Biology · Simulation and virtual lab Free
Virtual microscope: focus, measure and observe cells
Put a slide on the stage, choose an objective and focus. Measure the field of view and estimate the real size of cells.
- You will learn to
- Use a light microscope correctly: objectives, coarse and fine focus, light.
- Measure the field of view and calculate total magnification.
- Estimate cell size and calculate the mitotic index.
- You will measure
- Field of view (mm), number of cells across, cell size (µm), mitotic index (%).
- Time needed
- About 35–45 min
- Equipment
- Nothing extra. Real version: light microscope, transparent ruler, slides and cover slips, onion, iodine, methylene blue, prepared root tip slide.
- Price
- Free: no payment, no account.
Virtual lab
Follow the steps with the simulation above (go back to the simulation ↑). Each time you tap Record, a row is added to the table and a point to the graph.
Aim
To use a light microscope correctly, to measure the field of view, to estimate the size of cells and to calculate the mitotic index of an onion root tip.
What to do
- Choose the Transparent ruler and the ×4 objective. Turn the coarse focus knob until the millimetre lines are sharp. How many millimetres fit across the field of view? Record.
- Change to the ×10 objective and record again. How does the field of view change?
- Choose Onion epidermis with the ×4 objective and focus. Move to ×10, then ×40 using only the fine focus knob. Record each view.
- Count how many cells fit across the field of view and calculate the size of one cell: size = field of view ÷ number of cells.
- Try Cheek cells and Leaf lower epidermis. Compare plant and animal cells. Find the stomata and guard cells.
- Choose Onion root tip at ×40. Find the stages of mitosis. Count the dividing cells and all the cells in the field and calculate the mitotic index.
Results
Questions
Why do we always start with the lowest-power objective?
It has the widest field of view, so it is easy to find the specimen, and the most space between the lens and the slide, so there is less risk of damaging them.
Why must you not use the coarse focus knob with the ×40 objective?
The ×40 objective is very close to the slide. A big movement of the coarse knob can push the lens into the slide and break the slide or scratch the lens.
A microscope has a ×10 eyepiece and a ×40 objective. What is the total magnification?
Total magnification = eyepiece × objective = 10 × 40 = ×400.
The field of view at ×100 is 1.8 mm and 7 onion cells fit across it. Estimate the length of one cell.
1.8 mm ÷ 7 = 0.26 mm = 260 µm.
Name two structures seen in onion cells that are not seen in cheek cells.
A cell wall and a large central vacuole (onion epidermis cells have no chloroplasts because they are not exposed to light).
Why does the image move to the left when you move the slide to the right?
The lenses form an inverted image: it is upside down and reversed left to right.
For teachers: expected results and how the model works
Field of view with a ×10 eyepiece (field number 18): 4.5 mm (×40), 1.8 mm (×100), 0.45 mm (×400). Cell sizes in the model: onion epidermis about 260 × 66 µm, cheek cells about 60 µm, leaf epidermis cells about 50 µm with stomata, root tip cells about 35 µm. The root tip has about 79 % interphase, 10 % prophase, 4 % metaphase, 3 % anaphase and 4 % telophase, so the mitotic index is about 20 % (it varies with the part of the slide). Each slide has its own focus position; the depth of field gets smaller at higher power, so ×400 needs the fine knob. Moving the slide moves the image the opposite way.
Exam practice Exam set
National-exam style questions on this simulation (15 marks). Try each one on paper first, then open the marking scheme.
A biological drawing of a cell is 52 mm long. The real cell is 0.26 mm long. Calculate the magnification of the drawing. (2 marks)
Marking scheme: Magnification = size of drawing ÷ real size (1) = 52 ÷ 0.26 = ×200 (1).
In a root tip, 36 cells were counted in the field of view and 7 of them were dividing. (a) Calculate the mitotic index. (b) Explain why the root tip is used to study mitosis. (4 marks)
Marking scheme: (a) Mitotic index = 7 ÷ 36 × 100 (1) = 19.4 % (1). (b) The root tip contains meristem cells (1) that divide rapidly by mitosis to make the root grow, so many cells are dividing (1).
Describe how to focus a specimen at high power, starting with the specimen on the stage. (4 marks)
Marking scheme: Start with the low-power (×4) objective (1); looking from the side, bring the objective close to the slide with the coarse knob (1); looking through the eyepiece, move it away until the image is in focus (1); turn to the higher objective and use only the fine focus knob (1).
The field of view at ×40 total magnification is 4.5 mm. Calculate the field of view at ×400 and explain your method. (3 marks)
Marking scheme: Magnification increases 10 times (1), so the diameter of the field of view is divided by 10 (1): 4.5 ÷ 10 = 0.45 mm (1).
State two differences between a plant cell and an animal cell seen under the light microscope. (2 marks)
Marking scheme: Plant cells have a cell wall, animal cells do not (1); plant cells have a large permanent vacuole and a regular shape / may contain chloroplasts, animal cells do not (1).
Donat Sciences and MathsBiology · Virtual lab worksheet
School: ........................................ Name: ................................ Class: ........ Date: ............
Virtual microscope: focus, measure and observe cells
Aim: To use a light microscope correctly, to measure the field of view, to estimate the size of cells and to calculate the mitotic index of an onion root tip.
Apparatus (real laboratory): light microscope, transparent ruler, slides and cover slips, onion, iodine, methylene blue, prepared root tip slide.
