LABORATORY REPORT: OSMOSIS IN A POTATO
INTRODUCTION
After having done research on transport in the cell with methods such as diffusion, osmosis
and active transport, we were given the task of seeing the process of osmosis happen within potato
slices. Our control variables were the amount of water in a beaker (100ml in a beaker), our
independent variable was the amount of salt content in each of the four beakers (0g in the first, 2g
in the second, 4g in the third, 6g in the fourth) and our dependent variable was the length of the
potato slices (all cut to around 3.5cm in length). To do the experiment, we had 4 beakers, all filled
with 100ml of natural water. Every beaker would have 3 potato slices cut to 3.5cm, calibrated by a
ruler. Beaker 1 would have no salt, beaker 2 would have 2g of salt, beaker 3 would have 4g of salt,
and beaker 4 would have 6g of salt. My team (Nicola and Julie the visiting student) dropped the
potato sliced into the beakers at the same time, which made us start the timer for 20 minutes.
ANALYSIS
Our hypothesis was the beaker with no saline content would make the potato slices expand
as there would be more water molecules in the water surrounding the potato slice than in the
individual potato slices (the potato slice itself containing different molecules other than water
molecules). From this, we deduced that the potato slices in the salty water would shrink as the
surrounding content would presumably contain less water molecules than in the potato slices.
SALT
CONTENT
0g
2g
4g
6g
INITUAL POTATO
LENGTH
FINAL POTATO
LENGTH
3.5
3.5
3.5
3.5
3.5
3.5
3.5
3.5
3.5
3.5
3.5
3.5
3.8
3.8
4.0
3.5
3.5
3.1
3.3
3.5
3.5
3.5
3.5
3.1
CHANGE IN
LENGTH (%)
8.2%
8.2%
13%
0%
0%
-12%
-5.9%
0%
0%
0%
0%
-12%
AVERAGE
CHANGE (%)
9.8%
-4%
-2.0%
-4%
-> Fig 1
As we can observe from the table above (Fig 1), the potato slices didn’t change sizes as
expected: in the natural water, the potatoes did expand, which confirms our hypothesis. This is seen
by the average change of 9.8%, a positive increase compared to the other potato slices that were
put in the beakers with saline content. From this, we expected the potato slices placed in the salty
water to shrink down a gradient (e.g. those placed in 2g would have an average change by 2%, those
placed in 4g would have an average change of 4%, those placed in 6g would have average change of
6%, etc).
As we can observe from Fig 2, the
change does not decrease along a
gradient. Instead, we can see that potato
slice placed in beakers 2 (saline content
2g) and 4 (saline content 6g) had the same
average change in potato slice length and
beaker 3 (saline content 4g) seems to be
an anomaly.
0g=beaker 1; 2g=beaker 2; 4g=beaker 3;
6g=beaker 4
-> Fig 2
CONCLUSION
This experiment has disproved a part the initial hypothesis: the potato slices placed in the beaker
with water proved the hypothesis true as they expanded within the 20 minutes by 9.8%. The potato
slices placed in beakers with a higher saline content did shrink, but just not down a downward
gradient. Instead, the results with increasing saline content showed that the average change
decreased for beaker 2 (2g), increased in beaker 3 (4g) then decreased in beaker 4 (6g) again to the
same average change as beaker 2. Thus, we can conclude from these results that potato slices
placed in water with no saline content will expand because there is a higher concentration of water
molecules around the potato than in the potato. This leads to the surrounding water molecules
diffusing into the potato through a semi-permeable membrane, showing proof of osmosis. From this
we can conclude that potato slices in the salty water shrink as there is a higher concentration of
water molecules inside the potatoes than in the surrounding water, leading to the reverse of what
happened in the water with no saline content (the potato slices shrink).
EVALUATION
One weakness of this experiment is the time constraint: this led to there not being enough time to
observe the greater differences there could have possibly been in the size change of the potato
slices in the differing saline contents, leading to more ambiguous results and an unclear answer to
the hypothesis. To improve this, I would suggest leaving the potato slices in their differing beakers
for over a few days by booking the lab for two different days that are nearby or coming back to the
experiment for a few quick minutes at the end of the day just to get the end results to use for our
calculations back home.