BIOLOGY IN FOCUS
MAINTAINING A BALANCE
Chapter 2 Transport—dissolved nutrients and gases
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HSCCOURSE
Estimating the size of red and white blood cells
■
perform a first-hand
first-hand investigation using the light
microscope and prepared slides to gather information to
estimate the size of red and white blood cells and draw
scaled diagrams of each
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page 38
Guided method for estimating the size of red blood cells
1. Estimate the field of view under low power.
Place the mini-grid (or transparent ruler) on the microscope stage and view under the ×10
objective. This gives a magnification of 100 times (eyepiece × objective = 10 × 10 = 100).
Use the grid or ruler to estimate the diameter of the field of view in mm and μm (1 mm =
1000 μm) (see Fig. 2.4, step 1 and 2).
For example, if this field of view has a diameter of approximately 1.6 mm, it is 1600 μm. If
you are using a grid slide, each of the larger grids is 1 mm and the smaller are 0.1 mm (see
Fig. 2.4).
Figure 2.4 Sequence of steps to estimate the size of the field of view
1
Focus on large grid under s100.
2
field of view
grid line
at edge
of field
Focus under s400—cannot see
grid, therefore need to calculate.
diameter of
field of view
1 mm
grid lines on ruler
or minigrid slide
Mag. s100 (low power)
Field = 160 Mm
0.6 mm
Mag. s400 (high power)
160 Mm
Field =
= 400 Mm diameter
4
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2. Calculate the field of view under high power.
Swing the high power objective into place. You will no longer see the distance from one grid
line to the next and so you need to calculate the field of view. If your high power lens has
a magnification of ×40, everything you look at will appear four times bigger than under the
previous (×10) lens, and the diameter of the field of view that you can see will be four times less.
The field of view can therefore be calculated as follows (see step 2 of Fig. 2.4):
low power = ×100; high power = ×400
high power field of view = 100/400 = ¼ the size of your low power field of view.
3. View a prepared slide of blood and estimate the size (length) of red and white blood cells.
■ Place the prepared slide of a blood smear on the microscope stage under low power. Once
it is in focus, rotate the high power objective into place. (Do not change the focus before
doing this, as the school microscopes are parfocal—they remain in focus while you rotate
objectives and will not crush your slide.)
■ Distinguish between the numerous small red blood cells and the few, larger white blood
cells (the nucleus in the white blood cells takes up a stain and appears dark blue or purple
in colour). The nuclei in white blood cells may vary in shape (see Fig. 2.3b on the previous
worksheet).
■ Estimate the size of a red blood cell. Count or estimate the approximate number of red blood
cells that would fit across the diameter (imaginary centre line) of the field of view (using ×400
magnification). Using this number and the known diameter for the field of view, calculate the
size of each blood cell. (See Fig. 2.5.) For example, if about 50 red blood cells fit across the
field of view and the field of view = 500 μm, then each cell is 500/50 = 10 μm.
4. Improve accuracy and validity.
Repeat this process three times and find an average size for red blood cells.
Figure 2.5 Estimating the size of a red blood cell
(
Place blood smear under low power
then high power. Estimate number of
blood cells across diameter.
EXAMPLE ONLY
(Do not use these figures
in your practical)
Count: ±55 red blood cells
400 Mm (diameter of field of view)
55
(number of red blood cells)
= 7.3 Mm = estimated size of one
red blood cell
5. Estimating the size of white blood cells
■ Since there are so few white blood cells, it is not possible to count the number of white
cells across the diameter and therefore much more difficult to estimate how many would fit
across the diameter. Another method of estimating their size is to compare their proportions
with that of red blood cells. For example, is the white blood cell half the size or twice the
size—i.e. how much bigger or smaller is the white blood cell compared with the red cell?
■ Use this estimate to calculate their size.
■ To increase accuracy and validate the conclusion based on the data gathered, repeat the
process with three different white blood cells and obtain an average.
6. Draw a scale diagram of each type of blood cell as follows.
(a) Draw a line of a particular length (e.g. 1 cm or 2 cm). This will be your scale bar that
represents 10 μm.
(b) Using this scale, draw a circle on your paper to represent the average size of each cell.
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(c) Now add the detail to the cells, for example:
—red blood cell: represent the cytoplasm, cell membrane and use appropriate shading to
show its biconcave shape
—white blood cell: draw the nucleus, cell membrane and cytoplasm.