What to do (with the simulation on www.donatstem.co.rw)
- Choose the Transparent ruler and the ×4 objective. Turn the coarse focus knob until the millimetre lines are sharp. How many millimetres fit across the field of view? Record.
- Change to the ×10 objective and record again. How does the field of view change?
- Choose Onion epidermis with the ×4 objective and focus. Move to ×10, then ×40 using only the fine focus knob. Record each view.
- Count how many cells fit across the field of view and calculate the size of one cell: size = field of view ÷ number of cells.
- Try Cheek cells and Leaf lower epidermis. Compare plant and animal cells. Find the stomata and guard cells.
- Choose Onion root tip at ×40. Find the stages of mitosis. Count the dividing cells and all the cells in the field and calculate the mitotic index.
Results
| # | Slide | Objective | Total magnification | Field of view (mm) | Cells across | Size of one cell |
|---|
| 1 | | | | | | |
| 2 | | | | | | |
| 3 | | | | | | |
| 4 | | | | | | |
| 5 | | | | | | |
| 6 | | | | | | |
| 7 | | | | | | |
| 8 | | | | | | |
Graph
Draw one cell from each slide in the space below (pencil, clear outlines, labels on the right, title and magnification).
Questions
- Why do we always start with the lowest-power objective? [2]
- Why must you not use the coarse focus knob with the ×40 objective? [2]
- A microscope has a ×10 eyepiece and a ×40 objective. What is the total magnification? [2]
- The field of view at ×100 is 1.8 mm and 7 onion cells fit across it. Estimate the length of one cell. [2]
- Name two structures seen in onion cells that are not seen in cheek cells. [2]
- Why does the image move to the left when you move the slide to the right? [2]
- A biological drawing of a cell is 52 mm long. The real cell is 0.26 mm long. Calculate the magnification of the drawing. [2]
- In a root tip, 36 cells were counted in the field of view and 7 of them were dividing. (a) Calculate the mitotic index. (b) Explain why the root tip is used to study mitosis. [4]
- Describe how to focus a specimen at high power, starting with the specimen on the stage. [4]
- The field of view at ×40 total magnification is 4.5 mm. Calculate the field of view at ×400 and explain your method. [3]
- State two differences between a plant cell and an animal cell seen under the light microscope. [2]
Donat Sciences and MathsBiology · Virtual lab worksheet
School: ........................................ Name: ................................ Class: ........ Date: ............
Marking scheme: Virtual microscope: focus, measure and observe cells
Expected results
Field of view with a ×10 eyepiece (field number 18): 4.5 mm (×40), 1.8 mm (×100), 0.45 mm (×400). Cell sizes in the model: onion epidermis about 260 × 66 µm, cheek cells about 60 µm, leaf epidermis cells about 50 µm with stomata, root tip cells about 35 µm. The root tip has about 79 % interphase, 10 % prophase, 4 % metaphase, 3 % anaphase and 4 % telophase, so the mitotic index is about 20 % (it varies with the part of the slide). Each slide has its own focus position; the depth of field gets smaller at higher power, so ×400 needs the fine knob. Moving the slide moves the image the opposite way.
Answers
- Why do we always start with the lowest-power objective? [2]
It has the widest field of view, so it is easy to find the specimen, and the most space between the lens and the slide, so there is less risk of damaging them.
- Why must you not use the coarse focus knob with the ×40 objective? [2]
The ×40 objective is very close to the slide. A big movement of the coarse knob can push the lens into the slide and break the slide or scratch the lens.
- A microscope has a ×10 eyepiece and a ×40 objective. What is the total magnification? [2]
Total magnification = eyepiece × objective = 10 × 40 = ×400.
- The field of view at ×100 is 1.8 mm and 7 onion cells fit across it. Estimate the length of one cell. [2]
1.8 mm ÷ 7 = 0.26 mm = 260 µm.
- Name two structures seen in onion cells that are not seen in cheek cells. [2]
A cell wall and a large central vacuole (onion epidermis cells have no chloroplasts because they are not exposed to light).
- Why does the image move to the left when you move the slide to the right? [2]
The lenses form an inverted image: it is upside down and reversed left to right.
- A biological drawing of a cell is 52 mm long. The real cell is 0.26 mm long. Calculate the magnification of the drawing. [2]
Magnification = size of drawing ÷ real size (1) = 52 ÷ 0.26 = ×200 (1).
- In a root tip, 36 cells were counted in the field of view and 7 of them were dividing. (a) Calculate the mitotic index. (b) Explain why the root tip is used to study mitosis. [4]
(a) Mitotic index = 7 ÷ 36 × 100 (1) = 19.4 % (1). (b) The root tip contains meristem cells (1) that divide rapidly by mitosis to make the root grow, so many cells are dividing (1).
- Describe how to focus a specimen at high power, starting with the specimen on the stage. [4]
Start with the low-power (×4) objective (1); looking from the side, bring the objective close to the slide with the coarse knob (1); looking through the eyepiece, move it away until the image is in focus (1); turn to the higher objective and use only the fine focus knob (1).
- The field of view at ×40 total magnification is 4.5 mm. Calculate the field of view at ×400 and explain your method. [3]
Magnification increases 10 times (1), so the diameter of the field of view is divided by 10 (1): 4.5 ÷ 10 = 0.45 mm (1).
- State two differences between a plant cell and an animal cell seen under the light microscope. [2]
Plant cells have a cell wall, animal cells do not (1); plant cells have a large permanent vacuole and a regular shape / may contain chloroplasts, animal cells do not (1).