Microscopy and Cells

Objectives

After completing this exercise, you should be able to:

• Identify the parts of a compound light microscope and describe the function of each part.

• Use proper microscope technique to view slides.

• Calculate total magnification of an image.

• Create drawings that follow biological drawing guidelines.

• Differentiate between prokaryotic and eukaryotic cells.

• Identify organelles and other cell structures.

• Use a dichotomous key to identify organisms.

Key Terms

Introduction

The world around you teems with life—most of which you cannot see unaided. The macroscopic life visible to the naked eye is only a small portion of the planet’s biodiversity. Many organisms are microscopic and require specialized tools to be observed.

Microscopes allow scientists to visualize these organisms. In this lab, you will use a compound light microscope to explore the microscopic world.

Today’s focus is on eukaryotic cells, which contain a nucleus and membrane-bound organelles.
You (and most living organisms such as plants, fungi, protists, and animals) are eukaryotes—literally “true nucleus.”
In contrast, prokaryotic cells (e.g., bacteria) lack a nucleus and membrane-bound organelles.

Within eukaryotic cells, specialized structures called organelles perform essential functions:

• Chloroplasts (in plants) enable photosynthesis.

• Mitochondria generate most of the cell’s energy (ATP).

Figure 1. Parts of a Compound Light Microscope

Materials and Procedure

Materials

• Compound light microscope

• Lens paper

• Clean slide and coverslip

• Prepared slide: letter “e”

• Zoea of crab slide

• Volvox slide

• Two toothpicks

• DI water bottle

• Dropper bottle with methylene blue stain

• Elodea leaf

• Drop of pond water

General Rules for Microscope Use

1. Preparing the Microscope

1. Plug in and turn on the light source.

2. Adjust the oculars (eyepieces) to match the distance between your eyes.

3. Use the diopter adjustment ring to compensate for differences in eyesight.

4. Clean all glass surfaces (oculars, objectives, condenser) before and after use with lens paper only—never paper towels.

2. Using the Microscope

1. Place the slide on the stage and secure it with the stage clip.

2. Ensure the scanning objective (4x) is in place.

3. Use the coarse adjustment knob to raise the stage close to the objective lens.

4. While looking through the oculars, slowly lower the stage until the specimen comes into view.

5. Refine focus using the fine adjustment knob.

6. Center your specimen before increasing magnification.

7. Switch to low power (10x), then high power (40x). Use only fine focus at 40x.

8. Adjust contrast using:

   • The diaphragm, or

   • The light dimmer switch.

Caution: Never use the coarse adjustment knob with the 40x lens—this can crack the slide or damage the lens.

Exercise 1 – The Letter “E”

1. View the prepared “letter e” slide under low power.

2. Compare the specimen’s position on the slide (naked eye) vs. the image seen through the microscope.

   • How do they differ? ____________________________

3. Move the slide so half of the “e” disappears from the field of view. Switch to low power without moving the slide. What do you observe now? ____________________________

4. Return to scanning power, center the “e,” and move through all magnifications.

5. Choose the best magnification for clarity.

6. Draw your “e” following the Biological Drawing Guidelines below.

Guidelines for Biological Drawings

1. Use white, unlined paper (two figures per page, one side only).

2. Each drawing must include:

   • Figure number (e.g., Figure 1)

   • Descriptive title

   • Total magnification (ocular × objective)

3. Example: Ocular (10x) × High Power Objective (40x) = 400x Total Magnification

4. Draw accurately and neatly using pencil only.

5. Label structures outside the field of view with straight connecting lines.

6. Do not add colors (stains vary and are not standardized).

Exercise 2 – Depth of Field: Thread Slide

Use a prepared thread slide with overlapping colored threads.

1. Focus under 4x, then move to 10x and 40x.

2. Observe that not all threads can be focused simultaneously.

   • As you adjust focus, different layers become clear—illustrating depth of field.

3. This concept helps interpret cell layer depth in tissues.

Exercises 3 and 4 – Copepod or Zoea of Crab and Protist (Volvox)

1. View the arthropod (copepod or zoea) slide, then the Volvox slide.

2. Follow the previous microscope steps.

3. Sketch each organism following biological drawing guidelines.

Exercise 5 – Elodea

Elodea is an aquatic plant (Genus name: Elodea).

Identify and label the following on your drawings:

• Cell Wall – rigid outer boundary giving shape

• Cell Membrane – pressed against the cell wall

• Chloroplasts – small green spheres (site of photosynthesis)

• Central Vacuole – large clear space filled with water and solutes

• Cytosol – clear fluid portion of cytoplasm; look for cytoplasmic streaming (cyclosis)

Wet Mount Preparation

1. Place one Elodea leaf flat on a clean slide.

2. Add a drop of water.

3. Lower the coverslip at a 45° angle to reduce air bubbles (see Figure 3).

4. View at 400x total magnification.

5. Clean your slide and coverslip afterward.

Exercise 6 – Pond Water

1. Obtain a pond water sample with a pipette (include green algae and detritus).

2. Add 1–2 drops to a slide and place a coverslip.

3. Scan the slide systematically (see Figure 4) for organisms.

4. Draw two different organisms you find and note:

   • Type (protist, algae, etc.)

   • Movement or behavior

5. Clean all materials when finished.

Exercise 7 – Human Cheek Cells

This exercise involves biohazardous material. Follow all disposal instructions carefully.

1. Place one drop of water on your slide.

2. Add a small amount of methylene blue stain using a toothpick and mix.

3. Using a new toothpick, gently scrape the inside of your cheek.

4. Swirl the sample in the stained drop.

5. Add coverslip and view under microscope.

6. Identify and label:

   • Nucleus

   • Cell Membrane

   • Cytosol

Exercise 7 – Clean-Up Procedure

1. Dispose of toothpicks in the red sharps biohazard container.

2. Place slides and coverslips in the biohazard sharps container.

3. Use disinfectant spray and Kimwipes to clean microscope surfaces.

4. Clean glass areas with lens paper only.

5. Dispose of all contaminated materials in biohazard bins.

Do not use biohazard bins for regular trash.

Guidelines for Proper Microscope Care

Data and Drawings

Review Questions

1. List several differences between plant and animal cells based on your observations.

2. Compare and contrast prokaryotic and eukaryotic cells.

3. Can you identify each cell type and its organelles observed in this lab?

4. Identify the parts of the microscope and describe their functions.

Licenses and Attribution

CC Licensed Content, Original:

Other Licensed Content Included:

• Adapted with permission from BioScience I Laboratory Manual by Jamie Colson-Moon and Denise Bristol. Per request from these original authors, this content is now licensed (CC BY-NC-SA).