Technologies to Study Cell Structure (HSC SSCE Biology): Revision Notes

Technologies to Study Cell Structure

Introduction to microscopy

Scientists have been curious about viewing tiny structures for centuries. In the 1500s, they began using handheld magnifying glasses to examine objects more closely. However, to see even finer details, they needed more powerful tools.

The breakthrough came with the invention of the compound microscope. This early device consisted of two convex lenses positioned at opposite ends of a barrel. It was the ancestor of the modern light microscope we use in laboratories today.

Over time, scientists made improvements to these compound microscopes. They added stands to hold them steady, focus knobs for adjusting the image, and light sources for better illumination. These enhancements allowed scientists to study the structure of organisms in much greater detail.

Robert Hooke's discovery

In the 1660s, a scientist named Robert Hooke used an improved compound microscope to examine a thin slice of cork. When he looked at the cork under magnification, he saw tiny compartments that reminded him of small rooms. He called these compartments "cella" (the Latin word for small room), which eventually led to our modern term "cell."

Modern microscopes

As technology advanced, microscopes became increasingly sophisticated. Today, we have several types of microscopes, each with unique capabilities for studying cells.

Light microscopes

The compound light microscope is the type you typically use in school science classes. Understanding how it works helps us appreciate what we can see with it.

How light microscopes work

In a compound light microscope, light from a source passes upward through a condenser lens. This light then travels through the thin specimen you're examining. After passing through the specimen, the light enters a convex objective lens, which magnifies the image. Finally, you view the magnified image through the ocular lens (eyepiece).

Key specifications

Magnification refers to how much larger the image appears compared to the actual object. Light microscopes can magnify objects up to $1500 \times$, depending on the lenses used.

However, magnification alone isn't enough to see cellular details clearly. Resolution (also called resolving power) is equally important. Resolution is the ability to distinguish between two separate objects. It represents the smallest distance between two objects where each can still be observed as separate entities.

For compound light microscopes, the maximum resolution is $200 \text{ nm}$ (nanometres). This means the best light microscopes can only distinguish two separate structures if the distance between them is $200 \text{ nm}$ or more. If two objects are closer than $200 \text{ nm}$ apart, they will appear as a single object.

Advantages of light microscopes

One significant advantage of light microscopes is their versatility. Both living and non-living specimens can be viewed using a compound light microscope. This makes them invaluable for observing living cells and their behaviours.

Fluorescence microscopes

Fluorescence microscopes are similar to light microscopes but have special features that allow scientists to target and observe specific parts of cells.

How fluorescence microscopy works

To use a fluorescence microscope, scientists first label the sample with a fluorescent substance. This substance is carefully chosen to attach to the specific structures the scientist wants to observe.

When the sample is illuminated with high-intensity light, the fluorescent substance absorbs this light and then emits its own light (fluorescence). The emitted fluorescent light passes through special filters that separate it from surrounding light. This allows the viewer to see only the areas of the sample that are fluorescing.

Applications

This technique is particularly useful for:

• Identifying specific proteins within cells
• Observing cellular structures beyond the normal resolution limit of light microscopes
• Tracking the location of particular molecules
• Studying cellular processes in real-time

Electron microscopes

Since the 1950s, electron microscopes have revolutionised our understanding of microscopic structures. These powerful instruments use completely different technology compared to light microscopes.

Basic principles

Instead of using light rays and glass lenses, electron microscopes use:

• An electron beam instead of light
• Electromagnets instead of glass lenses

The interaction between electrons and the specimen creates a viewable image on a screen.

Why electron microscopes are superior

Electron microscopes achieve much greater magnification than light microscopes. They also have substantially higher resolving power because electrons have much shorter wavelengths than visible light.

These capabilities have revealed structures at both the cellular and subcellular levels. Many cell organelles were seen for the first time using electron microscopes. Materials previously thought to have little internal structure were shown to have elaborate organisation. Features as small as one-tenth of a nanometre (one ten-billionth of a metre) can be observed, including individual atoms.

Transmission electron microscope (TEM)

The transmission electron microscope (TEM) is the most common type of electron microscope. In a TEM, electrons are transmitted through (pass through) the specimen.

Key specifications:

• Produces two-dimensional images
• Can magnify up to $1,500,000 \times$
• Has a resolution of approximately $2 \text{ nm}$

The high resolution allows scientists to see internal structures of organelles in remarkable detail.

Scanning electron microscope (SEM)

The scanning electron microscope (SEM) works differently. It bombards solid specimens with a beam of electrons. These electrons cause secondary electrons to be emitted from the surface layers of the specimen.

Key specifications:

• Creates three-dimensional images of surfaces
• Has slightly lower resolution (approximately $10 \text{ nm}$) than TEM
• Provides excellent surface detail

The three-dimensional images from SEMs are particularly useful for studying surface structures and textures.

Disadvantages of electron microscopes

Despite their powerful capabilities, electron microscopes have some limitations:

Computer-enhanced technology

Modern technology has added another dimension to microscopy. Digital processing of microscope images allows scientists to view cells in innovative ways.

Digital image processing

Computer software can create three-dimensional representations of cell structures, providing a much deeper understanding of how cells are organised and function. Advanced computer technologies also help construct models showing how molecules interact during cellular reactions.

Confocal laser scanning microscopy

Confocal laser scanning microscopy is a sophisticated technique that creates three-dimensional images of specimens.

How it works

A laser produces a narrow, intense beam of light focused to a pinpoint on the sample. The microscope captures only the light from this precise focal point, while excluding all out-of-focus areas.

This focusing process occurs many times at different levels throughout the specimen. An image reconstruction program then combines data from all these different levels to construct a three-dimensional image.

Advantages

Confocal laser scanning microscopes are particularly useful for:

• Imaging structural components of cells
• Creating detailed 3D models
• Studying intact specimens without destroying them

Comparison of microscope types

Microscope Type	Magnification	Resolution	Living Specimens?	Image Type
Light	Up to $1500 \times$	$200 \text{ nm}$	Yes	2D
Fluorescence	Up to $2000 \times$	$200 \text{ nm}$	Yes	2D (specific structures)
TEM	Up to $1,500,000 \times$	$2 \text{ nm}$	No	2D
SEM	Up to $10,000,000 \times$	$10 \text{ nm}$	No	3D
Confocal	Variable	Better than standard light	Yes	3D