(d) Label all parts of each cell.
Results
■ Estimation of size of red and white blood cells.
■ Scale diagrams of red and white blood cells.
Record your results at the end of this worksheet under the section ‘Results’. You are required
to record all estimates, show your working for any calculations and follow the rules for drawing
scientific diagrams, a skill learnt in the Preliminary Course.
Discussion and conclusion
Answer all discussion questions. Re-read the aim and use your results to arrive at a valid
conclusion.
Investigation report: A microscopic investigation to estimate
the size of red and white blood cells
Aim:
Materials:
■
■
■
■
Safety:
Method:
1. Place the mini-grid (or transparent ruler) on the microscope stage and view under the ×10
objective. This gives a magnification of 100 times (eyepiece × objective = 10 × 10 = 100).
2. Estimate the field of view under low power.
Use the grid or ruler to estimate the diameter of the field of view in mm and μm (1 mm =
1000 μm) (see Fig. 2.4, steps 1 and 2).
3. Calculate the field of view under high power.
Using the ×40 objective, calculate the field of view (¼ the size of your low power field of view).
4. View a prepared slide of blood and estimate the size (length) of red and white blood cells.
■ Estimate the size of a red blood cell. Count or estimate the approximate number of red blood
cells that would fit across the diameter (imaginary centre line) of the field of view (using ×400
magnification). Using this number and the known diameter for the field of view, calculate the
size of each blood cell (see Fig. 2.5).
■ Repeat this process three times and find an average size for red blood cells.
■ Estimate the size of white blood cells compared with the red cell. Use this estimate to
calculate the size. Repeat with three different white blood cells and obtain an average.
5. Draw a scale diagram of each type of blood cell as follows:
(a) Draw a your scale bar that represents 10 μm.
(b) Using this scale, draw a red blood cell and a white blood cell. Label all parts of each cell.
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Results
1. Field of view ×40 = _______mm or _______ μm
Field of view ×100 = _______mm or _______ μm
Field of view ×400 = _______mm or _______ μm
2.
Blood cell type
Estimate 1
Estimate 2
Estimate 3
Average
3. Scale diagram of a red blood cell
4. Scale diagram of white blood cell
Conclusion
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Discussion questions
1. Describe how you could distinguish between red cells and white cells under the microscope.
2. Which are more numerous, red cells or white cells? Explain
Explain, using evidence gathered in the
investigation.
3. Compare the size of a red blood cell and a white blood cell. Assess the reliability of the
measurements that you obtained.
4. Explain why it was necessary to repeat the estimation process three times and find an average
for each cell type
5. Assess the accuracy of your results for each cell type and justify. (Re-read the background
information on measurement in science before attempting to answer this question.)
6. Discuss and report on the difficulties you had in carrying out this investigation.
7. Describe how you could improve the accuracy and reliability of your investigation.
8. From this investigation and your knowledge of biology, prepare a table to outline the similarities
and differences in structure between red blood cells and white blood cells.
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Extension work
1. Describe the ratio that Anton Van Leeuwenhoek used to estimate the size of red blood cells (see
the background information on pages 37–38 of the textbook).
2. Outline the method he would have had to use to arrive at this conclusion.
3. Using your knowledge of the actual size of a red blood cell, calculate the size of the grain of
sand that he used for comparison with a red blood cell. Show your working. For this to be true,
what assumptions do you need to make?
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Answers
Discussion questions
1. Describe how you could distinguish between red cells and white cells under the microscope.
■ Colour: white blood cells have no pigmentation, whereas red blood cells are red (appear
pale pink under the microscope).
■ Nucleus: white blood cells have a nucleus that may be irregular in shape and stained purple,
whereas red blood cells do not have a nucleus.
■ Size and shape: white blood cells are larger and round or slightly irregular in shape; red
blood cells are smaller and flatter with a biconcave shape.
2. Which are more numerous, red cells or white cells? Explain
Explain, using evidence gathered in the
investigation.
Red cells are more numerous.
Evidence: within the high power microscopic field, there are many hundreds of red blood cells
and only one or two white cells visible.
3. Compare the size of a red blood cell and a white blood cell. Assess the reliability of the
measurements that you obtained.
White blood cells are larger—approximately one and a half times the size of red blood cells.
The reliability of the results is increased by obtaining three sets of results for red cells and
averaging this, then obtaining three sets for white cells and finding an average for each. These
results were similar and so the results are fairly reliable.
