Learning Objectives

• Identify how common techniques of microscopy and biochemistry are used in cell biology.

• Compare and contrast prokaryotic and eukaryotic cells, as well as animal and plant cells.

• Describe the structure and function of the nucleus, chromosomes, and ribosomes.

• Compare and contrast the structures and functions of mitochondria and chloroplasts.

Concept 6.1: Biologists Use Microscopes and Biochemistry to Study Cells

Microscopy

Cells are the fundamental units of life, but they are usually too small to be seen by the naked eye. Microscopes are essential tools that allow scientists to visualize and study the structure and function of cells.

• Microscopes use lenses or electron beams to magnify and resolve cellular structures.

• Understanding microscopy is crucial for interpreting cell biology experiments and observations.

Light Microscopy (LM)

Light microscopes use visible light passed through a specimen and glass lenses to magnify images for observation.

• Lenses refract (bend) light, enlarging the image for the eye or a camera.

Three Important Parameters of Microscopy

• Magnification: The ratio of an object's image size to its real size. Light microscopes can magnify up to approximately 1,000 times the actual size of a specimen.

• Resolution: The measure of image clarity or the minimum distance between two distinguishable points. The minimum resolution of a light microscope is about 200 nanometers (nm), roughly the size of a small bacterium.

• Contrast: Visible differences in brightness between parts of a sample. Contrast can be enhanced by staining or labeling cell components.

Scale of Biological Structures

Biological structures range in size from meters (e.g., human height) to nanometers (e.g., proteins, lipids). The range of visibility for the unaided eye, light microscopy, and electron microscopy is illustrated below:

• Unaided eye: Can see objects down to about 0.1 mm (100 μm), such as frog eggs.

• Light microscopy: Effective for most plant and animal cells, nuclei, and some bacteria (down to ~200 nm).

• Electron microscopy: Required for visualizing viruses, ribosomes, proteins, and smaller structures (down to ~2 nm).

Limitations of Light Microscopes (LM)

• Light microscopes cannot resolve the internal anatomy of cells well, especially organelles (membrane-bound structures in eukaryotic cells).

Electron Microscopes (EM)

Electron microscopes use beams of electrons to achieve much higher resolution than light microscopes, allowing scientists to study fine cell structures.

• Resolution is inversely related to the wavelength used in imaging: shorter wavelengths provide better resolution.

• Electron beams have much shorter wavelengths than visible light, resulting in finer resolution.

• Theoretical resolution of modern EMs: nanometers (nm).

• Practical resolution limit: about 2 nm.

Types of Electron Microscopy

• Scanning Electron Microscopy (SEM): Used to study the surface structure or topography of a specimen. The sample is coated with a thin film of gold to enhance imaging.

• Transmission Electron Microscopy (TEM): Used to study the internal structure of cells. Thin sections of the specimen are stained with heavy metals to increase contrast, which bind to specific cellular structures.

• Cryo-electron microscopy (cryo-EM): Allows imaging of specimens at cryogenic temperatures, preserving native structures.

Limitations of Electron Microscopy

• Sample preparation for EM is complex and kills cells, so live cells cannot be observed.

Advanced Light Microscopy Techniques

• Confocal microscopy and deconvolution microscopy produce sharper images of three-dimensional tissues and cells.

• Other techniques include phase-contrast, differential interference contrast (Nomarski), fluorescence, and super-resolution microscopy.

Summary Table: Types of Microscopy and Their Applications

Key Terms

• Magnification:

• Resolution: The minimum distance two points can be separated and still be distinguished as two points.

• Contrast: The difference in brightness between the light and dark areas of an image.

Example

• To observe the movement of live cells, a light microscope is used. To study the detailed structure of a ribosome, a transmission electron microscope is required.