Assessment Task – Year 11 – Term 1, 2016
Human Biology General Unit 1
Task Description: Estimating cell size
In this investigation you will be making a wet mount microscope slide using onion cells. You will then be
estimating the size of an onion cell by calculating the field of view of the microscope using a minigrid. The field
of view is the circle of light that you see when looking through a microscope. You will be required to answer
the questions on this worksheet.
Background:
Microscopy skills are an important aspect of science inquiry. Science literacy skills are essential for
communication and development of scientific ideas. This practical builds on the science literacy skills
developed over the course of the Year 7-10 Science Curriculum and the numeracy skills developed over the
course of the Year 7-10 Mathematics Curriculum
Learning
Outcomes
 Science
understanding
 Science
inquiry skills
 Science as a
human
endeavour
Assessment
Description
PART A:
Groupwork:
 Aim & hypothesis
Introduction
PART B:
Microscopy skills
Practical:
 Making a wet mount using onion cells.
 Estimating cell size using a minigrid
 Recording & calculating
 Questions
GRADE
24
Student: _____________________________________
Teacher: Mrs Brown
PART A
AIM
1. Discuss and develop the aim for this investigation.
(2 marks)
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VARIABLES
2. Discuss and decide on the independent variable and the dependent variables for this
investigation.
(2 marks)
Independent variable
_________________________________________________________
Dependent variable
_________________________________________________________
HYPOTHESIS
3. Discuss and develop an hypothesis for this investigation
(2 marks)
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MATERIALS & EQUIPMENT
4. Discuss and decide what materials and equipment you would need for this investigation
(3 marks)
PART B
METHOD
PART 1
Measuring the field of view
Microscopes are expensive. Take care to follow these instructions.
1. Set the microscope to the lowest objective lens. This will either be x4 or x10.
2. Place minigrid on the stage underneath the lens.
3. Centre the minigrid on the stage using the stage clip adjustment knob.
4. While looking at the stage, bring the specimen up as close as possible to the objective lens
using the coarse adjustment knob. Take note of which way the knob is turning to move the
stage up.
5. Now looking through the eyepiece, bring the stage down using the coarse adjustment knob
until your specimen is in focus. Fine-tune with the fine adjustment knob.
6. Start measuring from the left hand side by positioning a millimetre line on the edge of the field
of view and measuring along the diameter of the circle.
7. Count the number of millimetres along the diameter. Estimate the fraction of the last
millimetre. It may be ½, ⅓ or ¼ of a millimetre.
8. Record the results.
9. To calculate the field of view for higher magnifications, use the following formula:
lower magnification
higher magnification
X
lower measurement
METHOD
PART 3
Using the microscope
Microscopes are expensive. Take care to follow these instructions.
1. Turn the microscope light on.
2. Turn to use the 4x objective lens.
3. Place the slide on the stage between the stage clips.
4. Centre the specimen on the stage using the stage clip adjustment knob.
5. While looking at the stage, bring the specimen up as close as possible to the objective lens
using the coarse adjustment knob. Take note of which way the knob is turning to move the
stage up.
6. Now looking through the eyepiece, bring the stage down using the coarse adjustment knob
until your specimen is in focus. Fine-tune with the fine adjustment knob.
7. To view the specimen at a higher magnification, turn to the 10x objective lens. From now on,
do not use the coarse adjustment knob. While looking through the eyepiece, use the fine
adjustment knob to focus the specimen.
8. To view the specimen at an even higher magnification, turn to the 40x objective lens.
Remember, do not use the coarse adjustment knob. While looking through the eyepiece, use
the fine adjustment knob to focus the specimen.
9. Never use the 100x objective lens. It needs emersion oil and will be damaged otherwise.
10. To finish, turn the 4x objective lens back to its original position. Now you can safely remove
the slide. Remember to turn the microscope light off.
eyepiece
(x10 magnification)
objective lenses
(x4, x10 & x40
magnification)
coarse adjustment
stage with clips
fine adjustment
light source
stage clip adjustment
RESULTS
1. Field of view at low power (LP)
a) Field of view diameter in mm = ___________________
b) Field of view diameter in µm
= ___________________
(2 marks)
2. Estimating size of an individual cell
To estimate the size of an individual cell, count the number of cells across the field of view and then divide the
number of cells by the diameter of the field of view. For example: if five cells fit across a field of view with a
diameter of 4000 µm each cell would be 800 µm in length as shown below.
<
a)
b)
c)
d)
e)
4000 µm
>
Place the prepared slide on the stage and observe the cells under LP.
Focus the microscope according to the instructions provided in the METHOD
Adjust the slide until a complete cell is at the edge of the field of view along the diameter.
Estimate the number of cells that fit across the field of view.
To estimate the size of an individual cell, divide the number of cells counted by the diameter of the field
of view.
Record the results:
a) Number of cells
___________
b) Field of view
___________
c) Size of an individual cell (µm)
___________
(2 marks)
3. High power (HP) field of view
Changing from LP to HP increases the magnification. As the magnification increases, the field of view
decreases. For example if you increase the magnification from 100x to 400x the field of view would be reduced
by ¼. The field of view under HP needs to be calculated from the diameter of the LP field of view because the
magnification under HP is too great for the mm spacing of a ruler to be seen.
Calculate the field of view under HP. Show all working out.
(2 marks)
QUESTIONS
1. Convert the following units to micrometres (µm)
2.4 mm =
0.2 cm =
4.2 mm =
1.7 mm =
0.5 cm =
3.2 mm =
(3 marks)
2. When you change from LP to HP what happens to the field of view? Explain why this happens.
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(2 marks)
3. A student found the field of view under LP (10 x 10) to be 1.5mm and counted five cells across the
diameter.
(a) Estimate the size of an individual cell in µm.
(2 marks)
(b) The student then changed to HP (10 X 40). Calculate the field of view under HP and suggest how many
cells would now be seen across the diameter. Show your working.
(2 marks)