Question

Question 4 Steven incubated rhubarb epidermal peel cells in water with increasing concentrations of sucrose for a period of 1 h and observed the extent of plasmolysis under the microscope and recorded the data in following table. Using the results obtained and the formula provided, calculate the osmotic pressure the cell exerts on the walls. Table: Plasmolysis gradient results after 1h of incubation % cells plasmolysed Sucrose concentration Number of cells /25 plasmolysed 0 0.0 M Sucrose 0.10 M Sucrose 0.20 M Sucrose 0.25 M Sucrose 0.30 M Sucrose 0.35 M Sucrose 0.40 M Sucrose 0.45 M Sucrose 0.50 M Sucrose 0.60 M Sucrose 6 9 7 12 16 19 23 24 25 0 24 36 28 48 64 76 92 96 100 Osmotic pressure-CRT where R is the gas constant (8.3144 J mol'K, or more conveniently, 0.0083144 L MPa mol K-), T the absolute temperature in Kelvin, and the concentration of solutes in the water in mol L or similar units. Consider room temperature is 21°C. Answer: -

Answer 1

This experiment is performed to see plasmolysis and calculate the osmotic pressure of the cell. For this we incubate the epidermal peels of the plants in increasing concentrations of sucrose.We then observe the cells for the rate of plasmolysis under the microscope. An observation is made on the number of cells plasmolysed within the microscopic field for all the concentrations of sucrose. When a graph is plotted molarity of sucrose solution (X axis) vs Percentage of cells plasmolysed(Y axis).The concentration of sucrose at 50% of plasmolysis is measured.We consider the concentration at 50% because we take an average of initial and final plasmolysis stages.This is taken as the isotonic concentration.Plasmolysis occurs just above the isotonic concentration hence 50% concentration is taken as the Concentration to determine the osmotic pressure. In this case, the Concentration of sucrose at 50% plasmolysis is 0.30-0.31M. That is when 12.5 cells out of 25 cells are plasmolysed.Hence we take 0.31M as C.

The other values are given as R= 0.0083144 and T= 21 °C +273 = 294 K.

therefore the osmotic pressure of the cell = CRT

= 0.31* 0.0083144* 294= 0.7577 ~ 0.76 MPa.