4. Explain why it was necessary to repeat the estimation process three times and find an average
for each cell type.
Repetition of the process gives more reliable results because several estimates give a greater
percentage coverage of the field of view.
5. Assess the accuracy of your results for each cell type and justify. (Re-read the background
information on measurement in science before attempting to answer this question.)
(Students should compare their results with those found in scientific literature and then comment
on the accuracy. The degree of confidence in the estimate also affects accuracy and students
should mention this). A sample answer is given below.
The accuracy using this method is not very high—it is simply an estimate of the size of red blood
cells, based on the number across the diameter of the field. Our estimate of the size of a red
blood cell was 10 μm, whereas scientific literature shows that it should be closer to 7.5 μm.
Limitations in the accuracy of our measurements arose when:
■ measuring the field of view (use of a ruler was less accurate than use of a mini-grid—smaller
calibrations would have given a more accurate estimate of the diameter field of view, so our
less precise measurements led to some inaccuracy
■ counting/estimating the number of cells across the diameter of the field—we are not very
confident that we counted correctly as the cells are very small and we had some gaps
across the diameter and had to estimate the number of cells that would fill them
■ comparing the size and proportion of white blood cells with that of red blood cells.
6. Discuss and report on the difficulties you had in carrying out this investigation.
Difficulties could include:
■ how precisely the diameter of the field of view could be measured (e.g. ruler, no mini-grids
availalble)
■ difficulties trying to observe and count how many red blood cells fit across the diameter—
cells are very small and we had some gaps across the diameter and had to estimate the
number of cells that would fill them.
7. Describe how you could improve the accuracy and reliability of your investigation.
Reliability can be increased further by comparing results with other students in the class, or
even averaging the results of all students in the class.
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Accuracy could be improved using more precise instruments of measurement, higher
magnification (smaller microscopic field), measuring instrument with smaller calibrations or
simple more experience in microscope work.
It is important to note that results can be reliable without being accurate. This is because
repeatedly getting the same result does not necessarily ensure that it is accurate (close to the
correct measurement). Once accuracy has been determined (by comparing the measurements
obtained in this practical with results in scientific journals), steps should be taken, using
more precise instruments such as mini-grids and greater care when counting. Accuracy could
be greatly improved if the technology used were upgraded—for example, using an electron
microscope and digital measuring equipment—but this is not feasible at school level.
8. From this investigation and your knowledge of biology, prepare a table to describe the
similarities and differences in structure between red blood cells and white blood cells.
Cell type
Density (per
mL blood)
Size (diameter)
Where produced
Structure
Function
Red blood cells
5–6 million
7–8 µm
Bone marrow
(destroyed in liver and
spleen)
Small, round,
flattened–biconcave,
no nucleus, red
(contain haemoglobin—
250 million molecules
in each RBC)
Transport oxygen:
4 oxygen molecules
combine with each
haemoglobin to form
oxyhaemoglobin
White blood
cells
4 000–
11 000
~ 12 µm
Bone marrow
(destroyed in liver and
spleen)
Larger than red blood
cells, no pigmentation
(i.e. no colour), contain
a nucleus which may
be unusual in shape
Function as part of
the immune system
in defence against
disease
Platelets
400 000
~ 3 um
Bone marrow
Disc-shaped cell
fragments, 50%
smaller than red blood
cells, non-pigmented
Release a
clotting enzyme,
thromboplastin, to
seal blood vessels
and prevent excessive
blood loss
Extension work
9. Describe the ratio that Anton Van Leeuwenhoek used to estimate the size of red blood cells
(see background information at the beginning of this investigation).
‘25,000 times smaller than a fine grain of sand’, i.e. 1: 25 000.
10. Outline the method he would have had to use to arrive at this conclusion.
To do this, he would have had to understand the magnifying power of the microscope and
lenses that he was using: estimate the field of view, count how many grains of sand fit across
the field of view, then divide the size of the field of view by the number of grains of sand to get
the size of each grain of sand.
11. Using your knowledge of the actual size of a red blood cell, calculate the size of the grain of
sand that he used for comparison with a red blood cell. Show your working. For this to be true,
what assumptions does one need to make?
Red blood cell = 7.5 μm.
Grain of sand is 25 000 times larger, i.e. 7.5 × 25 = 187.5 μm.
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