BackA Tour of the Cell: Structure, Function, and Microscopy
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Chapter 4: A Tour of the Cell
Overview: The Fundamental Units of Life
All living organisms are composed of cells, which are the basic units of structure and function in biology. Despite the diversity of cell types, all cells share certain fundamental features.
Cell: The simplest collection of matter that can be alive.
All cells are related by their descent from earlier cells.
Cells can differ substantially from one another but share common features.
Microscopy and the Study of Cells
Concept 4.1: Biologists Use Microscopes and the Tools of Biochemistry to Study Cells
Cells are generally too small to be seen by the unaided eye. Microscopes are essential tools for observing cells and their structures.
Light Microscope (LM): Uses visible light passed through a specimen and glass lenses to magnify images of cells.
Lenses refract (bend) light, magnifying the image.
Images produced by microscopes are called micrographs.
Parameters of Microscopy
Magnification: The ratio of an object's image size to its real size.
Resolution: The measure of image clarity; the minimum distance between two distinguishable points.
Contrast: The difference in brightness between the light and dark areas of an image.
Light microscopes can magnify up to about 1,000 times the size of the specimen.
Staining or labeling samples increases contrast and enables visualization of cell components.
Subcellular structures, such as organelles, are often too small to be resolved by light microscopy.
Advances in Light Microscopy
Fluorescent markers improve visualization of specific molecules or structures.
Confocal and other advanced techniques provide sharper images of tissues and cells.
Super-resolution microscopy allows visualization of structures as small as 10–20 μm.
Cryo-electron microscopy (cryo-TEM): Preserves specimens at extremely low temperatures, allowing visualization of protein structures in their cellular environment.
Types of Light Microscopy
Bright field: Involves staining; used to observe dead cells.
Simple phase contrast: Does not require staining; used to observe live cells.
Differential interference contrast (Nomarski): No staining; observes live cells with higher resolution than simple phase contrast.
Fluorescence microscopy: Uses fluorescent markers to visualize specific cell components.
Confocal microscopy: Provides optical sectioning for clearer images of thick specimens.
Size Range of Cells
Cells and their components vary greatly in size, from large nerve cells to small organelles and molecules.
Structure | Approximate Size | Microscopy Type |
|---|---|---|
Human height | ~1.7 m | Unaided eye |
Chicken egg | ~4 cm | Unaided eye |
Most plant and animal cells | 10–100 μm | Light microscopy |
Nucleus | ~5–10 μm | Light microscopy |
Mitochondrion | ~1–2 μm | Light microscopy |
Smallest bacteria | ~0.2 μm | Super-resolution microscopy |
Viruses | ~50–100 nm | Electron microscopy |
Ribosomes | ~20–30 nm | Electron microscopy |
Proteins | ~5–10 nm | Electron microscopy |
Types of Electron Microscopy
Scanning Electron Microscope (SEM): Focuses a beam of electrons onto the surface of a specimen, producing 3D images.
Transmission Electron Microscope (TEM): Focuses a beam of electrons through a specimen to reveal internal structures.
Electron microscopes use electron beams and magnetic lenses, requiring a vacuum and providing higher resolution than light microscopes.
Cell Fractionation
Cell fractionation is a technique that breaks up cells and separates their components using centrifugation. This allows scientists to study the function of organelles and correlate cell structure with function.
Components separate based on size and density.
Combines biochemistry (study of metabolism) and cytology (study of cell structure).
Example: Application of Microscopy
Fluorescence microscopy is used to visualize the location of specific proteins within cells by tagging them with fluorescent dyes.
Electron microscopy is essential for studying the detailed structure of organelles such as mitochondria and ribosomes.
