Investigation Lab: Cell Structures
Learn
In this investigation lab, you will use microscopy techniques to observe and compare plant and animal cells. Understanding how to properly use a microscope and identify cellular structures is a fundamental skill in biology.
Lab Objectives
- Prepare wet mount slides of plant and animal cells
- Use a compound microscope to observe cells at different magnifications
- Identify and compare organelles in plant and animal cells
- Create detailed scientific drawings with proper labels
- Calculate total magnification and estimate cell size
Background: The Compound Microscope
A compound microscope uses two sets of lenses to magnify specimens. The eyepiece lens (ocular) typically provides 10x magnification, while the objective lenses provide additional magnification (usually 4x, 10x, 40x, or 100x).
Total Magnification = Eyepiece magnification x Objective magnification
For example, using a 10x eyepiece with a 40x objective gives you 400x total magnification.
Key Structures to Identify
| Structure | Plant Cells | Animal Cells | Function |
|---|---|---|---|
| Cell membrane | Present | Present | Controls what enters/exits cell |
| Cell wall | Present | Absent | Provides rigid support |
| Nucleus | Present | Present | Contains genetic material (DNA) |
| Cytoplasm | Present | Present | Gel-like substance holding organelles |
| Chloroplasts | Present (green cells) | Absent | Photosynthesis |
| Large central vacuole | Present | Small or absent | Storage, maintains cell pressure |
Safety Considerations
- Handle glass slides carefully to avoid breakage and cuts
- Never touch the microscope lenses with your fingers
- Use iodine stain carefully - it can stain skin and clothing
- Dispose of materials properly as directed
Examples
Example 1: Preparing a Wet Mount Slide
Procedure for onion epidermis (plant cell):
- Obtain a clean glass slide and coverslip
- Place one drop of water in the center of the slide
- Use forceps to peel a thin, transparent layer from the inner surface of an onion scale
- Place the onion tissue flat in the water drop
- Add one drop of iodine stain to enhance visibility of structures
- Lower the coverslip at a 45-degree angle to prevent air bubbles
Example 2: Calculating Total Magnification
Problem: You are viewing cheek cells using a microscope with a 10x eyepiece and the 40x objective lens. What is your total magnification?
Solution:
Total magnification = Eyepiece x Objective
Total magnification = 10x x 40x = 400x
Example 3: Creating a Scientific Drawing
Requirements for a proper scientific drawing:
- Title including specimen name and magnification
- Drawing fills most of the space (at least half the page)
- Clean, single lines (no sketchy or broken lines)
- Labels with straight lines pointing to structures
- No shading - use stippling (dots) for darker areas if needed
- Scale bar or size estimate included
Example 4: Estimating Cell Size
Problem: The field of view diameter at 100x magnification is 2000 micrometers. You observe that 4 cells fit across the diameter. What is the approximate width of one cell?
Solution:
Cell width = Field of view diameter / Number of cells
Cell width = 2000 micrometers / 4 = 500 micrometers
Practice
Complete the following practice problems to demonstrate your understanding of microscopy and cell structures.
1. Calculate the total magnification when using a 15x eyepiece with a 40x objective lens.
2. A student observes cells under a microscope and notices a thick outer boundary and green oval structures inside. Is this most likely a plant cell or an animal cell? Explain your reasoning.
3. Why is it important to lower the coverslip at an angle when preparing a wet mount slide?
4. The field of view at 40x magnification is 4500 micrometers. If you switch to the 100x objective (keeping the same 10x eyepiece), what would you expect the new field of view to be approximately?
5. A student looks through the microscope but sees only darkness. List three possible reasons for this problem.
6. You observe that 8 onion cells fit across the field of view. If the field of view is 1600 micrometers, what is the approximate width of one onion cell?
7. Compare and contrast what you would expect to see when viewing an onion cell versus a cheek cell. Name at least two differences.
8. Why do we use iodine stain when viewing onion cells? What structures does it help us see more clearly?
9. A microscope has objective lenses of 4x, 10x, and 40x, and an eyepiece of 10x. Which combination would you use to first locate a specimen on the slide? Explain why.
10. Explain why plant cells maintain their shape even when the cell membrane pulls away from the cell wall (plasmolysis), while animal cells would collapse in the same situation.
Check Your Understanding
Question 1: Which structure would help you determine if a cell is from a plant or an animal?
Question 2: If you are using 400x total magnification with a 10x eyepiece, what objective lens are you using?
Question 3: Why should you always start with the lowest magnification objective when viewing a new slide?
Next Steps
- Complete your lab report with scientific drawings and observations
- Review the differences between plant and animal cell structures
- Practice calculating magnification with different lens combinations
- Continue to the next lesson: Data and Graphs