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The Effects of Salt Water and Fresh Water on an Onion Skin Cell Wall
Background
A cell’s membrane has many functions. One of its functions is that it forms a barrier
between the inside of the cell, and everything that is outside of the cell, so that the chemical
environments on the two sides can be different. The cell membrane controls those differences for
the chemical machinery inside the cytoplasm. It also makes changes in the interior of the cell by
moving solutes across the cell membrane.
Turgor is the main pressure of the cell contents against the cell wall in plant cells and
bacteria cells. Turgor is the force that is caused to go to a plant cell wall by the water that is
contained in the cell. If a plant gets enough turgor than it will stand upright, and if it does not get
enough it will be droopy. If a plant cell has too much turgor than the cell will explode.
Osmosis is the diffusion of a solvent, mostly water, through a semi permeable membrane,
from a low solute concentration to an area of high solute concentration. If a solution has low
solute concentration, than it has high water concentration, and if a solution has high
concentration, than it has low water potential. This is the process of solvent movement without
energy across a semi permeable membrane, and separates two solutions with different
concentration. When water molecules pass through the cell, they have to go through the cell
membrane from an area of low concentration outside of the cell to an area of higher
concentration inside the cell. When the membrane has pure water both inside and outside of the
cell, then the water molecules pass in and out of the cell at the same rate. If there is a solution on
one side of the membrane and pure water on the other side of the membrane, then they will move
in and out of the cell at the same rate, but some of the solute molecules will not pass through the
cell membrane.
Osmotic potential is the opposite of water potential. Osmotic potential is the degree to
which a solvent would want to stay in liquid form. Osmosis may be prevented by increasing the
pressure in an area of high concentration. The pressure needed to prevent water passage through
the membrane to a solution of higher concentration is known as osmotic pressure. Osmotic
pressure is necessary for many plant functions. It is the resulting turgor pressure that makes
plants stand upright, and lets plant regulate their stomata. Hyper tonicity is a solution that makes
a cell shrink. It may give the inside of the cell higher osmotic pressure, and causes the cell to
swell. It may have a lower osmotic pressure than the interior of the cell. Isotonic is a solution that
makes no change in the cell’s volume.
Hypothesis
If you put salt water and fresh water on two different onion skins the cells of the onion
skin with salt water will have smaller cells than the onion skin with fresh water.
Variables
The independent variable for this lab is the salt concentration in the water solution. The
dependant variable is the size of the onion cell.
Materials
For this lab experiment you will need a couple of materials to complete the procedures
correctly. The first material that you will need is the very thin top layer off a purple onion
skin. The second material that you will need is a light microscope to observe the onion skin at
different magnifications. Next you will need any amount of fresh water, and any amount of fresh
water with a small amount of table salt in it. The next thing that you will need is two glass slides
to put the onion skins on and observe them. You will also need two eyedroppers to put the drops
of fresh water and salt water on the slides. To go along with the glass slides, you will need two
small glass slides covers to put over the onion skin and the fresh- water and the salt- water. The
last thing that you will need, but is optional, are labels to label the onion skin slide that has fresh
water, and the onion skin slide that has salt water.
Procedure
The first thing that you will need to do is take the purple onion, and peel the very thin
transparent film off the top layer of the onion, and tear it into two small pieces that fit on the
glass slide. Second, you have to place the transparent onion skin on the glass slide, and make
sure that it is not folded, bumpy, or wrinkled. You will have to do this procedure twice on two
different glass slides, for the fresh- water and the salt- water. On the first glass slide, you will
apply two drops of fresh- water on the transparent film off the onion skin. And on the other glass
slide, you will apply once again two drops, but of salt- water on the transparent film off the onion
skin. On the next procedure step, you will have to make sure; once again, that both the freshwater onion film and the salt- water onion film are free of wrinkles, folds, creases and bumps, so
that the image comes out clear when you look through the microscope. And cover them carefully
with the small thin transparent glass covers. Next, if you chose to have labels, you will label the
two slides; salt water, and fresh water, just so that you do not get them confused. Now that you
have completed the first setoff procedures, you will start observing and writing about it. First you
will observe the salt water slide starting at one- hundred magnification, and work your way all
the way to four- hundred magnification. You will repeat that step, but this time you will do it
with the fresh water slide. When you are done observing both slides, you will illustrate the cell
walls and count the number of cells that fit in the known field of dimension. Then you have to
measure the length of the cells that fit in the field of dimension. On the second to last step of this
procedure, you will have to graph the differences of the volumes between the salt- water onion
cells, and the fresh- water onion cells from the experiments. Lastly you will have to write a
conclusion of the differences and outcomes of both treatments.
Data Collection
Magnification 400X
Field Diam 500µm
400µm x 77µm
Nucleus
Cell Wall
Cytoplasm
Fresh Water
222 µm x 33 µm
Salt
Figure 1: Onion cell areas in µm2
Fresh Water
Salt Water
30769
7407
Data Analysis
The effects of varying solutions on onion cell size.
35000
30000
25000
Cell areas in 20000
µm2
15000
10000
5000
0
Fresh Water
Salt Water
Conclusion
Analyzing the hypothesis
Osmosis is the other factor that makes the salt water cells shrink. Diffusion makes the
cells shrink by diffusing the solvent through the semi- permeable membrane. The osmosis makes
the salt go into the cell slower than the water comes out of the cell, so it shrinks. Since there is a
solution outside of the cell; the salt water; and water inside of the cell, then they move in and out
of the cell at the same rate, but some of the solute molecules do not pass through the semipermeable cell membrane. The solute molecules do not pass through the semi- permeable cell
membrane cause less turgor pressure, which makes the plant cells get droopy or shrink. Hypertonicity makes the cells shrink, and gives the cells higher osmotic pressure, Osmotic potential is
the degree to which a solvent would want to stay in a liquid. The cells shrunk because the
pressure of the water was decreased. Because the salt- water, the cells have low osmotic
potential, so they do not let water pass through the cell membrane and empties the inside of the
cell and deflates it, or lets it shrink.
Explaining the data
When I observed the cells, I noticed that there was a difference in their size. The cells
with the salt- water were much smaller than the cells with just fresh- water. I could have told that
the cells were different sizes because there were more salt- water cells in the field dimension
than the fresh- water cells. The cells in the salt- water are smaller than the fresh- water cells
because of turgor and osmosis. Osmosis is the diffusion of a solute, usually water, from a semipermeable membrane. The other factor that made the salt- water cells smaller was turgor. Turgor
is the main force of the cell’s contents against the cell walls in plant and bacteria cells.
Turgor made the cells smaller because the salt water has low solute concentration and the
cells of the onion skin have high solute concentration. Since the water has low solute
concentration, it has high water concentration, and since the onion skin has high solute
concentration, it has low water concentration. Because of this, the cells shrink.
Lab design improvements