3: Movement of Substances Across Cell Membrane - Problems Flashcards
In the human small intestine, food molecules move across the cell membrane of the cells in the inner wall of the small intestine and enter the cells. Some food molecules, eg. fatty acids can move across the cell membrane through the phospholipid bilayer, while other food molecules, eg. amino acids, cannot.
With reference to the structure of the cell membrane described by the fluid mosaic model, explain why there is such a difference. (4)
The core of the phospholipid bilayer is hydrophobic.
It is permeable to non-polar substances but impermeable to polar substances and ions.
Fatty acids are small and non-polar. They dissolve in the phospholipid bilayer and move across the membrane.
On the other hand, amino acids are small but polar. They are repelled by the phospholipid bilayer and cannot move through it.
In the human small intestine, food molecules move across the cell membrane of the cells in the inner wall of the small intestine and enter the cells. Some food molecules, eg. fatty acids can move across the cell membrane through the phospholipid bilayer, while other food molecules, eg. amino acids, cannot.
Suggest how amino acids move across the membrane. (1)
They are transported by channel proteins or carrier proteins.
With reference to the diagram, state one similarity and one difference between the structure of the cell membrane described by the sandwich model and the fluid mosaic model. (2)
Similarity: A phospholipid bilayer is present in both models.
Difference: The proteins in the sandwich model are located on the surfaces of the phospholipid bilayer, while those in the fluid mosaic model are interspersed among the phospholipid molecules.
Explain why the following finding does not support the sandwich model: Protein molecules extracted from the cell membrane are mainly hydrophobic. (2)
Hydrophobic protein molecules are repelled by water.
Thus it is unlikely for them to be located on the surface of the phospholipid bilayer, where they come in contact with water (the aqueous environments on either side of the cell membrane).
Explain why the following finding does not support the sandwich model: The cell membrane is permeable to polar molecules. (2)
Polar molecules are repelled by the hydrophobic core of the phospholipid bilayer, and cannot move through it.
Channel proteins and carrier proteins which transport them across the membrane are absent in the ‘sandwich’ model.
An artificial membrane is produced and its structure is shown below. The table shows the permeability of the artificial membrane and the cell membrane to oxygen molecules and sodium ions.
(Artificial membrane permeability, Cell membrane permeability)
Oxygen: (Highly permeable, Highly permeable)
Sodium ions: (Impermeable, Slightly permeable)
Explain why the permeability of the two membranes to oxygen molecules is similar. (2)
Oxygen molecules are small and non-polar.
They dissolve in the phospholipid bilayer and move across the membranes at a similar rate.
(Since the phospholipid bilayer is present in both the cell membranes and artificial membrane, both membranes are highly permeable to oxygen.)
An artificial membrane is produced and its structure is shown below. The table shows the permeability of the artificial membrane and the cell membrane to oxygen molecules and sodium ions.
(Artificial membrane permeability, Cell membrane permeability)
Oxygen: (Highly permeable, Highly permeable)
Sodium ions: (Impermeable, Slightly permeable)
Explain why the permeability of the two membranes to sodium ions is different. (4)
Sodium ions are small but polar. They are repelled by the hydrophobic tails of the phospholipid bilayer,
and cannot move through the membrane directly.
Carrier proteins and channel proteins are interspersed on the cell membrane, and they provide a pathway for sodium ions to travel across the membrane, thus the cell membrane is slightly permeable to sodium ions.
However, channel proteins and carrier proteins are absent on the artificial membrane, thus the artificial membrane is not permeable to sodium ions.
(DSE 2018 IB Q2)
Thee leaf of an aquatic plant was places in a concentrated sucrose solution and observed under a light microscope. Photomicrographs A and B show the appearance of the cells at the beginning of the experiment and after 5 minutes respectively.
Comparing the two photomicrographs, state two observable changes in the appearance of the cells after 5 minutes. (2)
- The cell membrane / cytoplasm of the leaf cells has detached from the cell wall (the cell is plasmolysed)
- Chloroplasts condense to the centre of the cell
- The vacuole shrinks
(DSE 2018 IB Q2)
The leaf of an aquatic plant was places in a concentrated sucrose solution and observed under a light microscope. Photomicrographs A and B show the appearance of the cells at the beginning of the experiment and after 5 minutes respectively.
