Investigating the Effect of Osmosis on Potato Cylinders
Aim: To investigate the effect of osmosis on the potato cylinders of a
certain size in different concentrations of solution.
· Water will flow from the low concentration to the high
· The cylinders in high concentration solution will gain and increase
· The cylinders in low concentration solution will loose and decrease
Osmosis, which is regarded as a form of diffusion, is defined as the
net movement of water or any small molecule across a partially
permeable membrane from a region of low concentration to a region of
Partiallypermeable membranesare very thin layers of material (cell
membranes are partially permeable) which allow some things to pass
through them but prevent other things
from passing through.
Partially permeable membranes will allow small molecules like Oxygen,
water, Carbon Dioxide, Ammonia, Glucose, amino-acids, etc. to pass
through. Cell membranes will not allow larger molecules like Sucrose,
Starch, protein, etc. to pass through.
When osmosis occurs in a plant cell after it has been watered the
water (which is less concentrated) enters the vacuole which therefore
results in that the vacuole is erected to its full size, this is
called turgidity. When all cells in the plant are turgor this brings
about a noticeable change in the plant, it stands in a healthy way
spreading its leaves out like an umbrella.
When the plant is put in a solution that is more concentrated than it,
it loses the water stored in its vacuoles and therefore becomes
Plasmolysed. When this happens, which never occurs naturally, the
plant becomes weak, dry and limp
The flow of water across a membrane in the process of osmosis
generates a pressure across the membrane called osmotic pressure.
Osmotic pressure is defined as the hydrostatic pressure required to
stop the flow of water, therefore osmotic and hydrostatic pressures
Equations related to Osmosis:
The osmotic pressure P of a dilute solution is approximated by the
P = RT (C1 + C2 + .. + Cn)
Where R is the gas constant (0.82 liter-atmosphere/degree-mole), T is
the absolute temperature, and C1 ... Cn are the molar concentrations
of all solutes.
Similarly, the osmotic pressure across of membrane separating two
P = RT ([IMAGE]C)
Where [IMAGE]C is the difference in solute concentration between the
A few key words related to osmosis:
Isotonic: The solutions being compared have equal concentration of
Hypertonic: The solution with the higher concentration of solutes.
Hypotonic: The solution with the lower concentration of solutes
Upon the light of the scientific knowledge I conclude the predictions
Upon researching through written material on osmosis in biology, I
came to the prediction that if the potato is put in a solution with
less Molarity i.e. less concentration the osmotic effect would be that
the water will cross the partially permeable membrane from the
solution to the potato thus increasing its size; this is also true
As above the prediction concerning mass derived from the knowledge
received via research from books and the like is that as the Molarity
in which the potato is put increases as a result increasing the
concentration which would result in the transfer of water molecules
from the potato to the solution thus decreasing in weight; it is also
As it was predicted that there will be increase in size it is also to
be predicted that this is a result of the vacuoles filling up to their
utmost (turgidity). Other than the increase in size this being true
means that the potato that is bigger will also be harder. It is also
To produce a fair and efficient experiment I shall keep some variables
constantly the same throughout the experiment and to all the potato
cylinders. The variables that have to be decided by me as which will
be constant and which will vary from potato cylinder to potato
· Solution concentration
· Surface area
· Solution volume
· Duration of experiment
· Type of solution
· Mass of potato cylinders
I will vary the concentration of the solution from potato to potato
because the aim of the experiment is to see the effect of osmosis when
allowed to process in different concentrations of solution.
I will keep the surface area the same as to minimise the probability
that one potato cylinder will have more space for osmosis to take
place, as this will allow a greater amount of water to pass through
the partially permeable membrane in the limited time thus destroying
the purpose of the experiment. For the same reason I shall keep the
solution volume the same.
I will keep the duration of the experiment the same for all the potato
cylinders so as to prevent one potato having more time to move
molecules of water across the partially permeable membrane.
