Mitochondria are organelles that generate most of a cell’s chemical energy. A light microscope is a standard tool for observing cells, which leads to the question of whether it is possible to view something as small as a mitochondrion with this instrument.
The Challenge of Visualization
The primary obstacle in viewing mitochondria is their size and the resolution limits of microscopes. Resolution is the ability to see two close objects as separate things. A mitochondrion measures between 0.5 and 1.0 micrometers in diameter, which is technically within the resolving power of a high-quality light microscope that has a resolution limit of about 0.2 micrometers.
The main problem is that mitochondria are almost completely colorless and transparent. They have nearly the same refractive index as the surrounding cytoplasm. This means light passes through them similarly to how it passes through the rest of the cell, making them invisible without special preparation.
Techniques for Viewing Mitochondria
To overcome the lack of contrast, scientists employ specific techniques that make mitochondria stand out. One method involves the use of vital stains, which are dyes that can color parts of a living cell without immediately killing it. A classic example is Janus green B, which accumulates in mitochondria and stains them a blue-green color because of enzyme activity within the mitochondrial electron transport chain.
A more modern approach is fluorescence microscopy. This technique uses fluorescent dyes, called fluorophores, that absorb light at one wavelength and emit it at a longer one. Scientists use dyes like MitoTracker, which bind specifically to parts of the mitochondria in living cells. When illuminated, these dyes glow brightly, making the mitochondria clearly visible against a dark background and allowing for observation of their shape, distribution, and movement.
Light Microscopy vs. Electron Microscopy
Using staining and fluorescence techniques, a light microscope reveals mitochondria as small, often dot-like or rod-shaped, objects moving within the cell. It is possible to observe their general shape, number, and location. However, no internal features can be discerned.
In contrast, an electron microscope provides a more detailed view. Because electron microscopes use beams of electrons instead of light waves, their resolution is thousands of times greater. An image from an electron microscope shows the mitochondrion’s ultrastructure, including its smooth outer membrane and its highly folded inner membrane, which forms structures called cristae. The internal compartment, known as the matrix, is also clearly visible.