Finding the Water Potential of Potato Cells

Finding the Water Potential of Potato Cells

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Finding the Water Potential of Potato Cells

Aim: Our aim was to discover the water potential of potatoes cells
using different concentrations of sucrose solution

Science:

Osmosis is 'the movement of water molecules from a region of higher
water potential to a region of lower water potential through a
semi-permeable membrane'

www.s-cool.co.uk

'Water can move between cells (unligninified) freely as the cell walls
are permeable. However its movement is controlled by a number of
factors, which are given names; solute potential ([IMAGE]), Pressure
potential (+ Matrix potential =0 normally)([IMAGE]) and water
potential ([IMAGE]).
Solute potential ([IMAGE]): this refers to the amount of substance
dissolved in the cytoplasm. This effects water movement because water
always moves from a less concentrated medium to a more concentrated
medium.
Pressure potential ([IMAGE]): this refers to the pressure exerted by
the cell walls. In a plant, the cell will fill with water until the
inner membrane is pushing on the outer membrane to the same force as
the wall is pushing back; they then are equal forces and cancel out.
Therefore there is no more movement of water.
Water potential ([IMAGE]): this is a figure worked out from the
equation; water potential = solute potential + pressure potential ([IMAGE]).
The figure represents the tendency of a cell to give out water.

http://www.restoringearth.co.uk/site/ossmosis.htm

Water moving out of cells causes them to lose turgor pressure - cell
membrane detaches from cell wall and shrinks (wilting) - plasmolysis
Water moving into cells causes them to become turgid, swollen -
deplasmolysis
http://biomicro.sdstate.edu/Hutchesh/bio101/Graphics/lab4/tsld011.htm

The is a dilute solution inside the potato cells due to it's cell sap,
this is a 'Dilute fluid found in the large central vacuole of many
plant cells. It is made up of water, amino acids, glucose, and salts.
The sap has many functions, including storage of useful materials, and
provides mechanical support for non-woody plants'
http://www.tiscali.co.uk/reference/encyclopaedia/hutchinson/m0029874.html

Prediction: From the research I have done into osmosis I have been
able to make some predictions about the results I will get.

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I predict
that as the concentration of sucrose increases (the water potential
decreases) Osmosis is the movement of water molecules from a region of
higher water potential to a region of lower water potential, through a
partially permeable membrane, therefore I predict that the potato
cylinders in the weaker sucrose solutions- i.e high water potential,
will gain mass. This is because the solution inside the potato cells
has a lower water potential so there will be a net movement of water
into the potato cells, making them turgid, so they will gain in mass
(due to the extra water) and will be firmer. I think that the potato
cylinders in the more concentrated sucrose solutions, i.e the lower
water potentials, will lose mass. This is because the solution inside
the potato cells has a higher water potential so there will be a net
movement of water out of the potato cells making them flaccid. The
potato cells in the most concentrated solution may lose enough water
that the cell membrane becomes detached from the cell wall and is said
to be plasmolysed. From previous coursework I have done at GCSE, when
I did a similar experiment investigating osmosis in potatoes, I
predict that the potato cells will have a water potential of the
equivalent of around 0.3M of sucrose because this was the solution
that had little net movement of water, this water potential is -900kPa
(ref- teacher support: coursework guidance OCR- graph).

Variables:

· Concentration of sucrose solution - as the concentration of sucrose
increases, there will be more solute molecules and less water
molecules, therefore the water potential will be lower, and so water
will move out of the cells and the cells will lose mass, as the
concentration of sucrose increases, the opposite will happen and the
cells water will move into the cells making them gain mass. This is
the variable that I will investigate; I will use a standard 1M sucrose
solution and dilute it with distilled water to get different
concentrations.

· Size of potato cylinders- the lengths and diameter of the potato
cylinders will affect the surface area of the potato cylinders, a long
thin potato cylinder will have a large surface are to volume ratio
than a short fat cylinder, this will mean there is more area for water
to move into and out of the cells so a long, thin cylinder will show
the effects of osmosis in a short period of time. I will control this
by using the same cork borer for all my cylinders (so they will all be
the same diameter, and I shall cut them all to exactly 3 cm long.

· Species of potato- Different species of potato will have slightly
different contents in their cell sap, this means the solutions within
the cells will be different so there water potential will be slightly
different, therefore so will their osmotic effects. I will control
this by using the same potato to cut all my cylinders.

· Length of time the potatoes are left in the solutions- The longer
the potatoes are left in the solutions, the longer time there will be
for the net movement of water, so the movement of water will be
greater, after a while the movement of water will end, as both the
solutions have the same water potential, but if not left for long
enough, the effects of osmosis may not be finished. I will control
this by ensuring all the cylinders are put into their solutions at the
same time, and are taken out at the same time. The potato cylinders
will all be left in their solutions for 24 hours, which should be
enough time for osmosis to have occurred fully, and there will be no
net movement after that time.