I will keep the temperature constant because osmosis could work faster
in certain temperatures, due to the fact that particles move faster in
hotter temperatures thus unbalancing my experiment.
I will have the same type of solution and will keep the mass of all
potato cylinders the same at the start of the experiment.
Below is the finished table of the variables and non-variables:
Plan of Investigation:
I will use the concentrations 1 M, 0.8 M, 0.6 M, 0.4 M, 0.2 M, and 0.0
M (distilled water).I will use Sucrose as the solvent and I will put
it in 1L of distilled water. To find out how much sucrose to use I do
the following calculations keeping in mind that the Molar Mass of
sucrose is 342.3g:
Concentration (M) / Volume (L) x Molarity (g)
1 M: M/L x g = 1/1 x 342.3 = 342.3 g
0.8 M: M/L x g = 0.8/1 x 342.3 = 273.84 g
0.6 M: M/L x g = 0.6/1 x 342.3 = 205.38 g
0.4 M: M/L x g = 0.4/1 x 342.3 = 136.92 g
0.2 M: M/L x g = 0.2/1 x 342.3 = 68.46 g
0.0 M: M/L x g = 0.0/1 x 342.3 = 0 g
I will use 11 cm 3 g as the volume of my potato cylinder with a mass
of 10g, I will do my experiment in room temperature and the experiment
will last for 50 minutes.
· Stop watch - to time the experiment
· Knife - to cut the potato
· Measuring cylinder - to make solution in
· Distilled water - part of the experiment
· Sucrose - part of the experiment
· Potatoes - part of the experiment
· Stand - to support the potato
· Tissue - to place potato on after experiment
· String - to hold the potato to the stand
· Balance - to weigh the potatoes
· Beakers - where experiment will take place
· Cork borer - to cut out potato cylinders
· Ruler - to measure the volume of the potato
1 I took two potatoes and checked to make sure they were healthy and
2 Using a cork borer and a ruler and I cut out potato cylinders that
were 35mm long. I had to be very careful whilst cutting as the knife
is very sharp. I made 6 cylinders, one for each solution.
3 Taking 6 stands I placed 6 beakers underneath them and then labelled
them 0 M, 0.2 M, 0.4 M, 0.6 M, 0.8 M, and 1 M
4 Using a measuring cylinder I measured out the previously calculated
amounts of sucrose solution and distilled water of which I then poured
200 ml into the beakers - each solution to its specified beaker.
5 I then weighed every potato cylinder on an electronic balance and
made sure the weights were 10g.
6 With the help of a partner I put the 6 potato cylinders in the
beakers and instantly started my stopwatch.
7 Whilst waiting for the 50 minutes to expire I set out some tissues
to prevent the table from getting wet, and I drew up a basic table for
8 After the 50 minutes were over I pulled out the potato cylinders and
placed all the chips on the tissues. I wrote the Molarity of the
solution which the cylinders had come from on their tissues as not to
confuse them with each other.
9 I observed the potato cylinders wrote my observations and results in
the observation part of this essay.
During the experiment I observed that the potatoes in the higher
concentrations of solution did not sink but floated at varying heights
from the bases of the beakers, in an unmistakable pattern: The greater
the concentration of the solution it was in the higher the potato
cylinder was from the base of the beaker.
Upon inspecting the potato cylinders after the experiment I came up
with the following observations:
1. Difference in mass:
2. In size and turgidity:
3. Under the microscope:
I therefore conclude upon the light of the observations from my
experiment that my predictions and hypothesis were correct and that
water flows from the less concentrated to the more concentrated
solution, and therefore the mass increases in the more concentrated
Upon finishing and observing my experiment I saw flaws that could be
patched in future experiments to improve the results and outcomes:
1. I could do more than one experiment for each concentration of
solution and take an average of the results to increase the accuracy
of the results obtained.
2. I could do experiment in another vegetable to see if the pattern of
osmosis changes across different organic material.