The Effects of Osmosis on Potato Cells


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The Effects of Osmosis on Potato Cells


Introduction

The aim of this investigation is to see the effect of varying
concentrations of glucose solution on the amount of osmotic activity
between the solution and a potato chip. An investigation into
Plasmolysis in onion cells was undertaken prior to this experiment, in
order to gain some knowledge of osmosis to enable a hypothesis to be
formed.



Preliminary Work and Scientific Knowledge
=========================================

Osmosis is defined as the net movement of water molecules from a
region in which they are highly concentrated to a region in which they
are less concentrated. This movement must take place across a
partially permeable membrane, like a cell wall, which allows small
molecules like water through but does not let larger molecules
through. The molecules continue to diffuse until the area in which
they are found has an even distribution of molecules all round, inside
and outside of the cell. Naturally, an environment that is identical
inside and out of the cell is the best condition to survive (i.e. the
'perfect' environment).

In the case of the onion cells, when placed with a drop of water,
after 15 minutes, the cell had a turgid, or swollen, appearance under
the microscope and felt fairly strong and sturdy, as the water was
diffused through osmosis into the cell. This is because pure water has
a 100% concentration of water, and the onion cell has much less, so
the water diffused from a higher concentration to a lower
concentration. With the 1% glucose solution, the cell had a flaccid
appearance and the onion felt particularly floppy, because the cell
membrane is pulled into the cell when the vacuole shrinks. This is
what Plasmolysis is, the shrinkage of cell cytoplasm, with the
membrane being pulled in. This happens because the water inside the
vacuole moves outside the cell as the 1% glucose solution is a
stronger solution than the solution in the vacuole, resulting in there
being a higher concentration of water inside than out.

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Variables

To ensure that this investigation is a fair test, certain aspects of
the experiment will have to be kept the same while one key variable is
changed. To gain a result related to the aim of the experiment, the
strength of the glucose solution will have to be changed. This will
give a set of results from which a conclusion can be drawn, but if a
non-variable is changed, the experiment would not be fair. For example
if the potato chips are all different lengths, the longer ones will
have more surface area enabling more osmosis to occur which could
affect the result.

For this investigation, in order to have the same temperature for each
experiment, they will all be done at room temperature. The volume of
the glucose solution in each boiling tube must be the same and must
also completely cover the chip, and to ensure this, a measuring
cylinder will be used and 25cm3 of each solution will be used. The
mass and length of the potato are dependent variables and so will be
measured throughout the experiment. The mass will be measured in
grams, using the same set of scales each time. Measuring the mass will
allow us to see if osmosis has taken place because if it has then
water will have either entered or left the potato. The length will be
measured in millimetres using a ruler. The duration of the experiment
must also be the same and so each solution test will be done at the
same time and left for 15 minutes.

To ensure reliability, the experiment will be repeated in that each
solution will have two potato sticks in it. Also, the exact same
length of the potato chip, in millimetres, on all four sides cannot be
guaranteed, and as a change of only a few millimetres is expected, a
corner of the side to be measured will be cut out to make sure the
same side is measured at the end of the experiment.

Apparatus

* Slab of potato object being examined

* Scalpel to cut the potato with

* White tile to cut the potato on

* Range of solutions part of experiment

(0.2M, 0.4M, 0.6M, 0.8M glucose solution)

* Distilled Water part of experiment

* 5 Boling Tubes to put sticks in for observation

* Clock to time the experiment

* Electric Balance to weigh the potato

(0.01g)

* Ruler (mm) to measure the length of the potato

* 5 Measuring Cylinders to measure the solution

* Tissue Paper to dry the potato

* Forceps to remove the potato from the boiling tube safely

* Boiling Tube Rack to keep the boiling tubes steady for 15 minutes

The balance to 0.01g and a millimetre ruler was chosen because the
differences in mass and length may be miniscule, so a less accurate
scale will not show the differences.

Planned Experiment

This experiment is pretty straightforward and there isn't anything too
difficult to do. Ten potato chips need to be prepared. They will be
cut using a scalpel and should be 3cm long and 1cm wide. A surplus of
1 or 2 millimetres can be allowed but no more as the surface area for
osmosis would start to grow significantly. Then one corner will be cut
off each stick to indicate that this will be the side to be measured.
Each stick needs to be weighed accurately to 0.01g as any changes will
only be of 0.01 0r 0.02g.

25cm3 of each solution will be measured out in the measuring
cylinders, one cylinder for each solution. Each solution needs a
separate cylinder to make sure that the solutions don't mix with each
other. Each solution measured out will be put into a boiling tube
each, and the two chips will be put into each tube and left for 15
minutes. 15 minutes seems to be a good time to keep the chips in the
solutions for because this was how long the onion cells were kept in
the solutions in the preliminary work, and a result was obtained from
that, so we see it is enough to obtain a result.

After 15 minutes, the tweezers will be used to take out the potatoes
and they will be dried using tissue paper. They need to be dried to
take as much of the solution as possible, which will add to the
weight. Then the length of the chips will be measured and their masses
will be taken. The feel of the chip will also be noted because this
can give an indication of the turgidity of the cells.

The two sets of results should give a more reliable result, which can
be used to plot a graph, and from this a conclusion can be drawn.

Hypothesis

I think that with a solution with a lower sugar concentration (e.g. a
0.2 molar solution), the length and mass of the potato will be bigger.
This is because, according to osmosis, which was learned from the
preliminary experiment, water molecules pass from a higher
concentration to a lower concentration, so if there is less sugar in
the solution, then less water needs to be diffused into the solution
from the potato to establish a perfect environment, so the potato
loses less water, therefore the mass of the potato is greater, than
that of a chip in 0.4 molar solution.

The chips in a higher sugar solution, like 0.8 molar, will have a
lower mass and length because more water needs to pass out into the
solution, to create a perfect environment, as was seen with the onion
cells.

Concerning the strength of the cells, I believe that when the potato
is put in a weak solution, the cells will become turgid because
'turgid' means swollen and hard and the cells become turgid when water
enters them, and with the onion cells, they became turgid when put
into dilute water, because dilute water has a 100% water concentration
and so has a higher concentration of water than inside the cell and so
it will diffuse into the cell.

I think that when the potato is put in a stronger solution, the cells
will become flaccid because 'flaccid' is the opposite of turgid and
flaccidity occurs when water leaves the cell and with the onion cells,
they became flaccid when put into 1% Glucose solution, because there
was a higher concentration of water inside the cell than out so the
water from within the cell diffused to the solution outside.

Safety

Safety is an important aspect of every experiment whatever the initial
interpretation of danger. In this experiment a sharp scalpel is being
used and could injure someone if misused or picked up at the wrong
end, so great care needs to be taken when cutting the chips up. Also,
the affect of the solution to the body is unknown, so care must be
taken into making sure that none of the solution enters the body
through the mouth or eyes and at the end of the investigation, hands
must be washed thoroughly.


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