Determination of turgor pressure and study of the process of osmosis
In the process of osmosis, solvent moves from a region of lower salt concentration to a solution of higher concentration via a semi-permeable membrane. This process of osmosis when observed in plant tissue is important to botanists. A turgid potato becomes limp and decreases in size when placed in a solution of a higher the concentration of solute particles.
Turgor pressure is the pressure exerted against the cell wall by contents of the cell. Turgor pressure is important for plant support and maintaining shape. If the solution is hypertonic, the cell will plasmolyze and die from lack of water. In an isotonic solution, the plant cell does not have enough turgor pressure to prevent wilting and will result in possible death. The information gained about turgor pressure is important in understanding the effects of different solutions on organisms in our environments, including ourselves. For a plant cell, the ideal solution is a hypotonic solution because the cell takes in water increasing turgor pressure. Students in grades K-11 through 12 learn about the process of osmosis and reverse osmosis. Turgor pressure and osmosis are inter-related. In a hypotonic solution, osmosis takes place when water flows from lower concentration into the plant cells. As a result the plant becomes turgid. The turgid plant cells exert a certain amount of pressure known as tugor pressure. Using potato cores a quantitative as well as a qualitative study of osmosis and turgor pressure can be achieved. The study assists the student to predict plant cell behavior under different conditions of solute concentrations. The weight of the potato cores change when placed in pure water. Since potatoes contain sugars in the form of carbohydrates and some amount of salts they are at a higher concentration. If plant cells are placed in pure water (a hypotonic solution) water will initially move into the cells. After a period of time, the cells will become turgid. At first most water movement is into the cell. As the turgor pressure increases water will begin to diffuse out of the cell at a greater rate, eventually equilibrium will be reached and water will enter and leave the cell at the same time. Water always moves from high water potential to low water potential. Water potential is a measure of the tendency of water to move from high free energy to lower free energy. Distilled water in an open beaker has a water potential of 0 (zero). The addition of solute decreases water potential. In cells, water moves by osmosis to areas where water potential is lower. A hypertonic solution has lower water potential rate. In a hypertonic solution, cells lose water due to osmosis of water out of the cells into the solution (exo-osmosis). This causes the potato to become limp and to decrease in size. The higher the concentration of solute particles, the more water is lost from the potato cell, so more weight is lost in proportion to increasing solute concentration.
This study can be conducted in the laboratory where the water potential of the potato cores can be determined in the presence of varying concentrations of sucrose as well as common table salt. Naturally, potatoes contain both, some amount of sucrose as well as salt. Hence, at concentrations where the potato has a higher salt and sugar concentration, the cores gain weight. However, when the ambient concentration of the salt and sugar is increased, the potato cores begin to lose weight as solvent moves from the cores to the outside. This loss in weight can be quantitatively expressed in the form of water and solution pressure. Water potential is a measure of the tendency of water to move from high free energy to lower free energy.
Water Potential = Osmotic Potential + Pressure Potential
By convention, the water potential of pure water at atmospheric pressure is defined as being zero (Ψ = 0). For instance, it can be calculated that a 0.1 M solution of sucrose at atmospheric pressure (Ψp = 0) has a water potential of -2.3 bars due to the solute (Ψ = -2.3). The solute potential of this sucrose solution can be calculated using the following formula
i = ionization constant (for sucrose this is 1.0 because sucrose does not ionize in water)
C = Molar concentration
R = Pressure constant (R = 0.0831 liter bars/mole oK) (determined above) T = Temperature oK (273 + oC of solution)
Knowing the solute potential of the solution (Ψs ) and knowing that the pressure potential of the solution is zero (Ψp = 0) allows you to calculate the water potential of the solution. The water potential will be equal to the solute potential of the solution.
Ψ = 0 + Ψs or Ψ = Ψs
The water potential of the solution at equilibrium will be equal to the water potential of the potato cells. Water potential values are useful because they allow us to predict the direction of the flow of water. Water always flows from an area of higher water potential to an area of lower water potential. Turgor pressure is a colligative property which is directly proportional to the amount of solute and inversely proportional to the molar mass of the solute.
The van’t Hoff factor ‘i’ is defined as observed molar mass/calculated molar mass. When a solute neither undergoes association nor dissociation, then the van’t Hoff factor equals 1. This is observed in the case of sucrose
But with a solute like sodium chloride which dissociates i>1. The experimentally determined turgor pressure of the potato cores is different from the calculated turgor pressure, this is called abnormal turgor pressure or abnormal colligative property.
‘i’=Vant Hoff factor = observed colligative property/calculated colligative property
i= observed concentration/experimental concentration
For a student, this project helps understand turgor pressure, osmosis, colligative properties and van’t Hoff factor. The graphs plotted from the data assist the students to graphically as certain the water potential and compare them with the calculated results. The experiment is simple but the information obtained is a useful and important study.