A Comparison of the Water Potential of Potato and Sweet Potato Tubers


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A Comparison of the Water Potential of Potato and Sweet Potato Tubers

Aim

The aim of this experiment is to compare the water potential of two
different plant tissues, potato and sweet potato tubers, by measuring
the gain or loss of water when samples of the tissue are placed in a
range of concentrations of sucrose solutions.

Background information

Osmosis is the movement of water molecules from a region of their
higher concentration to a region of their lower concentration through
a semi permeably membrane. If the cell is surrounded by pure water
(which has a water potential of zero), or by a solution that has a
lower concentration and therefore a higher water potential, than that
of the cell’s contents water flows into the cell by osmosis and the
cell would swell up (become turgid). The external solution is said to
be hypotonic to the solution in the cell (hypo meaning lower than and
applies to the solute concentration).

If the reverse was to occur and the cell was surrounded by a solution
whose concentration is higher and whose water potential is lower then
that of the cell, the water would flow out of the cell into the
solution. In this case the external solution would be known as
hypertonic to the solution in the cell. If the cell has the same
solute concentration and water potential as the surrounding solution,
there would be no flow of water into or out of the cell so the cell
stays the same size. In this case the external solution is called
isotonic, meaning it is the same as.

We have seen evidence of using hypo and hypertonic solutions during an
experiment using onion cells. Both onion cells were placed onto a
microscope slide, a drop of water (hypotonic solution) was then added
to slide A, and a salt solute (hypertonic solution) to slide B, below
are photos that were taken of the cells during this experiment.

[IMAGE][IMAGE]

Slide A Slide B

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As you can see from the images above, slide A has become turgid. This
is due to the fact that the water we placed on the slide has a higher
water potential then the cells. There fore the flow of water is from
the external solution into the cell causing it to swell, unlike animal
cells, plants cell have a rigid cell wall and this stops them from
bursting. As the water flows into the vacuole, the turgid cell begins
to oppose the uptake of water, called the pressure potential.

Slide B is the opposite to slide A and the cells have begun to shrink
(plasmolysis). This time the cells had the higher water potential then
the salt solute that we put on them was hypertonic. As the cell had
the higher water potential, the water from within the cell diffuses
into the solute. When the onion cell loses water, the pressure of the
water within it is reduced and this is the reason that the cells
membrane begins to shrink and the cell would begin to feel flaccid.
The point where the membrane just starts to leave the cell wall is
called incipient plasmolysis.

[IMAGE]

To test the results of this onion investigation further I am going to
conduct another experiment this time using sweet potato and normal
potato. During this time I am going to place both the sweet Potato and
normal potato into solutions of different sucrose concentrations and
also one with distilled water. From here I hope to draw conclusions to
the water potential of plant tissue and to see if any of my
predictions are correct.

Predictions

* From what I have learned about water potential I would expect
distilled water to have to highest water potential and therefore
both the sweet potato and normal potato to take up the more
molecules of water and have a larger end mass then what it began
with in distilled water. The two samples of potato should also
feel more turgid because of the extra water that has diffused into
its cells.

* I think that when I use sucrose solution of 0.5m that the sweet
potatoes mass should not change or only have a very small change.
I believe this will happen because; as there should be an equal
amount of sugar and water in the solution, and the potato. This
should cause the particles to flow in and out of the potato
evenly.

* As the normal potato contains starch this is not going to be able
to move out of the cells as the molecules are too large. When the
normal potato is in a solution with a high concentration of
sucrose, the potato itself will have a higher water potential then
the solution and will therefore end up lighter then its starting
mass, because it has a higher water potential then the solution.

* Overall I believe that sweet potato will have a heavier end mass
then that of normal potato because the normal potato will have a
higher water potential and therefore lose mass when in solutions
of a high sucrose concentration.

* I think that the normal potato will become isotonic with the
solution, when the solution is around 0.25m in strength because
once the solutions get any stronger then this then the water is
going to flow from the potato into the solution.

Planning

When doing this experiment I will only be changing one variable and
this going to be the concentration of the solution. I am going to be
using 5 different strengths of solution; these will be distilled
water, then 4 sucrose solutions of strengths; 0.25m, 0.5m, 0.75m and 1
molar (these solutions will have a negative water potential). To try
and make the experiment fair I also kept some of the variables the
same these included the size of the potato (as much as possible), the
amount of the solution and the time that the potato was left in the
different solutions.

There were not any major safety issues’ when doing this experiment,
the main one was taking care when using the knife to cut the potato.
To do this I placed a tile under the potato so that the surface was
protected, and also took care while handling the knife so to protect
myself and others.

Once the potato samples were cut, I weighed them and then placed them
into test tube with 20cm3 of different strength solutions. The potato
was then left in the solution for approximately 2 hours 30 minutes. On
taking the potato out of the solution I blotted it and reweighed it,
to find out the new mass.

Results

On the next page can see two tables of results, these are from the
experiment that I done and also a copy of the results supplied by
Imogen Freeman. The reason that I also used Imogen’s results was
because I did not have time to conduct another experiment, so by using
these results it is going to allow me to draw better conclusions, and
also allow me to compare my results, and to look for consistencies or
differences that might stand out.

Please see attached graphs for experiment one and two, where I have
compared the percentage change in mass against the different solution
strengths.

