뒤로A Tour of the Cell: Microscopy and Cell Structure
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Chapter 6: A Tour of the Cell
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 for visualizing cells and understanding their structure and function.
Microscopes magnify and resolve the details of cells and their components.
Understanding microscopy is crucial for studying cell biology.
Light Microscopy (LM)
Uses visible light passed through a specimen and glass lenses.
Lenses refract (bend) the light, magnifying the image for observation.
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: A 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
The following diagram (not shown) illustrates the size range of biological structures and the types of microscopy used to observe them:
Unassisted eye: human height, chicken egg, frog egg
Light microscopy: most plant and animal cells, nucleus, most bacteria, mitochondrion
Electron microscopy: smallest bacteria, viruses, ribosomes, proteins, lipids, small molecules, atoms
Additional info: 1 cm = 10-2 m; 1 mm = 10-3 m; 1 μm = 10-6 m; 1 nm = 10-9 m.
Limitations of Light Microscopes (LM)
Cannot resolve the internal anatomy of cells well, especially organelles (membrane-bound structures in eukaryotic cells).
Electron Microscopes (EM)
Use a beam of electrons to study fine cell structures.
Provide much higher resolution than light microscopes due to the shorter wavelength of electrons.
Resolution and Wavelength
Resolution is inversely related to the wavelength used in imaging: shorter wavelengths yield better resolution.
Electron beams have much shorter wavelengths than visible light, allowing for finer resolution.
Theoretical resolution of modern EMs: ~0.002 nm; practical resolution limit: about 2 nm.
Types of Microscopy Techniques
Microscopy Type | Main Purpose | Sample Preparation | Resolution | Live/Dead Sample |
|---|---|---|---|---|
Brightfield (unstained/stained) | General cell observation | Minimal or stained | ~200 nm | Live or fixed |
Phase-contrast/Differential interference contrast | Enhance contrast in unstained cells | Unstained | ~200 nm | Live |
Fluorescence/Confocal/Deconvolution/Super-resolution | Specific labeling, 3D imaging | Fluorescent dyes/labels | ~200 nm or better | Live or fixed |
Scanning Electron Microscopy (SEM) | Surface structure/topography | Coated with gold | ~2 nm | Dead |
Transmission Electron Microscopy (TEM) | Internal cell structure | Thin sections, heavy metal stains | ~2 nm | Dead |
Cryo-electron microscopy (cryo-EM) | High-resolution, near-native state | Rapid freezing | ~2 nm or better | Dead (but close to native state) |
Additional info: Table summarizes the main microscopy techniques and their applications, as depicted in the provided images.
Summary of Key Points
Light microscopes are suitable for observing live cells and general cell structure, but have limited resolution.
Electron microscopes provide much higher resolution, allowing visualization of organelles and fine cell structures, but require complex sample preparation and can only be used on dead cells.
Advanced light microscopy techniques (e.g., confocal, deconvolution) improve image clarity and allow for 3D imaging of cells and tissues.
