3: Movement of Substances Across Cell Membrane - Practical Flashcards
3.1: Temperature and organic solvents’ effects on membrane permeability
Why is it necessary to make sure that there is no peel left on the beetroot strips? (3)
Diffusion cannot take place across the beetroot peel because the peel is impermeable.
Any peel left on the beetroot discs will decrease the total surface area of the beetroot strip available for the diffusion of pigment, which causes a decrease in rate of diffusion of pigment from the vacuole of the beetroot cells to the bathing solution.
This results in an underestimation in the colour intensity of the bathing solution, and an underestimation in the permeability of the cell membrane.
3.1: Temperature and organic solvents’ effects on membrane permeability
Why is it necessary to rinse the beetroot discs?
To wash off the pigment on the surface of the beetroot discs. Any pigment left on the surface will affect the results.
3.1: Temperature and organic solvents’ effects on membrane permeability
Explain the effect of temperature on the relative colour intensity of the bathing solutions of the beetroot strips. (4)
The higher the temperature, the higher the intensity of red colour in the solution.
When temperature increases, the phospholipid molecules in the cell membrane and the vacuole membrane have more kinetic energy.
Therefore there is more lateral movement of the phospholipids; they move faster and pack less closely together.
The red pigment can move across the phospholipid bilayer of the cell membrane more easily; there will be more diffusion of red pigment from the vacuole of the cell, out of the vacuole and out of the cell, into the bathing solution.
3.1: Temperature and organic solvents’ effects on membrane permeability
Explain the effect of alcochol concentration on the relative colour intensity of the bathing solutions of the beetroot strips. (3)
The higher the alcohol concentration, the higher the intensity of red colour in the solution.
Alcohol is an organic solvent, which dissolves the phospholipid bilayer making up the cell membrane and the vacuole membrane, and damages the cell membrane.
As a result, the permeability of the membrane increases, which causes the rate of diffusion of the red pigments to increase. Therefore, more red pigment diffuses out from the vacuole of the cell, out of the vacuole and out of the cell, into the bathing solution, resulting in a higher colour intensity.
3.1: Temperature and organic solvents’ effects on membrane permeability
Four tubes of beetroot strips are added to alcohol solutions of different concentrations. What are the expected results if the tube were left overnight? (2)
If the tubes were allowed to stand overnight, the diffusion of the red pigment in the three tubes containing alcohol would reach equilibrium.
The concentration of the red pigment would be the same in the liquid of the three tubes, so the colour intensity of the surrounding liquid would also be the same.
3.2: Demonstration of osmosis using dialysis tubing
Why is a capillary tube, instead of a tube with a wide bore, used in the demonstration?
A narrower lumen shows a more obvious change in liquid level.
3.2: Demonstration of osmosis using dialysis tubing
Why should the outside of the dialysis tubing containing sucrose solution be rinsed with distilled water before putting it into the beaker of distilled water? (4)
This is to ensure the sucrose on the outer side of the dialysis tubing is washed away. If the sucrose is not washed away, there will be sucrose present in the beaker of water, decreasing the water potential of the solution in the beaker.
There will be a decrease in steepness of the water potential gradient between the solution in the dialysis tubing and the solution outside the dialysis tubing.
Therefore, there will be a decrease in net movement of water from the the outside of the beaker to the the inside of the tubing through the differentially permeable dialysis tubing by osmosis. The rate of osmosis decreases.
Therefore, the final liquid level will be lower, which means that the final liquid level is underestimated.
3.2: Demonstration of osmosis using dialysis tubing
What is the purpose of the control set-up (filling a dialysis tubing with distilled water and putting it into a beaker of distilled water)?
It is to show that any change in the liquid level in the capillary tube in the experimental set-up is due to the presence of sucrose in the sucrose solution inside the dialysis tubing.
3.2: Demonstration of osmosis using dialysis tubing
Explain the changes in liquid level in the capillary tube of the experimental set-up.
The water level increases because the water potential of the sucrose solution inside the dialysis tubing is lower than that of the bathing distilled water, so water molecules move through the differentially permeable membrane by osmosis from the beaker into the tubing.
(The water level keeps increasing at the end of the experiment because the water potential of the sucrose solution inside the dialysis tubing is always lower than distilled water since the tubing is impermeable to sucrose.)
3.2: Demonstration of osmosis using dialysis tubing
Explain the changes in the liquid level in the capillary tube of the control set-up.
The water level drops due to hydrostatic pressure of the liquid column, which forces the water to exit the dialysis tubing.
3.2: Demonstration of osmosis using dialysis tubing
Explain the changes in liquid level in the capillary tube of the experimental set-up if a more concentrated sucrose solution is put in the dialysis tubing. (3)
The water potential gradient between the sucrose solution inside the dialysis tubing and the bathing distilled water will be steeper.
There is a higher rate of net movement of water from the beaker of distilled water to the inside of the dialysis tubing theough the differentially permeable dialysis tubing by osmosis, so the liquid level in the capillary tube will rise at a higher rate.
The final liquid level will be higher.
3.2: Demonstration of osmosis using dialysis tubing
Explain the changes in liquid level in the capillary tube of the experimental set-up if distilled water is put in the dialysis tubing and sucrose solution is put in the beaker. (3)
The liquid level in the capillary tube will drop, and the dialysis tubing will eventually shrink.
This is because the water potential of the distilled water inside the dialysis tubing is higher than that of the sucrose solution inside the beaker,
so there is a net movement of water molecules from the distilled water inside the dialysis tubing to the bathing sucrose solution inside the beaker through the differne tially permeable dialysis tubing by osmosis.
3.3: Study of osmosis at cellular level
Describe the changes in the appearance of the cells when the concentration of the sucrose solution that surrounds the cells decreases.
The cytoplasm swells up gradually until the cell membrane presses tightly against the cell wall, and the cell becomes turgid.
3.3: Study of osmosis at cellular level
Do all cells change in appearance at the same rate when the concentration of the bathing sucrose solution decreases?
No, the cells have different initial water potentials so the rate of osmosis is different. Also, the diffusion distance is shorter for cells near the periphery, and they become turgid first.
3.4: Study of osmosis at tissue level
Why is it necessary to make sure that there is no peel left on the potato strips? (3)
Osmosis cannot take place across the potato peel because the peel is impermeable to water.
Any peel remaining on the potato strips will decrease the surface area for the net movement of water molecules between the bathing solution and the potato cells in the potato strips. The rate of osmosis will decrease.
Therefore, the final lengths and/or masses of potato strips to be measured will be affected.
