Summary Biology ib IA about osmosis in sweet and strong onions
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Course
Biology
Institution
Kings College London (KCL)
This document is a full IA that achieved a grade 7 in IB biology. It offers a good structure for all lab experiments and can help in paper 3. It can also help in the igcse papers.
Introduction and Background information
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Osmosis is the movement of water through a semipermeable membrane from a dilute solution to
a more concentrated solution. When a cell is in a solution with a higher salt concentration than its
cytoplasm, water is therefore lost by osmosis. Gaps will appear between the cell wall and the cell
membrane after osmosis occurs for any time, also called plasmolysis. We utilize plasmolysis to show
the concentration of a cell’s cytoplasm. When 50% of the cells show signs of plasmolysis in a given
solution, we can estimate that the solute concentration of the solution must be equivalent to that
of the cell’s cytoplasm. In this lab, we test sweet and strong onion cells in different solute
concentrations. By utilizing classical/traditional scatter graphs of the results, I will estimate the
solute concentration when exactly 50% of the cells show plasmolysis.
To find out if there is a higher concentration of salts in the cytoplasm of strong onion cells
compared to sweet onion cells using osmosis and plasmolysis.
2. Hypothesis
Alternative hypothesis: As the concentration increases, so will the percentage of plasmolyzed cells
in consideration. As the concentration increases, so will the percentage of plasmolyzed cells in
consideration.
Null Hypothesis: The higher concentration of salts in sweet onion plasmolyzes more cells than
higher concentration of salts in strong onion cells.
3. Research question
How does the concentration of a surrounding solution affect the % plasmolysis of red and white
onion cells (strong and sweet onion cells)?
1. Materials
− 6 microscope slides and cover slips.
− 6 concentrations of sodium chloride solution (0.0M, 0.25M, 0.5M, 1.0M, 1.5M and 2.0M).
− 1 Dropping pipette and paper tissue
, − Epidermis samples of Red and White Onion
− Tweezers for taking epidermis off onion
− Light Microscope
− 6 beakers (100ml)
2. Procedure
A. Label six slides with the six salt concentrations (0.0, 0.25, 0.5, 1.0, 1.5, 2.0).
B. Put two drops of each salt solution onto each slide.
C. Add a small piece of epidermal tissue from an onion to each slide, and then a cover slip.
D. Study the onion cells on the first slide under the microscope at x100 magnification.
E. Wait 2 minutes to allow osmosis to take place.
F. Count the total number of cells in the eyepiece view.
G. Count the number of plasmolyzed cells in the same view.
H. Repeat steps 6 & 7 at two different places on the slide, so you have three repeats.
I. Repeat steps 6 to 8 for each concentration of salt solution.
J. Record all your results in a table.
K. Write a short paragraph of qualitative observation, ie. Things you observed about the cells
during the experiment in addition to the number of cells.
Plasmolyzed = (Number of Plasmolyzed Cells ÷ total no. of cells)
Explanation of how to calculate % of Plasmolyzed Cells and Average of % Plasmolyzed Cells
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During the real experiment, there wasn’t enough time to do real trials, which is why I’m using
other data, and using it to our advantage to calculate % of Plasmolyzed Cells and Average of %
Plasmolyzed Cells.
, The % of cells plasmolyzed is found by dividing the overall number of cells plasmolyzed found in
the field of view by the total number of cells found in that same FOV. Just like every calculation, you
have multiplied it by 100 to get the percentage. The average percentage of plasmolyzed cells in the
Field of view is calculated by adding all 3 percentage values for the three samples discovered from
the experiment and then dividing it by three to get the mean percentage for the three samples.
Here’s the formula:
Finding the mean can be done with the calculator or excel. I personally will be using excel to write
down data and make graphs. The calculation results will later be seen in the tables and figures
Rational for Using Average % Plasmolyzed Cells vs. Salt Concentration
________________________________________________________________________________
Finding the average of the % plasmolysis of the cells of the three trials will help us interpret the
whole set of data more easily because we don’t have to assess each of the trials individually.
Moreover, the mean considers every value of a set. This allows us to discover the overall correlation
between the 3 samples and the concentration they were tested with, using a single value.
Unfortunately, there're limitations to using the average which will be addressed in my conclusion.
Variables Category How Variable will be
controlled/manipulated/moni
tored
Concentration in each Independent We can use 0.0M
solution concentration to 2M
concentration, these
concentrations don’t rely on
anything else and to
manipulate it we simply
change the concentration.
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