뒤로Fundamental Units of Life: The Cell and Microscopy
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Fundamental Units of Life
Introduction to Cells
Cells are the basic structural and functional units of all living organisms. Every organism is composed of one or more cells, and all cells share certain fundamental characteristics.
All organisms are made of cells: This is a central tenet of cell theory.
The cell is the simplest collection of matter that can be alive: Cells carry out all necessary life processes.
Cells are related by their descent from earlier cells: All cells arise from pre-existing cells through division.
Cells can differ substantially from one another but share common features: Despite diversity, all cells have a plasma membrane, cytoplasm, genetic material, and ribosomes.
Microscopy and the Study of Cells
Microscopes in Cell Biology
Because most cells are too small to be seen with the naked eye, biologists use microscopes and chemical tools to study their structure and function.
Microscopes are used to visualize cells: They magnify and resolve cellular structures.
Light Microscope (LM): Uses visible light passed through a specimen and glass lenses to magnify images. Lenses refract (bend) the light, producing a magnified image.
Key Parameters of Microscopy
Three important parameters determine the effectiveness of a microscope:
Magnification: The ratio of an object's image size to its real size. For example, a 400x magnification means the image appears 400 times larger than the actual object.
Resolution: The measure of image clarity, or the minimum distance between two distinguishable points. Higher resolution allows finer details to be observed.
Contrast: Visible differences in brightness between parts of the sample, which can be enhanced by staining or labeling cell components.
Scale of Biological Structures
Cells and their components vary greatly in size, and different types of microscopy are required to observe them. The following table summarizes the size ranges and the appropriate methods for visualization:
Structure | Approximate Size | Visualization Method |
|---|---|---|
Human height | ~2 m | Unaided eye |
Chicken egg | ~4 cm | Unaided eye |
Most plant and animal cells | 10–100 μm | Light microscope (LM) |
Nucleus, most bacteria, mitochondrion | 1–10 μm | Light microscope (LM) |
Smallest bacteria | ~0.1 μm | Electron microscope (EM) |
Viruses | ~100 nm | Electron microscope (EM) |
Ribosomes | ~20 nm | Electron microscope (EM) |
Proteins, lipids, small molecules, atoms | 1 nm and below | Electron microscope (EM), super-resolution microscopy |
Summary of Microscopy Techniques
Light Microscopy (LM): Suitable for most cells and larger organelles; limited by resolution (~200 nm).
Electron Microscopy (EM): Used for much smaller structures such as viruses, ribosomes, and molecules; much higher resolution than LM.
Example: Application of Microscopy
Studying cell structure: Light microscopes are used to observe living cells and tissues, while electron microscopes are used to study detailed structures such as organelles and macromolecules.
