Investigate if osmosis occurs in potatoes and whether varying the concentration of sucrose solution affects the water/mass gain or loss in the potato chips when kept in a constant environment.

Fair Testing- Controlling variables.

In order to get fair, accurate results I will:

• Make sure that all of the potato is covered by the sucrose solution so the whole surface area of the potato can under go osmosis.
• Measure all variables accurately including solution volume and potato size and mass using correct equipment- ruler, measuring cylinder.
• Place all the potato chips at the same time into the solution and immediately starting the timer. Also stopping the timer as soon as the 30 minute period is over so reducing the chances of one chip staying longer in the test tube than the other- all chips stay in solution for 30 minutes.
• Try to reduce the amount of contact with the test tube as heat may be transferred from my hand to the test tube via convection, consequently making one test tube hotter and therefore breaking the rule that all variables shall remain constant.
• Use the same type of test tube- same volume, as different volume test tubes may mean different amounts of potato exposed to sucrose solution.
• Make the 5 potato chips from the same potato as different potatoes may have a different permeable membrane or one that is different to the others.

• Do the experiment at room temperature.
• Keep the water content in the potato the same by using chips from the same potato and by not washing or exposing them to water.
• Use the same mass balance as some balances may have worn out over time making them read masses incorrectly or different manufactures of balances may measure the sizes of the units differently making measurement slightly different between scales.

Safety Precautions

• I will take care using the knife as it is very sharp and is designed to be used to cut hard substances such as potatoes, which will easily pierce my skin or other students around me.
• I shall also take care when handling the solutions as they may get in to our bodies internally and as we are not aware of the capability of the solution and what harm it can do to us.  

Scientific Knowledge

Osmosis is the movement of water particles from a higher concentration to a lower concentration across a partially permeable membrane. A partially permeable membrane is just a membrane with tiny holes in them that only allow small enough particles to pass through them. Other molecules such as glucose, which are too big, will not be able to pass through the membrane and stay at the same side of the membrane. Osmosis requires no respiration- energy, although it requires a living cell membrane for osmosis to occur in cells, but will happen with suitable non living membranes such as visking dialysing membrane. Water entering a solution through a selectively permeable membrane exerts a pressure that can be measured using a manometer. In order to prevent plant cells from taking up too much water and causing a high pressure to be exerted (which would cause them to burst), cells osmoregulate which is just the regulation of osmosis.

        As osmosis is the movement of water from a higher concentration to a lower concentration, when a substance (potato) -high concentration of water, with a semi permeable membrane is placed in a high concentrated solution of glucose- therefore low concentration of water, then according to the theory of osmosis water should move from the potato (high concentration of water) to the solution of glucose (low concentration of water). So if the concentration gradient is lower in the potato, the water will then transfer from solution to the potato and vice versa.

Extra Scientific Knowledge

When plant cells take up water they become turgid and exert a pressure. They do this when placed in a dilute solution where water is taken up. Turgid means when the cells are fat and hard- full of water. The content of the cell pushes against the cell wall and causes turgidity to happen. As the pressure within the cell increases the water pressure or hydrostatic pressure works in the opposite direction of osmosis. The pressure of turgid cells work together to hold up the plant and it acts as “bones” to keep the plant facing the sun and able to photosynthesize. Turgidity also plays a part in transpiration as turgid guard cells open stomata cells and allow water vapour to be removed from the plant- therefore more taken up by the roots.

        The opposite of turgid is flaccid- when cells begin to wilt and cause the entire plant to droop towards the floor. This is also called plasmolysis

When plants are in an in-between state of turgid and flaccid the correct name to call this is incipient plasmolysis which means “about to be” plasmolysed.

Prediction

Looking at the scientific knowledge, I think it would be true to say that the potato chip in the 2M concentration of sucrose solution will loose water and therefore mass, as the scientific knowledge states that osmosis is the net movement of water across a selective permeable membrane from a high to a low concentration. So as there is a low concentration of water in the sucrose solution and a high concentration of water in the potato then in theory water should move from the potato to the solution-so there is a decrease in mass. Likewise if the potato chip is in a 0.5M sucrose solution then according to the rules of osmosis water should move from the solution to the potato. As there is a higher concentration of water in the solution and a lower concentration in the potato then due to osmosis water will enter the potato and cause a mass increase. If my prediction is true then the graph that I will draw from my results will look like this rough one:

        

                

                          Time (minutes)

See Key on next page

        

        Key

                                           Solution with low concentration of sucrose,

                                           Solution with high concentration of sucrose

                

This graph gives us the basic idea of how osmosis in the potato should occur. The solution with a low concentration of sucrose must have a high concentration of water so consequently percentage mass change should be positive and an increase. Whereas the solution with a high concentration of sucrose must have a low water concentration, so water will pass from the potato to the low concentration of water making water content balance. This will then cause the percentage mass change to decrease as shown by the red line of the graph.

             Taking into consideration, we did a preliminary test of the experiment to see whether we could improve the final run. We can see from the preliminary results in the Results section that the prediction was fairly accurate and supported the scientific knowledge, although we learnt from the preliminary work that we should use smaller sized potato chips as the got stuck in the test tube.