Variables:
Controlled variables: Size of chip; volume of sucrose solution; the time that the chips were left in the solution;
Independent Variables: The range from distilled water to 8m solution.
Dependant Variables: % change of mass
In order to calculate the percentage change of mass, we had to use the following equation-
Difference in Mass
----------------------- X 100
Original Mass
Objective:
The objective of this experiment is to find the optimum solution in which potatoes are well preserved but not affected excessively by osmosis.
Introduction:
Chip shop workers have to prepare their food for the lunchtime rush a few hours in advance because it would be impossible to peel potatoes, slice them and then cook them at the time. It would be extremely difficult to satisfy the demand for chips. So workers prepare their potatoes well before the busy part of the day. If chips are left in air, an oxidation reaction takes place; the cells give out their energy, this in turn discolours the potato and renders them almost useless. To prevent this most workers leave the potatoes in water. But this is where our problem lies – OSMOSIS.
Theory:
The illustration below shows a concentrated sugar solution, separated from a dilute sugar solution by a membrane.
The cell membrane has holes in it called pores, which are big enough for water molecules to pass through but are too small for other molecules to pass through, for example: salt molecules. This is why it is called a SELECTIVELY PERMEABLE MEMBRANE.
There is a higher concentration of sugar molecules on the right hand side of the membrane than on the left hand side. If the membrane was not there, the sugar molecules would diffuse from the concentrated solution to the dilute one until they were evenly spread out. However they cannot do this because the pores in the membranes are too small for them to get through.
There is also a concentration gradient for the water molecules. On the left hand side there is a high concentration of water molecules. On the right hand side there is a lower concentration of water molecules because the sugar molecules are taking up a lot of the space. The molecules are not evenly spread out so the water molecules diffuse through the pores to the right hand side. They can do this because the pores are large enough for them to fit through.
As the diagram illustrates, water has diffused from the dilute solution, through the selectively permeable membrane, into the concentrated solution. The concentrated solution will become more dilute, because of the extra water molecules coming into it. This process is called osmosis.
Definition:
Osmosis is the diffusion of water from a place of high concentration to a place where the water molecules are in a lower concentration through a selectively permeable membrane.
Unlike animal cells, plant cells do not burst in pure water. The illustration above shows a plant cell in pure water. A wall surrounds potato cells, which is fully permeable. This means it will allow any molecules to go through it, so osmosis will not occur across it. A potato has a cell membrane just like an animal cell. The cell membrane is selectively permeable. If the potatoes are left in pure water to preserve them, the cells in the potato will take in water. Water will diffuse into the cytoplasm, and vacuole and they will increase in size. However the cell wall is very strong and it stops the cell from bursting. The cytoplasm will push against the cell wall but it resists and presses back on the cytoplasm. A plant cell in this state is said to be TURGID. The turgidity of the cells makes them very hard to fry and it will leave them with a watery taste. Therefore workers add salt to the water counteract osmosis.
The illustration below shows a plant cell in a concentrated solution. Plant cells lose water in concentrated solutions through osmosis. This would happen in a high salt concentration – water would diffuse out of the cytoplasm and vacuole through the selectively permeable membrane. The cytoplasm and vacuole reduce in size. As the cytoplasm shrinks, it stops pushing out on the cell wall, and becomes FLACCID. A bit like a tyre when some air has leaked out. It is floppy. If the solution is very concentrated, a lot of water will diffuse out of the cell, the cytoplasm and vacuole will continue to shrink further into the centre of the cell the cell wall gets left behind. The membrane will tear away from the cell wall and the cell will be PLASMOLSED.
When a cell is plasmolysed, it dies because the cell membrane is damaged in the process. If the cells become plasmolysed, the potatoes will shrivel up and become useless, as the cells inside are dead.
Link:
So if a chip shop owner pre-cuts chips and stores them in water before they are served to the customers. The owner has found that by storing the chips in water, it causes them to taste soggy when cooked. To prevent this he adds salt to the water, however on the other hand he has found that by adding too much salt it will also have an effect on the taste of the chips.
I am going to investigate this by placing slices of potato in different concentrated solution and examine the changes to the potato. I am going o try to find the best concentration of solution that least affects the taste of the potato.
Prediction:
I think that as the chips are placed in distilled water, the mass will increase because the water will diffuse into the cells of the potato. I predict that if the potatoes are placed in a salt solution, the mass will decrease because the water from the cytoplasm will flow out through the selectively permeable membrane along the concentration gradient into the salt solution, in other words the cell will lose water and possibly be plasmolysed. I think that this will vary as the concentration of the solutions change.
