There are distinct structural as well as physiological differences between plant and animal cells. In this exercise we will examine human cheek cells and compare them with cells obtained from an onion.
Show
ImageJ application.
Go
to File, Open Samples and open the image CheekC.
There are a couple of cells shown along with a scale in the lower left corner.
This scale was obtained by photographing a micrometer slide (shown on the
left) under the microscope at the same magnification as the onion cell.
Then a 10 micron portion of the scale was selected with the Rectangle Selection
tool 
and copied using
the command under Edit in the Menu. It was then pasted on the cheek cell
image using the Paste under the Edit Menu. The Text A tool was used
to put on the label. Observe the nuclei present in the two cells.
Calibrating the image.
Go to your
Line 
tool. Drag
the tool across the scale bar. Then go to Set Scale under
Analyze. Put micrometers for "Unit of Measurement"
and 10 for "Known Distance".
Then go to Set Measurements under Analyze and only check "Area" and "Perimeter".
Using this
same Line tool
measure the greatest length of the upper cell. (Once you have drawn the
line select Measure under Analyze and you will
have the length in micrometers.) Record your results in Table 1.
Do the same thing for the lower cell. Then use the Freehand 
Tool and trace around the two cells to calculate the area. Record
results in Table 1.
Close
CheekC and Open Images Ep1000 and Ep1500.
The cheek cells that you have been looking at are called Epithelial Cells. They are the cells that cover your skin and line the internal cavities of the body. These are scanning electron micrographs SEMs of cheek cells that are magnified either 1000 or 1500 times. Calibrate Ep1500 using the technique that was described above.
Measure the greatest length of the cell shown under 1500X and enter the information in Table 1.
1. Compare this length with your other two lengths.
2. What is the percent difference between this measurement and an average of the other two values? (Percent difference is determined by dividing the difference between the SEM length and the calculated the average of the two light microscope values and then multiplying this difference by 100.) (Show your calculations)
Onion Cell:
Open
the the folder Cells and then open the image OnionC.
3. What is the most prominent cellular difference between these plant cells and the cheek cells.
4. If this is a plant cell, what two structures are not present?
If you look closely you may be able to see several nucleoli inside each nucleus.
Keep
OnionC open and also open Scale This Scale
image is a photograph of a micrometer slide that you saw earlier on this
page. The large gap is 0.1 mm. or 100 microns. The closely spaced lines
on the left are 10 microns apart. Use the Rectangle Selection 
tool and copy a 10 micron area. (It should look like the piece that
is immediately above the label 10 microns. Notice how the selection is made
through the middle of the micrometer line to middle for the next line.)
5. Why is the copy portion selected in this manner?
Paste
the 10 micron portion in the lower left portion 
of
the OnionC image.
Calibrating the image. Follow the same procedure that you used earlier.
Measure the length and perimeter of the cell that is in the center and Record results in Table 2.
Measure the perimeter of the nucleus of this center cell.
6. Compare the nuclear area to the area of the entire cell as a percentage. How accurate a method is this for determining the size of a nucleus in the cell?
Results
Table 1:
1. Compare this length with your other two lengths.
2. What is the % difference between the light microscope measurements and the EM measurements? (Show your calculations)
Comment on possible reasons for the difference.
3. What is the most prominent cellular difference between these plant cells and the cheek cells.
4. If this is a plant cell, what two structures are not present?
5. Why is the copy portion selected in this manner?
Table 2 :
6. Compare the nuclear area to the area of the entire cell as a %. How accurate a method is this for determining the size of a nucleus in the cell?
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