At point A the graph shows that no change in mass, of the potato, would have have occurred had we used a 0.2 (m) sucrose solution. This suggests that the concentration of water inside the potato would have been equal to the solution outside the potato.
At point B (plain water), there is no indication that the cell is increasing in mass. This is because the cell is fully turgid and no more water can enter.
At point C (highest sucrose – lowest water concentration), there is no indication that the cell is decreasing further in mass. This is because the potato has become flaccid and no more water can leave the cell. The potato is said to be plasmolysed.
Conclusions:
There is a pattern on the graph, and data table, which shows that as the concentration of the sucrose solution increases, the potato’s percentage change in mass decreases.
The almost straight line from 0.5m to 1.0m, on the X axis, shows that the pieces of potato are becoming as flaccid as they possibly can. This indicates that the change in mass of the potato, per molar of sucrose concentration, is getting closer and closer.
The graph shows that the potato cells increase in mass in solutions with a high water/low sucrose concentration and decrease in mass in solutions with a low water/high sucrose concentration.
The graph shows that water transfer has occurred across the potato cell membranes in a process known as osmosis.
From the graph an estimate to the concentration of the potato can be made as 0.2 M, as this is the theoretical point where the potato would not increase or decrease in mass. This is the point where no osmosis is taking place; both the potato and the solution have an identical molar concentration.
At 0.25 M the potato pieces loose a mean average of -0.8 g in mass. This shows that the water potential of the sucrose solution in the Petri dish is weaker than that of the potato.
At 0.50 M, the potato looses -0.23 g in mass. Indicating that the sucrose solution has an even weaker water potential than 0.25 M and that osmosis took place. This is why the potato lost even more mass, and it shows that the water potential in the Petri dish is less than that of the potato.
This trend continues through the graph as more water moves out of the potato into the sucrose solution, resulting in a loss of mass of potato.
Ways in which the experiments might be improved.
- The experiments could have been carried out by individuals, instead of groups, to obtain more data.
- The same sized cork borer should have been used by all of the class. This would insure that potato sizes were exactly the same.
- A mechanical cutter, such as a guillotine, could have been used to cut the potato pieces uniformly.
- Potato pieces should have been left to dry in air rather than dried with paper towels. This would cut down the instances of ‘over’ or ‘under’ drying the pieces.
- More potato pieces and a longer range of sucrose concentrations would have increased the amount of data collected. This may have shown a relationship between the potato mass loss and the concentration of sucrose, such as an inverse proportional relationship.
- Distilled water could have been used instead of tap water. Tap water may contain chemicals, such as chlorine and salts that could interfere with the water potential.
- Using a percentage of mass loss rather than the actual mass loss may have given more accurate results on the graph, due to the differing masses of the potato pieces. (One for the Maths boffins!)
- The potatoes could have been completely submerged in the solutions so that all the surface area was exposed to the liquid e.g. placed in covered test tubes.
- The potatoes could be handled by mechanical devices e.g. tweezers so that different body heats and contamination by people’s hands was eliminated.
- The potato could have been sliced into smaller pieces. A pin could be pushed through the pieces, maintaining the spaces, and the whole submerged in the solution. This would increase the surface area and speed up the osmosis.
