Observing Osmosis OBSERVATIONS

OBSERVATIONS
By adding concentrated salt water to the cells of red onions, it is
possible to make the cell’s contents shrink. Plasmodesmata
may become visible as well.
By Oliver Kim
P
lasmolysis of red onions belong
to one of the easiest introductory
microscopy experiments. By
adding salt water to the onion cells, the
cell’s contents starts to shrink to the
point that the cell membrane starts to
separate from the surrounding cell wall.
Plasmolysis is due to the diffusion of
Observing Osmosis
water out of the cell, by a process called
osmosis. The salt ions (Na+ and Cl-)
bind the water molecules and immobilize them. For this reason the probability of a water molecules diffusing into
the cells is smaller than the probability
of one diffusing out, which results in a
net movement of water out of the cell.
Obtaining onion skin
To make the process visible, it is
necessary to use red onions. The natural
pigmentation of the cells makes the process visible the best.
Cut out a square piece of a red onion
and tear off (do not cut) the strongly
Figure 1: The skin of the red onion
peeled off by tearing.
Figure 2: High magnification image of
plasmolyzed red onion cells. You can
see the strands of cytoplasm extending from the dark red region towards
the cell wall. These plasma strands
are referred to as plasmodesmata
and are responsible for cell-to-cell
communication. During plamolysis,
the shape of the cell wall is not affected.
14 - MicrobeHunter Microscopy Magazine - January 2014
pigmented epidermis. Tearing will result in a single layer of cells. I found it
easiest to use a dull knife to first cut into
the “meat” of the onion and then tear off
the red epidermis. This process will
break many of the cells open and these
cells will lose their cell contents (and
their red color). In Figure 1 you can see
the torn epidermis on the bottom. Some
parts of the epidermis are clear, these
are the parts that are not useful for observation. The thick parts of the onion
were cut away and the remaining onion
epidermis was then placed on a microscope slide with water and cover glass
and observed normally at low to medium power.
Observing Osmosis
OBSERVATIONS
Plasmolyzing the cells
I first made a saturated salt water
solution. I did not weigh the salt, but
simply added excess salt to some water.
It is then necessary to allow the solution
to settle in order to minimize the disturbing effect of small salt crystals on
the image.
Excess water is then removed from
the wet mounted slide with some tissue
paper. Several small drops of salt water
are then added to the edge of the cover
glass. The salt water is pulled beneath
the cover glass by capillary action and
comes into contact with the onion cells.
Observing plasmodesmata
The red cell content started to shrink
quickly. The color of the cell content
became more intense, an indicator that
the cell membrane did not allow the
pigment to pass (Figures 3 and 4). The
cell membrane started to separate from
the cell wall, but continued to be attached on some placed to the cell wall.
Thin strands extended from the cytoplasm of the cell towards the wall. This
phenomenon can be seen well in Figure
2. These plasma strands are important
for cell to cell communication and connect at the cell wall at places called
plasmodesmata.
After some time the process came to
a halt, because the salt solution became
more and more dilute. In this case it was
necessary to add more salt water to the
cells.
■
Figure 3: Original onion cells before
addition of salt water. Some cells
were pigmented red, others were not.
This was because the tearing process destroyed some of the cells and
caused the red pigment to spill out.
On other parts of the onion epidermis, all cells were pigmented.
Figure 4: The plasmolyzed cells. The
cell membrane (surrounding the pigmented contents) remained attached
to the cell wall at certain places, the
plasmodesmata.
January 2014 - MicrobeHunter Microscopy Magazine - 15