The microscope is the tool that allowed for the discovery and study of cells, the fundamental units of life. Every living organism is composed of one or more cells, making their visualization essential for understanding biology. Early scientists in the 17th century first used simple magnifying lenses to observe these miniature structures. Modern instruments now allow for the observation of not only the entire cell but also the intricate components within it.
The Standard Tool: Compound Light Microscopy
The most common instrument used to view cells is the compound light microscope, which utilizes a system of glass lenses and visible light to magnify a specimen. This type of microscope achieves a total magnification between 40 times and 1,000 times the actual size of the object. High magnification alone is insufficient; resolution, the ability to distinguish two closely positioned points as separate entities, is equally important. The resolution of a standard light microscope is limited by the wavelength of visible light, meaning it cannot resolve structures smaller than about 200 nanometers.
The compound light microscope uses an objective lens near the specimen and an ocular lens in the eyepiece to achieve total magnification. For instance, combining a 10x ocular lens with a 100x objective lens produces a 1,000x magnified image. To view specimens clearly, light must be focused and passed through the sample. When viewing a transparent cell, contrast must be introduced to make structures visible against the bright background.
Specimens must often be stained with colored dyes to increase contrast and highlight specific internal components, such as the nucleus. Stains like methylene blue or iodine bind to cellular structures, absorbing light and making them stand out. Another technique to enhance clarity involves placing immersion oil between the highest-power objective lens and the glass slide. This specialized oil has a refractive index similar to glass, which prevents light rays from bending and scattering, thereby improving image resolution. This combination of magnification, resolution, and staining allows researchers to see the basic morphology of cells.
Visualizing Cellular Structures
Through the compound light microscope, only the larger features of a cell are distinguishable. The outer boundary—the cell membrane in animal cells or the rigid cell wall in plant cells—is visible, delineating the cell’s perimeter. The cytoplasm, the jelly-like substance filling the cell, often appears as a clear or slightly granular area within this boundary.
The nucleus is usually the most noticeable organelle, appearing as a dense, spherical, or oval structure within the cytoplasm. It is easily seen because it contains the cell’s genetic material, which readily absorbs common stains. Plant cells, such as those from an onion or a leaf, display a regular, often rectangular shape due to the stiff outer cell wall. They also contain large central vacuoles and, if photosynthetic, are filled with numerous chloroplasts.
In contrast, animal cells, like human cheek cells, have irregular and flexible outlines because they lack a cell wall. While the nucleus is readily apparent, many smaller organelles within the cytoplasm, such as the mitochondria or the Golgi apparatus, are too small to be individually resolved. These components often blend together, appearing only as a general texture or slight granularity within the cytoplasm at maximum resolution.
Seeing the Unseen: Electron Microscopy
While the light microscope is excellent for viewing whole cells and large organelles, its 200 nanometer resolution limit prevents it from revealing the cell’s fine internal architecture. To overcome this limitation, scientists employ electron microscopes, which use a beam of electrons instead of light. Electrons have a much shorter wavelength than visible light, allowing the electron microscope to achieve a resolution up to 1,000 times greater than its light-based counterpart, resolving structures as small as 0.1 to 0.2 nanometers.
This leap in resolution allows for magnifications up to 1,000,000 times, revealing the cell’s ultrastructure. The Transmission Electron Microscope (TEM) passes the electron beam through an ultra-thin slice of the specimen, providing a detailed two-dimensional view of internal organelles. This technique allows for the visualization of the intricate folding of the inner mitochondrial membrane, the stacked sacs of the Golgi apparatus, and the ribosomes responsible for protein synthesis.
A second type, the Scanning Electron Microscope (SEM), scans the electron beam across the specimen’s surface, capturing reflected electrons to create a three-dimensional image. The SEM is useful for studying the surface topography of cells, bacteria, and viral particles, which are too small to be seen with a light microscope. Both TEM and SEM require the specimen to be in a vacuum and often coated in a thin layer of metal, meaning they cannot be used to observe living cells, unlike the light microscope.