Effect of Sucrose Concentration on the Rate of Osmosis

The effect of sucrose concentration on the rate of osmosis across a potato’s cell membrane submerged for 94 hours in the solutation.

Background Information

Osmosis is the movement of solvent molecules across a partially permeable membrane. They move from a region of low concentration (hypotonic) to a region of high concentration (hypertonic). The rate of osmosis across a eukaryotic cell membrane can be affected by different factors; including temperature, concentration gradient, water potential and the surface area for osmosis to occur and pressure exerted on either side of the semi-permeable membrane.

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A potato (Solanum Tuberosum) contains 92% water, which is an osmotically active component. The precise osmotic potential of sweet potatoes is however unknown. The osmotic potential of sucrose is know to be at 20 oC, the osmotic potential of sucrose is 2. 436 MPa2. The rate of osmosis will be calculated by measuring the percentage change in mass of a piece of potato, that has been submerged in a specific concentration of sucrose over a 94 h period.

Research Question

What is the effect of sucrose concentration on the rate of osmosis across a potato (Solanum Tuberosum) cell membrane? Hypothesis:

If the sweet potato cytoplasm is hypotonic relative to the sucrose solution, then water will move from the potato into the sucrose solution, resulting in a decrease in mass. If the potato cytoplasm is hypertonic relative to the sucrose solution, then water will move from the sucrose solution into the potato, resulting in an increase in mass of the sweet potato.

Variables

Independent| Concentration of sucrose solution: 1. 0 M0. 8 M0. 6 M0. 2 M| Dependent| Rate of osmosis, indicated by change in mass of sweet potato (±0. 01 g)/time (94h ±0. 01h)| Controlled| Temperature Water potentialPressure exerted on either side of cell membrane| Uncontrolled| Surface area for osmosis|

Method

1. Peel potato and cut chunks of uniform mass using a custom made potato corer.
2. Place two potato chunks on the balance and record the mass in a suitable table.
3. Put two chunks in a glass boiling tube.
4. Pour in enough 1 M sucrose solution to cover the potato. Record the concentration of sucrose used, and the initial mass of the two potato chunks, on the side of the beaker using an indelible pen. Mark the time, mark the ambient temperature recorded on the thermometer and start the stopwatch.
5. Precisely 24 h later, pour the sucrose solution off the potato chunks.
6. Dry the potato chunks carefully using a paper towel.
7. Place the potato chunks on the balance and record the mass in the raw data table.
8. Calculate change in mass and percentage change in mass (final mass – initial mass/initial mass)*100.
9. Repeat steps 1 – 8, substituting 0. 2 M, 0. 4 M, 0. 6 M, 0. 8 M sucrose for 1. 0 M sucrose.
10. Repeat steps 1 – 9 four times, so that 5 sets of two potato chunks are submerged in each specific concentration of sucrose.
11. Peel potato and cut chunks of uniform mass using a custom made potato corer.
12. Place two potato chunks on the balance and record the mass in a suitable table.
13. Put two chunks in a glass boiling tube.
14. Pour in enough 1 M sucrose solution to cover the potato. Record the concentration of sucrose used, and the initial mass of the two potato chunks, on the side of the beaker using an indelible pen. Mark the time, mark the ambient temperature recorded on the thermometer and start the stopwatch.
15. Precisely 24 h later, pour the sucrose solution off the potato chunks.
16. Dry the potato chunks carefully using a paper towel.
17. Place the potato chunks on the balance and record the mass in the raw data table.
18. Calculate change in mass and percentage change in mass (final mass – initial mass/initial mass)*100.
19. Repeat steps 1 – 8, substituting 0. 2 M, 0. 4 M, 0. 6 M, 0. 8 M sucrose for 1. 0 M sucrose.
20. Repeat steps 1 – 9 four times, so that 5 sets of two potato chunks are submerged in each specific concentration of sucrose.