· The amount of solution- the more solution there is the more solute
and water molecules there are so the movement of molecules will occur
quicker, therefore the net movement of water in osmosis will occur
quicker. I will control this by using a measuring cylinder and
ensuring all the solutions amount to 10cm .

· Temperature of the solutions- The higher the temperature of the
molecules, the faster they will move so the movement of water
molecules will increase, so the higher the temperature the higher the
net movement of water. I will control this by leaving all the
solutions in the same environment- the science lab, at room
temperature.

Pilot run

For my pilot run I decided to investigate the best range of sucrose
solutions to put my potato cylinders in, this will also give me a
chance to ensure the equipment I will use is appropriate and that the
lengths and diameters of the cylinders are suitable to measure the
effects of osmosis.

Risk assessment:

The Procedure

The Risk

The Prevention

Using a cork borer to cut out the cylinders of potato

The cork borer is sharp and needs force to be inserted into the potato
but then slices with relative ease, if pushed too hard it could cut
into hands

Always ensure hands are in a safe position (i.e not clasping the
potato) and cut down onto a while tile

Using a scalpel to cut the potato cylinders into equal lengths

The scalpels are very sharp and could cut hands

Using cautiously, making sure hands and fingers aren't in the way.
Also when walking with them ensure the blade is pointing towards the
floor

Walking around the lab with equipment

Bags on the floor could be tripped over

Ensure the path way is clear, with bags pushed under the tables.

Transferring the solutions into the McCartney bottles

The McCartney bottles are glass and if they topple over the will
smash, the shards of glass could cut someone

Hold the bottles steady whilst pouring liquid into them, and wear
goggles in case of glass shards

Using syringes to make the sucrose solutions

The sucrose could squirt out of the syringe and could get someone in
the eyes

Ensure goggles are worn to prevent solutions coming into contact with
the eyes

Apparatus:

· Distilled water

· McCartney bottles and lids

· Sucrose solution

· Potato

· Syringes

· White tile

· Scalpel

· Ruler

· Balances

· Cork borer

· Tissues

Method:

· Find the cork borer with a diameter of 0.8 cm to cut out the potato
chips

· Put a potato of a white tile to cut the potato- and NOT on your hand
as the cork borers are very sharp and will easily cut your hand.

· Cut out six cylinders of potato

· Ensure there is no skin left on the potato cylinders

· Line the six cylinders up and cut the all to 3 cm long.

· Weigh each cylinder making sure you know which mass applies to which
cylinder and then put the cylinders to the side

· Make the sucrose solutions; adding different amounts of 1M sucrose
and water can make these. For the 1M sucrose solution measure out 10cm
of the sucrose with no water, for the 0.8M measure out 8cm of sucrose
and add 2cm of water to make it up to 10 cm. Do the same for the 0.6M,
0.4M, 0.2M and the 0M (which is just water)

· Put each solution into a McCartney bottle.

· Add a potato cylinder to each of the bottles (making sure you know
which cylinder is in which solution, so the mass change can be
calculated)

· Ensure the lids are put on the McCartney bottles to prevent any loss
of water by evaporation.

· Label all the bottles and leave them overnight.

· Take the potato cylinders out of the solutions, making and
observation of their appearance

· Blot the potato cylinders to remove excess liquid and then reweigh
them.

· Record your results in a table and calculate the percentage change
in mass using the equation: (Change in mass/ original mass) x100

Results:

Concentration of sucrose solution (M):

1

0.8

0.6

0.4

0.2

0

Initial mass (g):

1.08

1.06

1.00

1.05

1.04

1.02

Final mass (g):

0.60

0.55

0.64

0,73

1.02

1.17

Change in mass (%)

-44

-48

-36

-30

-1.9

14.7

Conclusion: My pilot run was very successful, the equipment I used and
the size of the potato chips were appropriate. I choose this size of
potato cylinder- 3cm length and 0.8cm diameter, because it was the
variable I had investigated for my GCSE pilot run and, referring to my
GCSE coursework I felt it was the size potato cylinders which would
show the effect of osmosis best, my results were successful, so I
shall use the same measurements for my actual experiment. The
equipment I used was all appropriate and will be the equipment I use
for my actual experiment. The range of concentrations I used for my
pilot, which was the main aspect I was investigating, was, I feel, a
little too wide, all the results but one, showed a negative change in
mass, meaning they were all losing water, so the solutions were too
concentrated. For my actual experiment I think I will cut my range of
concentrations in half, and will experiment with using intermediate
concentrations, such as 0.15 and 0.25, so that I can get a closer
estimate of the potatoes water potential. For my real investigation I
shall use solutions between 0 and 0.4 M sucrose, using intermediate
values such as 0.15, so that I can get closer to finding the
concentration at which no net movement occurs.

