Exercises: Microscope observation

In the light microscope diagram, there are three parts labelled with "?". Write the name of each one and explain its function.

Put the steps for preparing an onion cell sample in order: (a) cover with the coverslip taking care to avoid air bubbles, (b) place a thin layer of onion on the microscope slide, (c) add a drop of iodine solution (lugol) to stain, (d) remove excess liquid with filter paper.

Why are many samples stained before observing them under a microscope? Give an example of a stain used in the school laboratory.

Name the two parts of the microscope that contain lenses and explain where you look through and where the specimen is placed.

Explain what the microscope slide and the coverslip are and what each one is used for when preparing a specimen.

Describe how a microscope should be carried from the cupboard to the work table without damaging it: which parts you hold and with how many hands.

What must be done when finishing a microscopy practical? Put these tasks in order and explain them: switching off the light, cleaning the lenses with the special paper, removing and disposing of the specimen, putting the microscope away, and washing your hands.

To observe something under the light microscope it must be cut into very thin sections. Explain why the specimen must let light through and what happens if it is too thick.

A microscope has an eyepiece lens of $10\times$ and objective lenses of $4\times$, $10\times$, and $40\times$. Calculate the total magnification achieved with each objective lens, and explain which one you should always start observing with and why.

When observing a sample, the image looks very dark and only a fragment of the specimen can be seen. State two possible causes for each problem and what you would do to correct them without changing the objective lens.

Why must you focus first using the lowest power objective lens and reserve the fine focus knob for higher power objective lenses? Explain what might happen if you use the coarse focus knob with the $40\times$ objective lens.

A microscope has an eyepiece lens of $10\times$ and an objective lens of $20\times$. Calculate the total magnification. If you switch to a $40\times$ objective lens, explain what happens to the size of the image, to the area of the specimen you manage to see, and to the amount of light.

The microscope image is inverted. If you gently push the specimen to the right while looking, reason in which direction the image will seem to move in the eyepiece and what practical consequence this has when you are following a moving microorganism.

You have focused a cell with the $10\times$ objective lens and switch to the $40\times$ one: the image is left slightly blurry and darker. Explain which focus knob you must use to refocus (and which one you must not touch) and which lighting adjustment can improve the image.

Describe how you would prepare a wet mount of a hair or of a fibre from your jumper to view it under the microscope: what materials you would use and in what order you would act. What is the difference between that homemade preparation and a permanent laboratory slide?

Name three safety rules for working with biological samples and stains in the school laboratory and explain the reason for each one.

With the $10\times$ objective lens (total magnification $100\times$), the field of view is about $1.5\ \text{mm}$ in diameter. If an onion cell is about $0.25\ \text{mm}$ long, estimate how many cells in a row would fit across the field of view, and explain what will happen to the diameter of the field of view when switching to the $40\times$ objective lens.

A research team wants to film in real time how a unicellular protozoan swims and captures bacteria in a drop of stagnant water, and at the same time, photograph in full detail the internal structure of the viruses that infect those bacteria. A student proposes using only a transmission electron microscope to achieve both goals. Evaluate their proposal with reasoned arguments: explain why it is not possible to observe the live protozoan with this device and determine which microscope should be used in each case and why.

During a practical activity observing cheek cell specimens stained with methylene blue, a group can only see shapeless blue smudges. Suggest at least three errors they might have made when preparing the sample and explain how you would repeat the experiment to get good results.

A classmate observes a slide under a microscope at $400\times$. The field of view with that objective lens measures $0.4\ \text{mm}$ in diameter, and the cell they are observing occupies approximately half of that diameter. Your classmate claims: "Since the microscope magnifies $400$ times and the cell occupies $0.2\ \text{mm}$, its actual size must be $0.2 \times 400 = 80\ \text{mm}$, it's almost eight centimeters long!".\na) Identify the conceptual error in their reasoning and explain the difference between actual size and magnified apparent size.\nb) Calculate the correct actual size of the cell in millimeters and micrometers ($1\ \text{mm} = 1\,000\ \mu\text{m}$).\nc) If you placed identical cells one after another in a row, how many would fit along one centimeter of a ruler?

You want to check whether the cells in the growth zone of an onion root are smaller than those in the bulb. Design the practical activity: which two specimens you will prepare, how you will estimate the size of the cells using the field of view, which data table you will use, and how many cells you will measure so that the comparison is reliable.

When observing the letter "e" cut out from a newspaper, the microscope shows it rotated 180° (upside down and back to front). Predict how the letter "p" will look before viewing it, explain why this inversion happens, and describe how you would test your prediction.

Analyze the following data on the limit of resolution (the minimum distance to distinguish two separate points) of three instruments:\n• Human eye: $0.1\ \text{mm}$ ($100\ \mu\text{m}$).\n• Compound light microscope: $0.2\ \mu\text{m}$.\n• Electron microscope: $0.0002\ \mu\text{m}$.\n\nYou have four biological samples:\n1. An onion plant cell ($100\ \mu\text{m}$).\n2. A bacterium ($2\ \mu\text{m}$).\n3. A flu virus ($0.1\ \mu\text{m}$).\n4. The plasma membrane of a cell ($0.008\ \mu\text{m}$).\n\na) Deduce the minimum instrument you need to distinguish each of the four samples.\nb) Justify, using these data, why cells and bacteria could already be observed in the 17th and 18th centuries, but the existence and structure of viruses could not be demonstrated until the invention of the electron microscope in the 20th century.

An online store advertises a "School optical microscope with magnifications of up to $50\,000\times$ that allows you to observe viruses and molecules for only €30".\na) Using the analogy of enlarging a low-resolution digital photograph (where you only get larger pixels, but no new details), refute why this advertisement is deceptive and predict what you would actually see when trying to force such high magnification using visible light.\nb) Explain what fundamental physical change the electron microscope introduces compared to the optical microscope to overcome this limit and sharply display structures measuring just a few nanometers.

Unit challenge. Imagine that you are collaborating with your region's science museum by designing an informative sheet on microscopic life in the home.

1. Search for real samples: Access the free image bank of INTEF (Ministry of Education) or the CSIC photo library. Download two real micrographs: a bacterium (Lactobacillus from yoghurt) and a plant cell (onion epidermis). Record the magnifications and the indicated scale bar.

2. Measurement and analysis: Measure the cell in the photo with your ruler and, applying the graphic scale, calculate its actual size in micrometres ($1\text{ }\mu\text{m} = 0.001\text{ mm}$). Prepare a comparative table that includes: calculated actual dimensions (e.g. $1.8\text{ }\mu\text{m}$ compared to $120.5\text{ }\mu\text{m}$), cell type (prokaryotic or eukaryotic), and the presence or absence of a visible nucleus, cell wall, and cytoplasm.

3. Final report: Write a reasoned proposal (maximum $1$ page) explaining why the cell is the unit of life in both organisms, why they differ under the microscope, and what advantages this instrument offers for studying them.

Assessment criteria:

• Rigorous identification and classification of prokaryotic and eukaryotic cellular structures.

• Accuracy in calculating actual cell size from the microscope scale.

• Conceptual clarity when justifying that the cell is the functional and structural unit of living things.