Name: ______________________________________________ Sec: _____________ Date:________________________
Lab 4: Measuring Using the Compound Microscope
Introduction: In today’s laboratory activity, you will measure the size of an onion skin cell. The units of measurement
you will be using are the millimeter (mm) and the micrometer (um). The smallest divisions on your metric ruler are
millimeters and micrometers are much smaller than that!
Objectives: In this activity, you will be able to:




Convert among units of the metric system.
Observe and count cells.
Measure field of view.
Calculate average cell size.
Materials: ruler, onion skin, tweezers, microscope, iodine, slide, coverslip
Part A: Pre-Lab
1. Why would it be a bad idea to measure cells using a regular ruler? _____________________________________
___________________________________________________________________________________________
___________________________________________________________________________________________
2. A meter is made up of 1000 millimeters. A millimeter is as thick as a dime. How many dimes would you have to
stack on top of each other in order for the pile to be 1 meter tall? _________
3. An egg from a frog is 1mm wide. Draw the actual size of a frog’s egg on the line. _______
4. Scientists use micrometers to measure things smaller than the frog egg you drew above. Given the following
information, answer the questions below.
a. A piece of paper is 100um thick.
b. A dime is 1000 um thick.
c. A bacteria cell is 1um thick.
How many bacteria cells (1um) make up the thickness of one piece of paper(100um)? ______
How many bacteria cells(1um) make up the thickness of one dime (1000um)?_____
5. To convert from mm to um: multiply by 1000. To convert from um to mm: divide by 1000.
A white blood cell is 0.02mm in diameter, what is its size in um?
The flu virus is 0.1um in diameter, what is its size in mm?
0.02mm = _______um
______ = 0.1um
How many flu viruses would fit across the diameter of a white blood cell? ________
PART B: Measuring Average Finger Width
Before we begin measuring onion cells using the microscope, we are going to first practice the technique measuring our
fingers.
1. Using your ruler, measure the diameter of the circle below.
Diameter = ___________mm = ___________um
2. Put your fingers across the diameter of the circle above. How many of your fingers cover the circle? ____fingers
3. Calculate the average width of your fingers using the equation below and the data you just collected.
Diameter of the circle
Average Finger Width =
_______mm
=
Number of Fingers that fit across
= _____mm = _____um
_______fingers
4. Now you have found the average width of your fingers. We are going to use the same technique for measuring
cells underneath the microscope. Follow the directions below.
PART C: Measuring Average Onion Cell Size
1. Create an onion cell wet-mount slide.
a. Pull off a small piece of onion skin and place it at the center of the slide.
b. Put one drop of iodine onto the center of the slide to stain the onion cell.
c. Place the over slip on top of the specimen by dropping at a 45* angle
2. Measure the diameter of the field of view on low power.
a. Turn the microscope to the low power objective.
b. Place the ruler on the stage beneath the objective lens.
c. Make sure that you can see the mm side of the ruler through the eyepiece.
d. In order to measure the field of view, COUNT THE SPACES between the lines. Do not count the lines.
__________= 2 spaces
__________ = 2 spaces
e. How many spaces did you count? ________ spaces
f. Each space is 1.0mm. What is the diameter of your field of view? ______mm = ______um
Diameter of Field of View
LOW POWER (40X)
3. Focus the onion cells under low power.
a. Each onion cell has a rectangular shape. Count the number of onion cells in a single row that spans the
diameter.
b. How many cells go across the field of view? _______
c. What is the average LENGTH of the onion cells?
Diameter of the field of view
Average Onion Cell Length
=
_______mm
=
Number of cells that fit across
= _____mm = _____um
_______cells
Average Onion Cell Length
LOW POWER (40X)
d. Count the number of onion cells (number of rows) from the top to the bottom of the field of view.
e. How many cells go up and down the field of view? ______
f. What is the average WIDTH of the onion cells?
Diameter of the field of view
Average Onion Cell Width
=
_______mm
=
Number of cells that fit up and down
= _____mm = _____um
_______cells
Average Onion Cell Width
LOW POWER (40X)
4. Measure the diameter of the field of view on high power.
a. Turn the microscope to the high power objective.
b. Place the ruler on the stage beneath the objective lens.
c. How many spaces did you count? ________ spaces
d. Each space is 1.0mm. What is the diameter of your field of view?
______mm = ______um
Diameter of Field of View
HIGH POWER (100X)
2. Focus the onion cells under low power.
a. Each onion cell has a rectangular shape. Count the number of onion cells in a single row that spans the
diameter.
b. How many cells go across the field of view? _______
c. What is the average LENGTH of the onion cells?
Diameter of the field of view
Average Onion Cell Length
=
_______mm
=
Number of cells that fit across
= _____mm = _____um
_______cells
Average Onion Cell Length
HIGH POWER (100X)
d. Count the number of onion cells (number of rows) from the top to the bottom of the field of view.
e. How many cells go up and down the field of view? ______
f. What is the average WIDTH of the onion cells?
Diameter of the field of view
Average Onion Cell Width
=
_______mm
=
Number of cells that fit up and down
= _____mm = _____um
_______cells
Average Onion Cell Width
HIGH POWER (100X)
DATA CHART: Go back through the lab and record all of the data you have collected in the box below.
LOW POWER (40X)
HIGH POWER (100X)
LENGTH OF FIELD OF VIEW
NUMBER OF CELLS ACROSS
NUMBER OF CELLS DOWN
AVERAGE CELL LENGTH
AVERAGE CELL WIDTH
CONCLUSIONS
1. How did the number of cells change as you switched from low power to high power? Circle one.
increased
decreased
stayed the same
EXPLAIN WHY? _______________________________________________________________________________
2. How did the size of cells change as you switched from low power to high power? Circle one.
increased
decreased
stayed the same
EXPLAIN WHY? _______________________________________________________________________________
3. The diagram below represents and organism viewed under the microscope on low power. What is the length of
this organism? Show your work using the measurement formula. Include units.
4. A red blood cell is 7.0um in diameter. How many cell would fit across the diameter of the low power field of
view? Show your work using the measurement formula. Include units.
5. Why does the method we used today only tell us the average size of cells as opposed to the actual size?