Sweet Potato Table of Results 1

Start mass

(g)

end mass

(g)

Change in mass

(g)

Change of mass in %

1 molar sucrose solution

1.53

1.49

-0.04

-2.61

0.75 molar sucrose solution

1.91

1.90

-0.01

-0.52

0.5 molar sucrose solution

1.77

1.99

+0.22

+11.11

0.25 molar sucrose solution

1.82

2.17

+0.35

+16.13

Distilled water solution

1.70

2.18

+0.48

+22.02

Potato Table of Results 1

Start mass

(g)

end mass

(g)

Change in mass

(g)

Change of mass in %

1 molar sucrose solution

2.81

1.99

-0.82

-29.29

0.75 molar sucrose solution

3.22

2.50

-0.72

-22.47

0.5 molar sucrose solution

3.29

2.78

-0.51

-15.50

0.25 molar sucrose solution

3.18

3.39

+0.21

+6.60

Distilled water solution

2.78

3.05

+0.27

+9.71

Sweet Potato Table of Results 2

Start mass

(g)

End mass

(g)

Change in mass

(g)

Change of mass in %

1 molar sucrose solution

1.02

0.96

-0.06

-6.25

0.75 molar sucrose solution

1.44

1.47

+0.03

+2.04

0.5 molar sucrose solution

1.76

1.95

+0.19

+9.74

0.25 molar sucrose solution

1.26

1.52

+0.26

+14.87

Distilled water solution

1.59

1.99

+0.4

+25.16

Potato Table of Results 2

Start mass

(g)

End mass

(g)

Change in mass

(g)

Change of mass in %

1 molar sucrose solution

1.74

1.23

-0.51

-29.31

0.75 molar sucrose solution

1.91

1.44

-0.47

-24.60

0.5 molar sucrose solution

2.28

1.95

-0.33

-14.47

0.25 molar sucrose solution

2.29

2.33

+0.04

+1.74

Distilled water solution

2.54

2.95

+0.41

+16.14

Analysis and Evaluation

As you can see from both graphs and as I predicted the sweet potato
had an overall heavier end mass then that of the normal potato. This
is due to the fact that the solution was hypotonic to the sweet potato
cells and therefore had a greater water potential. This in turn has
caused the cells mass to increase and its internal pressure to rise
making the cells themselves more turgid.

During my predictions I also thought that the sweet potato would
neither gain nor lose mass when in a 0.5 molar solution, from the
graphs you can see that this is actually wrong. The point on the graph
where the line cross zero is around 0.80 molar and 0.85 molar for
experiment 1 and between 0.70 molar and 0.75 molar for experiment 2.
To find the exact point that water potential is equal to that of
solute potential plus pressure potential, it would be a good idea to
try placing the sweet potato in solutions around these strengths but
with smaller strength increases.

On experiment 1 the normal potato looks like it has an odd result when
placed in a 0.25 molar solution, but because I have only got two
normal potato samples in this strength solution, it is very difficult
to tell if it really is an odd result. As I had predicted though the
normal potato has lost mass throughout most of the experiment. The
reason for this is that the sucrose solutions between 1molar and 0.5
molar are hypertonic to the potato cells. This causes the cells to
become flaccid and feel floppy to touch, and also means that the
potato cells have a higher water potential then the solution.

From looking at the results table for the normal potato it is hard to
tell what point the water potential becomes equal to the solute
potential plus the pressure potential, but once these results have
been transferred to the graph, it is easier to see the rough point at
which equilibrium is reached is approximately 0.2 molar for both
experiment 1 and 2. As with the sweet potato it would be a good idea
to try placing the potato in smaller increase of solution strength
from around 0.2 moles to 0.25 moles.

In both experiment 1 and 2 the normal potato that was placed in the 1
molar sucrose solution has lost almost exactly the same amount of
weight (just over 29% of its starting mass), leading me to believe the
both samples have become fully plasmolysised, have minimum hydrostatic
pressure and also feel flaccid. However because the sweet potato
already contains sucrose, when it is in the 1 molar solution it only
just begins to lose mass and is probably only at the point of
incipient plasmolysis, to find out when the sweet potato samples
become fully plasmolysised I would need to do another experiment with
stronger strengths of sucrose solutions.

When both samples of potato are placed in distilled water they both
gain in mass, however I don’t think that either of them has reached
the point of full turgor. However if the samples had been left in the
solution for longer they would have become fully turgid, and had
maximum hydrostatic pressure. When either of the samples becomes fully
turgid or total plasmolysised then they are not going to change any
more. This is because when the cell is turgid, the vacuole has reached
its maximum volume and no more water can enter the cell. When the cell
is in a state of total plasmolysis then the vacuole has almost
disappeared from the lack of water within the cell.

If I was to do this experiment again, there are several improves and
changes that I would make. The first thing I would do is to use
smaller increases in solution strengths, at the point where the line
on the graph appears to cross 0, this would allow me to find the exact
point that water potential is equal to solute potential plus pressure
potential. I would also repeat the experiment more times to enable me
to find an average result and also spot a spurious result. Something
else I could do, would be to leave both potato samples in the
distilled water for longer to see how long it takes for them to become
fully turgid.

When doing this experiment you are never going to be able to all the
samples of potato weighing and measuring exactly the same, and because
of this the experiment is not going to be totally reliable. Nor are
you going to be able to blot the potato sample’s the same why every
time you remove them from the solutions. Again this is going to cause
inconsistencies in the results.

References

Examination. (2004) Revise AS and A2 Biology. Uk: Hart McLeod

Indge, Rowland, Baker. (2000) An introduction to AS Biology. Italy:
Hodder and Stroughton.

Roberts, Reiss and Monger. (2000) advanced Biology. Spain: Nelson.

Rowland,M. (1992) Biology. China: Thomas Nelson and Sons Ltd.


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