Explain how the observable changes are brought about. (2)
The water potential of the concentrated sucrose solution is lower than that of the cell content.
There is a net movement of water molecules from the inside of the cell towards the bathing sucrose solution through the differentially permeable cell membrane by osmosis.
(DSE 2017 IB Q2)
Explain why a piece of pineapple preserved in a sugar solution is softer than fresh pineapple. (3)
The sugar solution has a lower water potential compared to the cell contents of the pineapple.
There is a net movement of water molecules from the inside of the cell towards the sugar solution through the differentially permeable cell membrane by osmosis. Water moves out of the pineapple cells.
The pineapple cells become flaccid and fail to press against one another. Hence, the texture of the pineapple preserved in sugar solution is softer than that of fresh pineapple.
(DSE 2014 IB Q7)
The vacuoles of beetroot cells contain a red pigment which will be released from the cells if the cell membrane and vacuole membrane are damaged. In an investigation, identical cylinders of beetroot tissues are put into 4 test tubes. Each tube contained the same volume of alcohol at different concentrations. The following photograph shows the appearance of the solutions bathing the beetroot cylinders after 30 minutes.
From the results of the above investigation, deduce which test tube contained the highest concentration of alcohol. (4)
Tube D should have the highest concentration of alcohol.
This is because the phospholipids of the membrane dissolve in alcohol.
The cell membrane and vacuole membrane of the beetroot tissue bathing in the test tube with highest concentration of alcohol would be most damaged.
The amount of pigment released to the bathing solution would be the highest, as indicated by the highest colour intensity.
(DSE 2014 IB Q7)
The vacuoles of beetroot cells contain a red pigment which will be released from the cells if the cell membrane and vacuole membrane are damaged. In an investigation, identical cylinders of beetroot tissues are put into 4 test tubes. Each tube contained the same volume of alcohol at different concentrations. The following photograph shows the appearance of the solutions bathing the beetroot cylinders after 30 minutes.
After three hours, the colour intensity of all tubes became the same. Suggest an explanation for this. (2)
When the cell membrane and vacuole membrane are damaged, the pigment leaks out of the vacuole by diffusion.
As time passes, it allows the diffusion of the red pigment in all 4 tubes to reach an equilibrium state at which the same concentration of red pigment are found in the bathing solutions. The bathing solution contains the same amount of red pigment in all 4 tubes, so they all have the same colour intensities.
(CE 2009 IA Q4)
The water potential of potato tissue is determined in an investigation. 5 identical potato strips were separately immersed into 5 beakers containing sucrose solution of different concentrations for one hour. The masses of the potato strips before and after the immersion were measured and recorded. The graph shows the results of the investigation.
“When the ratio of final mass to initial mass is 1.0, the potato tissue has the same water potential as the corresponding sucrose solution.” Explain the biological principle behind this statement. (3)
When the ratio of final mass to initial mass is 1.0, there is no change in mass of the potato strip in the experiment.
This shows that there is no gain or loss (net movement) of water molecules from the inside of the cell to the bathing solution through the differentially permeable cell membrane throughout the investigation
by osmosis. Therefore, the water potential of the cell content is the same as the bathing solution.
(CE 2009 IA Q4)
The water potential of potato tissue is determined in an investigation. 5 identical potato strips were separately immersed into 5 beakers containing sucrose solution of different concentrations for one hour. The masses of the potato strips before and after the immersion were measured and recorded. The graph shows the results of the investigation.
Describe the state of the potato strip after being immersed in 10% sucrose solution for one hour and explain. (3)
The potato strips become flaccid.
The bathing solution has a lower water potential than that of the cell content.
There is a net movement of water molecules from the inside of the cell to bathing solution through the differentially permeable cell membrane by osmosis.
The potato tissue loses water to the sucrose solution.
(CE 2009 IA Q4)
The water potential of potato tissue is determined in an investigation. 5 identical potato strips were separately immersed into 5 beakers containing sucrose solution of different concentrations for one hour. The masses of the potato strips before and after the immersion were measured and recorded. The graph shows the results of the investigation.
Describe the change in water potential of a fresh potato after it has been stored for a long time and explain. (2)
The water potential of the potato will become lowerafter storing for a long time
due to the evaporation of water in storage.
