in solutions of higher water potential and lose water, and mass, in solutions of lower water potential ". This is the reason as to why the prediction stated earlier has been made the theory behind it. The potato cylinders placed in the lower concentrations of the sucrose solution have a higher water potential and therefore, gain in mass because it gains water, and vice versa. The potato cylinder placed in the highest concentration of sucrose solution has a lower water potential and as a result the potato cylinder loses water and mass and as stated before, it would have a negative value. However, the potato cylinder which makes neither a gain or loss in mass is said to have the same water potential as the sucrose solution because it is stated that, " The water potential of the tissue is equal to that of the solution in which it neither gains nor loses mass ". Therefore, in this situation a higher or lower region of water potential does not exist, equilibrium already exists and as a result, no change is made in mass of the potato cylinder.
Apparatus:
. Six boiling tubes with stoppers
2. Boiling tube rack - to hold boiling tubes
3. Wax pencil - to mark each boiling tube with correct concentration level
4. Cork borer about 10mm in diameter - to obtain an accurate measurement
5. Razor blade - to cut cylinder to appropriate size
6. Filter paper - to absorb any surplus solution from potato cylinders
7. Forceps - to hoist potato cylinders
8. Balance capable of reading to the nearest 0.01g
9. Distilled water
0. Sucrose solutions (0.2, 0.4, 0.6, 0.8, 1.0 mol dm-3)
1. Potato tuber
Method:
* Label the six boiling tubes: Distilled Water (DW), 0.2, 0.4, 0.6, 0.8, 1.0 mol dm-3. Place approximately half a tube full of distilled water in the first tube and the appropriate sucrose solution in each of the other tubes. Firmly stopper each tube.
* Using a cork borer and razor blade, prepare six potato cylinders, each about 1mm in diameter and 50mm long. Place each on a separate sheet of filter paper, which you have labelled in pencil with the figures 0, 0.2, 0.4, 0.6, 0.8, and 1.0 respectively.
* Take the cylinders and the boiling tube rack to a balance. For each cylinder, record its mass on the filter paper, transfer it to one of the boiling tubes using the forceps, make a note of which tube it is in, stopper the tube and record the mass of the filter paper on its own. Calculate the initial mass of each cylinder once you have transferred them all to the tubes.
* After one hour remove the cylinders from the tubes in turn. Remove any surplus fluid quickly and gently with filter paper, using a standardised procedure. Do not squeeze the cylinders, or they will lose water. Then reweigh each cylinder and record the mass.
Risk Assessment:
Handling sharp objects is always dangerous, therefore, please take extreme caution when handling the razor blade.
Results:
To show weight of cylinders
Percentage change
Graph:
The graph enclosed with this document shows the percentage change of the potato cylinders when placed in the different concentration of sucrose solution.
Conclusion:
By analysing the results given it can be concluded that as the concentration of the sucrose solution increased, the potato cylinders lost more water and therefore, resulting in a loss in mass. For example the potato cylinder that was placed in the distilled water had a percentage change of 5.61%, whereas, the potato cylinder, which was placed in the solution with 1.0 mol dm-3 concentration, had a percentage change of -0.03%. Therefore, my prediction and theory stated previously agree with the result shown above. However, the results shown earlier are not entirely correct, which can also be seen in the graph. The last two results obtained do not agree with the rest of the results. As the percentage change should decrease as the concentration increases, but the final percentage change increases from -16.24% to -0.03%. Therefore, it is believed that a mistake may have been made when weighing the potato cylinder and/or filter paper or a miscalculation may have occurred, despite this no other mistakes were made.
Therefore, it is concluded that cells will gain water and mass in solutions of higher water potential, which exists in lower concentrations of solutions. Whereas, cells will lose water and mass in solutions with higher concentration, this is due to osmosis-"the movement of water molecules over a partially permeable from a region of high water concentration to a region of low region of water concentration". Also, it has been shown that when the tissue makes neither a gain nor loss in mass it is equal to the water potential, and no change is made. In this practical the potato cylinder which made no change was placed in a concentration of 0.6 mol dm-3 of sucrose solution and the percentage change was equal to 0%. Therefore, using the table below, the water potential of 0.6 mol dm-3 was found to be -1800kPa, and obviously the kPa is a negative number because all solutions have a negative value, because only pure water has the maximum water potential which equals to zero.
This experiment has been made successful due to the principles of osmosis. Although water molecules can move across a semi-permeable membrane randomly and without expending any energy, many dissolved substances cannot. This is because the membrane lets water molecules pass back and forth, but blocks certain solutes. In other words, the semi-permeable membrane limits diffusion of the solutes. However, osmosis is the flow of water through a semi-permeable membrane separating two solutions of different concentrations.
Osmosis is a special form of diffusion: the MOVEMENT OF WATER from a dilute solution to a more concentrated one through a PARTIALLY PERMEABLE MEMBRANE. This type of membrane (also called semi-permeable) allows only water, but not other (dissolved) substances to pass through.
Evaluation:
Due to the results I have obtained, this investigation proves to be a success. I procured a substantial amount of readings which makes this experiment fair, and I have also concluded a reasonable final statement, therefore, this is associated to the success of this investigation. The only main problem with my limitation, was the time. Even though I had a time period of three hours, I performed two experiments, both of which required one-hour each. This then left me to prepare and attain results in the space of one hour. The lack of time may be the cause of inaccurate results I obtained; also the timing for each experiments may not have been the same because a stopwatch has an error margin of 0.5 seconds, however, even though this seems insignificant in number, it is still relevant.
I believe that this practical was made a fair test as far as possible. This is because the same amount of solution was placed in each boiling tube. All potato cylinders were made the exact same size and shape, so surface area was the same. Only one variant was kept: - the concentration. However, when it came to removing the potato cylinders from the boiling tubes some took longer to remove then others as it was a slightly difficult task which may have also caused the potato cylinders to be squeezed slightly, resulting in the loss of water and mass. The only real problem within this experiment was the apparatus. Within my apparatus and method I stated "Using a cork borer and razor blade, prepare six potato cylinders, each about 1mm in diameter and 50mm long" however, due to inefficient equipment I changed the measurements of my potato cylinders too, length of 20mm and a diameter of 7mm.
If I was to perform this investigation again, the only thing I would change is the timing, because I had to hurry slightly, thus, may perhaps cause inaccurate results. In addition to a greater time period, more readings may be taken to make my experiment fairer.
For further research or evidence of osmosis taken place refer to the onion percentage plasmolysis investigation. This is simply where small thin strips of onion are placed under a microscope; where their cells are counted. Then the onion strips are placed within different sucrose concentrations (different moralities). After a set period of time, the onion strips are removed and once again placed under a microscope. Next you are required to count the number of cells plasmolysed. This experiment is made possible due to osmosis. If you put a plant cell in a solution (e.g. a concentrated sucrose solution) that has a lower (more negative) water potential than the cell's cytoplasm and vacuole, water leaves the cell by osmosis. The vacuole shrinks and eventually the protoplast (the living part of the cell) becomes detached from the cell wall. The point at which the protoplast is just about to become detached is called incipient plasmolysis. When it has become detached, the cell is said to be plasmolysed. Therefore, from this happening you are able to count the cells within this onion strip that has been plasmolysed, hence, proving the principles of osmosis.
By Jaikishan Sharma 12LVY
