Movement In and Out of Cells: Question 5
Syllabus 3.2
A freshwater aquarium fish is accidentally transferred into a marine (saltwater) aquarium tank. The salt water now surrounding the fish's gill cells is a much more concentrated solution than the cytoplasm inside those cells.
(a) State what is meant by osmosis, referring to water potential in your answer. [2]
(b) State whether the water potential of the salt water surrounding the gill cells is higher or lower than the water potential inside the gill cells. Explain your answer. [2]
(c) Predict what will happen to the gill cells by osmosis, and explain your reasoning in terms of water potential. [2]
(d) Gill cells, unlike plant cells, do not have a cell wall. Suggest why this makes gill cells more vulnerable than plant cells to severe changes in the water potential of their surroundings. [2]
Show worked solution Hide worked solution
Worked solution
Part (a): Defining osmosis using water potential
Osmosis is the net movement of water molecules from a region of higher water potential (a dilute solution) to a region of lower water potential (a more concentrated solution), through a partially permeable membrane.
Part (b): Comparing water potential inside and outside the gill cells
The salt water surrounding the gill cells is a much more concentrated solution than the cytoplasm inside the cells. A more concentrated solution has a lower water potential than a more dilute one, so the water potential of the salt water is lower than the water potential inside the gill cells.
Part (c): Predicting the effect on the gill cells
Water always moves, by osmosis, from a region of higher water potential to a region of lower water potential. Here, the water potential inside the gill cells is higher than the water potential of the surrounding salt water, so water molecules move out of the gill cells, through their partially permeable cell membranes, into the salt water. The gill cells will therefore lose water and shrink.
Part (d): Why gill cells are more vulnerable than plant cells
A plant cell is surrounded by a rigid cell wall as well as a cell membrane. If a plant cell is in a dilute solution and gains water by osmosis, the cell wall resists further stretching once the cell is full of water, so the cell does not usually burst. Gill cells, like other animal cells, have only a cell membrane and no cell wall to resist changes in volume. This means that when a large amount of water moves out of (or into) a gill cell by osmosis, there is nothing to limit the resulting shrinkage (or swelling), so the cell is far more likely to be damaged or destroyed than a plant cell facing the same change in water potential.
Final answers
- (a) Osmosis: the net movement of water molecules from a region of higher water potential to a region of lower water potential, through a partially permeable membrane.
- (b) The salt water has a lower water potential than the gill cells, because it is the more concentrated solution.
- (c) The gill cells will lose water and shrink, since water moves from their higher water potential to the salt water’s lower water potential.
- (d) Gill cells have no cell wall to resist the resulting change in volume, unlike plant cells, so they are more vulnerable to being damaged by large changes in water potential.