Osmosis Investigation.

I also predict that with a lower concentration the potato will become turgid. Turgid is the term referring to a plant cell once it has taken in water by osmosis but the thick cell wall will not allow it to burst so it becomes hard and firm.

Preliminary Investigation

Prior to the investigation I did preliminary work which I used to find out what would be a good range of variables to use to get results which would show that osmosis happens more in a lower concentration than a higher concentration. I set up 3 test tubes each with a strip of potato in. I had on test tube of low sugar concentration, one of medium concentration and one of high concentration.

Method

I intend to take 6 measurements ranging from strength of 0M to strength of 1M. I will used distilled water (0M) as a control and also note the change in length at 0.2M, 0.4M, 0.6M, 0.8M and 1M. I decided to use this range based on my findings in my preliminary work.

To keep the investigation safe I will use the equipment carefully and responsibly.

I will use 2 strips for each measurement and take an average of the change in length to try to avoid any unreliable results. This will make my conclusions more accurate and reliable.

Apparatus:

• Test tubes (6)
• Potatoes (2)
• Test tube rack
• Measuring cylinder
• Sucrose solution of different strengths from 0M to 1M
• Foil
• Tweezers
• Knife
• Cutting tile
• Ruler

Planned Method

1. Collect equipment and set it up as shown in diagram 1.
2. Cut 12 strips of potato, each 6cm x 1cm. Make sure that each has the skin taken off it to make it a fair test. Check the room temperature and check that it is consistent throughout the investigation.
3. Fill each test tube with 20 ml of the intended strength solution and mark on test tube rack with a washable pen which strength it is.
4. Put two slices of potato in each concentration of solution. This will give me an average to eliminate any poor or inaccurate results.
5. Cover each test tube in foil to prevent any evaporation during the investigation.
6. Leave the test tubes for 24 hours so that there is time for water molecules to pass into and out of the potato enough that our results will reflect that osmosis has clearly occurred.
7. Take the foil off of the test tubes and measure the length of each strip of potato. Record the length of each strip of potato in a table of results.

Skill Area O: Obtaining Evidence

I used two potato strips for each concentration of sucrose and took an average of the start and length this way I could eliminate any results that were unreliable or inaccurate and have firmer evidence. I made one change to my plan and used only five different strengths of sucrose and left out the 1M variable. This way I saved time and still had a wide enough range of results to show me some clear conclusions. I could not control the temperature in this experiment and, therefore, it varied slightly throughout the investigation. I was unable to measure the temperature throughout the 24 hours of the experiment when the potato chips were left in the solutions. The temperature would not have differed much and would have stayed very close to the 24ºC that the room temperature usually is in the specific room. I had to rely on this staying consistent through the investigation but do not believe it would have changed enough to severely affect my results. I recorded how flexible each chip was after the experiment so I could see a link between osmosis and turgidity. I recorded the temperature when each chip was put into the solution and sealed off with foil. This way I could be sure I was only changing one variable, the concentration, which meant my results would be more reliable.

Skill Area A: Analysis

I did not use many calculations in this investigation because it is a biological investigation and does not involve many calculations. It is more about observations than calculations. Biological processes are often natural and don’t always have a set equation or formula. However, to calculate percentage change from start to finish I used this calculation:

Percentage change= Average end length – Average start length             x 100         

                                      Average start length

The higher the concentration of sucrose, the smaller the percentage increase. This is because if there is a higher concentration of sucrose there are more water molecules passing from the potato into the water because in osmosis water molecules pass through the potato skin to an area which has a lower concentration of water molecules because there are more sucrose molecules. If there is water passing out of the potato because of a higher concentration outside it shrinks rather than increases in size so the percentage increase is lower, i.e. a negative number.

The graph of sucrose concentration against percentage increase in length is a curved line. This is because there is a point where, as sucrose concentration goes up the potato cannot carry on releasing water molecules because there are not enough to continue losing water molecules at the same rate. This is the same as if you had a lower concentration of sucrose the potato would not be able to carry on gaining water molecules at the same rate because the potato will just swell up until it cannot gain anymore water. I will become turgid and firm when concentration is low because a plant cell wall does not burst and the cell just continues to swell.

This supports my prediction that as sucrose concentration increases, percentage increase decreases. It also supports my prediction that as concentration increases the potato chips will become less firm and more flexible. However, I did not predict that the graph would be a curved line as a result of the fact that the chips can become saturated.

Skill Area E: Evaluation

The results for 0.2M and 0.4M do not completely fit the pattern shown by the other results. I would have expected them to have decreased more because they are slightly off of my curve of best fit. However, they are not very far out so I don’t think they would have affected the investigation to a significant extent.

None of my results were really very anomalous and were all fairly accurate. I repeated my results twice for each measurement so I don’t believe there were any measurement inaccuracies. I took an average of the two chips for each result in order to eliminate any poor results. The chips were left overnight so I could not measure the temperature throughout the investigation. This may have caused slight variations but nothing large enough to disrupt the experiment. When measuring the flexibility of the chips I did this by hand and there is no way to accurately measure flexibility so this may have not been entirely accurate. If the potatoes had not been fresh or they had a bit of skin on this would have prevented water passing into and out of the chips.

The fact that I repeated each test and the values were close together suggests that the results I got are reliable. Also as most of my results are very near my curve of best fit it is acceptable to say that it is confirmation that my prediction that as the concentration increases, the percentage increase decreases.

One of the main problems with my investigation was that I could not control the temperature or even check it throughout the experiment. This meant that I could not ensure that the temperature was constant throughout the experiment. To combat this I could leave the chips in a cooler environment overnight. This way even if the temperature did vary there would be no risk that it would go high enough to cause evaporation, which would affect the results because water would move through evaporation as well as osmosis.

I measured each strip by the length and width.  I think I should have measured each strip by mass as well because two potato strips of the same dimensions could have different masses.  I believe that the mass could affect the amount of osmosis more than length because a heavier chip would have a less water potential.

I could also have used a wider range of sucrose concentrations with a smaller gap between them so I could see more intimately the relationship. Also this would make the results a lot more accurate.

My investigation showed me that there is an obvious relationship between the concentration of sucrose and the amount of osmosis in a potato chip. To extend my investigation I could look to see if the amount of osmosis follows a similar pattern if a different solution was used instead of sucrose. It may also be interesting to investigate if the belief that keeping potatoes submerged in water is the most effective way to keep potatoes fresh and prevent them from going off. Also I could compare the rate of osmosis in a potato chip to the amount of osmosis in another object which has a partially permeable membrane.