Cell Theory and Microscopes
State the three parts of cell theory and explain how microscopy changed biology. Compare resolution, magnification, and scale for cells and organelles.
A lesson of Biology, in Cells and Cellular Processes, about 10 minutes.
By the end of this lesson, you will be able to state and explain all three parts of cell theory, compare light and electron microscopy, use magnification and scale, and explain how the development of the microscope transformed biology.
Cell Theory: The Foundation of Biology
The three parts of cell theory:
- All living things are composed of one or more cells.
- The cell is the basic structural and functional unit of life.
- All cells come from pre-existing cells (biogenesis).
Historical development: Hooke (1665) coined "cell"; van Leeuwenhoek observed microorganisms; Schleiden (plants) and Schwann (animals) proposed the first two tenets in 1838–39; Virchow added the third in 1855.
Types of Microscopes
Light (optical) microscope:
- Uses visible light and glass lenses
- Maximum useful magnification: ~1,000×
- Resolving power: ~200 nm (can distinguish structures 200 nm apart)
- Can observe live cells
- Most cell organelles visible (not ribosomes)
Transmission electron microscope (TEM):
- Uses electron beams; sample must be thin and dead
- Resolution: ~1 nm
- Shows internal ultrastructure (ribosomes, membrane details)
Scanning electron microscope (SEM):
- Scans surface with electrons
- 3D surface images; resolution ~10–20 nm
- Shows cell surface features
Magnification and Scale
$$\text{Magnification} = \frac{\text{image size}}{\text{actual size}}$$
$$\text{Actual size} = \frac{\text{image size}}{\text{magnification}}$$
Scale bars: always interpret with the scale bar provided, not assumed scale.
Common units:
- 1 mm = 1,000 μm = 10⁶ nm
- Typical animal cell: 10–20 μm
- Mitochondrion: 1–10 μm
- Ribosome: ~20 nm
- Virus: 20–300 nm
- Atom: ~0.1 nm
Example 1 — Scale Calculation
A cell image is 4 cm long under a ×1,000 microscope. What is the actual cell length?
$\text{Actual size} = 4\text{ cm}/1000 = 0.004\text{ cm} = 40\text{ μm}$
Why Microscopy Changed Biology
Before microscopes, scientists debated spontaneous generation (life arising from nonliving matter). Pasteur's microscopy-supported experiments definitively disproved it: microorganisms from air seeded broth, not spontaneous generation.