Plan
In this experiment my aim is to find out how the time that potatoes are left in different concentrations of sugar affect the rate of osmosis.
To carry out this experiment I will need the following equipment;
Potato, sugar solution, beaker, cork borer, white tile, ruler, stopwatch, weighing machine, knife, and a measuring cylinder.
Method: - Gather all the equipment and take the potato and the cork borer and punch out 18 cylinders of potato, this will ensure that all the potato tubes have the same cross – sectional area all the way through. Push the potato out of the cork borer using a thinner cork borer. Using a ruler measure the length of each potato and if they are not all the same use the knife to cut the potato tubes to size on the white tile. This will make sure that all the tubes are the same length and so this particular variable is kept constant. While the potatoes are being cut out, the solutions need to be made because other wise the potato will start to dry out and lose water. Starting with a 2 molar solution and diluting it down to each denomination by adding water can make the solutions, which are put into labelled beakers. The water and sugar solution will be measured with the measuring cylinder, the 2 molar will have 20ml of sugar solution and the 1.5 solution will have 15ml of sugar solution and 5ml of water, 1 molar will have 10ml water and 10ml sugar solution, the 0.5 will have 15ml water and 5ml sugar solution, the 0.25 will have 2.5ml of sugar solution and 17.5ml of water and then finally the 0 molar will just have 20ml of water. Once both the potatoes and the solution are sorted out, three potatoes can be put in to each beaker. After every two minutes we will weigh the different tubes in each different solution. The stopwatch will measure the time.
These are the different measurements and units for the experiment: -
The length of the potato tube – 4cm
The different concentrations of sugar solution – 0 molar, 0.25 molar, 0.5 molar, 1 molar, 1.5 molar and 2 molar.
The time left in the solution – Every two minutes one tube will be removed, the other two will stay in the solution to keep the test as fair as possible, each tube will be removed to be weighed in rotation. The total amount of time that the tubes will be in the solution is 12 minutes
The cross – sectional area of the potato tube – size goes here this is the size of the cork borer, because the metal is a complete cylinder the potato tubes will all be the same area all the way through the length of the tube.
The type of potato – The potato that we used was a new potato, the different kinds of potato could have affected the end result.
To make the test safe care must be taken when using both the knife and the cork borer. To make sure that no accidents can happen after the knife has been used, it will be returned to its place as soon as I have used it so that no – one can be harmed by it.
To keep the test fair and reliable all the variables need to be kept the same and as far as possible need to be controlled to the maximum.
Scientific Theory
Osmosis is the diffusion of water through a semi – permeable membrane. Diffusion is when water from an area of high concentration diffuses to an area of low concentration. Random particles hit the wall attempting to get through the selectively permeable membrane, after diffusion the gradient of the substance has reached equilibrium. Factors affecting the rate of diffusion are a) the diffusion gradient – the bigger the difference in concentrations, the steeper the gradient and therefore the quicker the diffusion, b) the form of the diffusion – the particles in solid vibrate, the particles in a liquid are able to move easily and the particles in a gas move extremely quickly, the faster the particles are moving, the more collisions there are likely to be and so therefore more chance of the particles getting through the membrane.
Plant cells can take different forms; they can be turgid or flaccid. When a plant cell is turgid is means that it is swollen and hard, all of the space in the cell has been taken up and so therefore there is more pressure. Because of the cell wall the cell can’t burst, but once it is completely full no more water can be passed into it. A flaccid cell is the opposite of turgid. When a cell is put into a concentrated sugar solution, water diffuses out of the cell, therefore making the cell shrink. When the cell is flaccid the cytoplasm has come away from the cell wall, this is called plasmolysis. Turgor in a plant is important because it holds the leaves up right and towards the sun enabling them to take in more sunlight. A plant only becomes flaccid when water is being passed out of the cell, because it is decreasing the pressure and so therefore it cannot stand up right.
From the above knowledge I would expect to find that the potato tubes put into the 100% water concentration would gain the most weight because no water has had to diffuse out to equal the sugar solution, instead water has diffused into the cell so that it can become more turgid. Potatoes have shoots that grow above the ground and so the leaves of these needs to be turgid, so that they can receive the maximum amount of sunlight.