Actual Method

Apparatus:

Choice

Reason

Distilled water

This was used rather than tap water because tap water may contain
traces of Chlorine and Fluorine which would lower the water potential
slightly making the results less accurate

McCartney bottles and lids x 6

1 bottle for each concentration, as the bottles are glass the potato
cylinders can be observed. The lids on the bottles will ensure no
water is lost through evaporation.

Sucrose solution (1M)

A 1M sucrose solution will be used to make the solutions by diluting
it with water to make the other sucrose solutions

Potato

A potato will be used to cut potato cylinders out of.

Syringes x 2 (10cm)

Two syringes will be needed to measure out water and sucrose; the use
of two syringes will prevent any cross contamination of the two
substances. The syringes are small and will allow a fairly accurate
measure.

White tile

This provides a hard surface fro the potato cylinders to be cut onto

Scalpel

This is used to cut the potato cylinders to equal lengths

Ruler

This is used to measure the lengths of the potato cylinders and will
need a cm scale on it.

Balances

This will be used to measure the mass of the potato cylinders before
and after, a digital balance showing the mass of the potato cylinders
to two decimal places for accuracy and this will prevent human errors
that may occur with manual scales

Cork borer (0.8)

This is used to cut the cylinders from the potato

Tissues

The potato cylinders should be blotted with a tissue before being
weighed to remove any excess liquid.

How to make the sucrose solutions:

Using 1M sucrose as a starting point, the different solutions can be
made. By diluting the 1M sucrose down with water, the molarity of the
solution can be altered by the ratio of water to sucrose: Here is a
dilutions table:

Concentration of solution (M)

Amount of sucrose (cm )

Amount of water (cm )

1

10

0

0.9

9

1

0.8

8

2

0.7

7

3

0.6

6

4

0.5

5

5

0.4

4

6

0.3

3

7

0.3

2

8

0.1

1

9

0

0

10

I will need at least 5 different concentrations to ensure my results
are correct and that the concentrations go in the right order,
therefore increasing the accuracy and I shall do 3 repeats to take
into account any anomalous results, so if one result is very different
it can be isolated, doing 3 repeats will allow me to take an average
which will be more accurate.

Accuracy:

I have made my results as accurate as I can be using accurate
measuring equipment, such as 10cm syringes, as they have the smallest
scale so will therefore be able to be measured to a more accurate
point, I also used a digital balance which measuring the mass to two
decimal points. Distilled water was used to reduce the amount of
impurities, which may affect the water potential, and reduce the
accuracy of the results. To avoid muddling, I used a tray, which I
labelled with the different concentrations, I put the cylinders under
each of the labels with their corresponding mass, these were kept here
until they went into the McCartney bottles, which had been labelled to
ensure the right cylinders went into the right solution.

Method:

1. Using a cork borer with a diameter of 0.8 cm cut 18 potato
cylinders onto the white tile. Push the cylinders out of the cork
borer and avoid handling as much as possible. 3 cylinders will be
needed for each concentration to reduce the risk of anomalous
results.

2. Using a scalpel cut all the potato cylinders down to 3 cm and
ensures there is no skin left on them, as the skin is less
permeable and will affect the movement of water.

3. Weigh each chip and record this on a table, ensure you know which
mass goes with which chip so that when they are re-weighed the
mass change can be calculated.

4. Make up the sucrose solutions using the method above as guide for
the different concentration.

5. Pour the solutions into the McCartney bottles and label
immediately to prevent any confusion later.

6. Add the potato cylinders to the solutions, ensuring you know
which cylinders are going into which solution, as three cylinders
will be going into each McCartney bottle, colour code them so you
can tell them apart and know which one weighs what.

7. Replace the lids on the McCartney bottles to prevent the loss of
water by evaporation, which would effect the sucrose
concentration.

8. After 24 hours, remove the cylinders from the solutions, blot
with a tissue and reweigh, do one solution at a time to reduce
confusion but don't take too long on each one. Record the results
immediately

9. Calculate the average change in mass and plot a graph of mass
change against concentration.

10. Find the point where the mass change is 0; find the
concentration, which corresponds to this and that is the
equivalent water potential. Using the graph of water potential
against Sucrose concentration, (ref- see appendix) the
concentration for no mass change can be converted into water
potential (kPa). This will give you the water potential of potato
cells.
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