Apparatus: List

• Two potatoes
• Vegetable peeler
• 250 cm3 1M dm-3 sucrose solution
• 250 cm3 0. 8 M dm-3 sucrose solution
• 250 cm3 0. 6 M dm-3 sucrose solution
• 250 cm3 0. 4 M dm-3 sucrose solution
• 250 cm3 0. 2M dm-3 sucrose solution
• 25 x 30 cm3 glass test tubes, 3 cm diameter Stopwatch
• Thermometer, accurate to ± 0. 5 oc
• Balance, accurate to ± 0. 01 g
• Indelible marker pen
• Paper towel
• Aluminium foil to cover glass beakers

Risk Assessment

The procedure involves minimal risk, since it uses Swiss organic potatoes and sucrose solution which is not toxic. Avoid ingestion of raw potato.

Results

Measurements recorded before and after experiment including percentage change in mass Sucrose concentration (M)

| Mean % Mass change| Standard Deviation % Mass change| Standard error % mass change| Sucrose concentration (M)| Mean Rate of osmosis (%/h)| SD Mean rate of osmosis| 0| -11. 4| 12. 14| 5. 43| 0| -0. 119| 0. 129| 0. 2| 1. 82| 13. 64| 5. 57| 0. 2| 0. 019| 0. 145| 0. 4| -3. 05| 5. 07| 2. 07| 0. 4| -0. 032| 0. 054| 0. 6| -11. 84| 5. 31| 2. 17| 0. 6| -0. 126| 0. 057| 0. 8| -14. 85| 8. 14| 3. 32| 0. 8| -0. 158| 0. 087| 1| -24. 45| 9. 63| 3. 93| 1| -0. 260| 0. 102|

Figure 1: Mean (± SEM) % mass change in chunks of potato (Tuberosum Solanum) immersed in varying concentrations of sucrose solution (0 – 1. 0 M) for 94 h Figure 2: Mean rate of osmosis in chunks of potato (Tuberosum Solanum) varying concentrations of sucrose solution (0 – 1. 0 M) for 94 h

Discussion

Both graphs support that all solutions containing sucrose apart from 0. 2 M are hypotonic, because as stated in the hypothesis when the mass decreases it means the water in the potato is moving out of the membrane into the sucrose trying to equalize both concentrations. For the 0. 2 M however, the mass increased which suggests that the potato is hypertonic relative to solution. Graph 2 shows the different osmotic potential possible for each concentration, even though the mass for is decreasing for all apart from 0. 2 M, both of these are considered osmosis.

They differ in the way, if either the potato or the solvent has the high or low concentration, the process however is the same. The decrease in mass would be called reverse osmosis as the goes against the nature way of osmosis. (2) I find a result s a little surprising in the way that the water is actually hypotonic, I would have expected it to me hypertonic. Possible errors could have occurred though that make the results less accurate. Some students believe that they might have mixed up their solutions, that would mean a huge error in the results.

Also the measurements were only taken before and after the 94 hours, to show the rate of osmosis results should have been taken more frequently. Another possible error could be the way the tube was closed; we put aluminium, which is not the most secure lit. It could have allowed some of the solutions to evaporate affecting the results as well. All of these could have been possible errors that either increased or decreased the percentage change of the different groups. I would suggest in order to make it a fair test; be sure to label the tube, use a crock to close the tube and record the data more frequently.

Conclusion

Possible factors that effect osmosis are temperature, concentration gradient, water potential and the surface area exposed for osmosis to occur. (1) For this experiment specifically the concentration gradient was changed, however other things might not been fully controlled either. The surface area is a factor not fully controlled. The data collected makes sense though, and clearly supports the hypothesis discussed earlier. To answer the research question (What is the effect of sucrose concentration on the rate of osmosis across a potato cell membrane? the results make it very clear that as the concentration increases so does the potential for osmosis to occur. Reasons behind extreme percentage change could be some of the errors stated before or it could indicate that if the solution was raised even further the rate of osmosis would be even greater.

Reference

1. Allot, A. and Mondorff, D. Biology Course Companion. 2nd Edition, Oxford. Text. 04 Feb. 2013.
2. "A COMPLETE RESOURCE GUIDE ON OSMOSIS. " A Complete Resource Guide on Osmosis. N. p. , n. d. Web. 04 Feb. 2013.

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