1a & 1b – To Investigate Cells Using a Light Microscope (LC 2027) (Leaving Cert Biology): Revision Notes

1a & 1b – To Investigate Cells Using a Light Microscope

Introduction

These investigations allow you to examine living cells using a light microscope. Investigation 1a focuses on observing animal cells (cheek cells), while Investigation 1b examines plant cells (onion cells). Both investigations follow similar procedures but use different staining techniques to make cellular structures visible.

Understanding magnification

Before starting any microscopy work, you need to understand how magnification works. The total magnification of your image depends on two lenses working together.

This calculation is essential for understanding how much larger your specimen appears compared to its actual size.

Parts of a light microscope

Understanding your microscope is crucial for successful investigations. Each component has a specific function that contributes to producing a clear, magnified image.

Key components include:

• Eyepiece lens - magnifies the image formed by the objective lens
• Objective lenses - provide the primary magnification of the specimen
• Stage - supports the microscope slide containing your specimen
• Condenser - focuses light onto the specimen from below
• Diaphragm - controls the amount of light passing through the specimen
• Coarse focus knob - allows major adjustments to bring the image into approximate focus
• Fine focus knob - enables precise focusing for sharp, clear images

Investigation 1a - examining cheek cells (animal cells)

Preparing the slide

The first step involves collecting and preparing your cheek cell sample:

1. Sample collection - Gently scrape the inside of your mouth using a cotton wool bud or wooden spatula to collect cheek cells
2. Cell distribution - Spread the collected cells evenly onto a clean microscope slide to prevent clumping
3. Adding water - Place a small drop of water onto the cells to prevent them from drying out
4. Staining - Add one or two drops of methylene blue stain, which colours the nucleus dark blue and the cytoplasm light blue, making cellular structures more visible
5. Cover slip application - Lower a cover slip at an angle to eliminate air bubbles and protect the objective lens from moisture

Examining under the microscope

Once your slide is prepared, follow these systematic steps:

1. Initial setup - Turn on the microscope light and adjust the mirror to ensure proper illumination
2. Stage positioning - Lower the stage completely and place your slide with cells positioned over the central opening
3. Low power examination - Begin with the lowest power objective lens to locate cells and get a general overview
4. Focusing - Use the coarse focus knob first, then fine-tune with the fine focus knob
5. High power observation - Once cells are located, switch to higher magnification for detailed examination
6. Light adjustment - Modify illumination using the condenser, diaphragm, or mirror angle for optimal contrast

Investigation 1b - examining onion cells (plant cells)

Preparing the slide

Onion cells require a different preparation method due to their plant origin:

1. Tissue preparation - Remove the outer, dry layers from a fresh onion and cut it into sections
2. Cell layer isolation - Use forceps or your fingers to peel off a thin, transparent layer of cells from the inner curve of an onion section
3. Slide preparation - Quickly place this thin strip of cells onto a microscope slide (avoid delays as the cells may curl up)
4. Hydration options - Add either water for unstained cells OR iodine solution for stained cells (iodine creates a red-yellow stain that makes the nucleus orange and cytoplasm yellow)
5. Cover slip placement - Apply a cover slip at an angle, similar to Investigation 1a

Examining under the microscope

The examination process follows the same systematic approach as Investigation 1a:

1. Progressive magnification - Start with low power to locate cells, then increase magnification for detailed observation
2. Comparative analysis - Examine cells under both low and high power to understand different levels of detail
3. Structure identification - Focus on identifying key plant cell features that distinguish them from animal cells

Key differences between cell types

Understanding the fundamental differences between animal and plant cells is crucial:

Exam tips