Magnification and scale of cells
Magnification and scale of cells
Cells are extremely small but knowing the sizes of objects viewed under
the microscope can be very useful. For example, a plant scientist might
want to compare the relative sizes of pollen grains from plants in the same
genus to help identify diff erent species.
Magni cation is defi ned as the ratio of the size of the image to the
size of the object:
magnification = size of image /size of object
With a compound microscope, the magnification is the product of both
lenses, so if a microscope has a ×10 eyepiece and ×40 objective, the total
magnification is ×400.
Printed images of structures seen with a microscope usually show a
scale bar or give the magnification, so that the size of an object can be
calculated. For example, the magnification of the micrograph in Figure
2.5 is given as ×165.
Many cells are roughly spherical in shape. The volume of a sphere is 4/3 πr3 and its surface area is 4πr2. Make a table similar to Table 2.1, this time for a sphere. Describe the relationship between surface area and volume in this case.
Cells that can be seen by the unaided eye (without a microscope) are typically large single cells or specialized cells in multicellular organisms. Here are some notable examples:
👁️ Cells Visible to the Naked Eye
1. Egg Cells (Ova)
- Human egg cell: ~0.1 mm in diameter — just at the limit of visibility.
- Bird egg (e.g., chicken egg): The entire yolk is a single cell — one of the largest known cells.
2. Algal Cells
- Acetabularia (mermaid’s wineglass): A single-celled green alga up to 10 cm long.
- Caulerpa: Another giant unicellular alga with leaf-like structures.
3. Some Protozoa
- Stentor: Trumpet-shaped protozoan that can reach up to 2 mm.
- Amoeba proteus: Can grow up to 0.5 mm — visible as tiny specks.
4. Plant Cells (in bulk)
- While individual plant cells are microscopic, onion epidermal layers or banana pulp cells can sometimes be seen as translucent sheets or dots.
📌 Why Most Cells Are Invisible
- Most cells range from 1–100 µm, far below the resolution of the human eye (~100 µm).
- Specialized large cells (like eggs or algae) are exceptions due to their unique functions or adaptations.
Would you like a diagram showing these large cells compared to typical microscopic cells for better visualization?
Cell size
One of the few cells large enough to be visible to the unaided eye is the mature human ovum, which has a diameter of approximately 150 μm. However, most cells are much smaller than this, and can only be seen using a microscope. Light microscopes, which can magnify up to 1000 times, reveal some internal structures such as the nucleus, but greater detail requires the use of more powerful microscopes such as the electron microscope, which magnifies up to 500 000 times.
Viruses can only be seen in the electron microscope, so the structure of viruses was unknown until the invention of electron microscopes in the 20th century. Even the electron microscope cannot distinguish individual molecules. Other techniques such as X-ray crystallography are needed to do this. Figure 2.3 indicates the relative sizes of some biological structures.
Surface area to volume ratio
Cells are very small, no matter what the size of the organism that they are part of. Cells do not and cannot grow to be very large and this is important in the way living organisms are built and function. The volume of a cell determines the level of metabolic activity that takes place within it. The surface area of a cell determines the rate of exchange of materials with the outside environment. As the volume of a cell increases, so does its surface area, but not in the same proportion, as Table 2.1 (page 16) shows for a theoretical cube-shaped cell.
SI units – International System
1 metre (m) = 1 m
1 millimetre (mm) = 10– 3 m
1 micrometre (μm) = 10– 6 m
1 nanometre (nm) = 10–9 m
1 centimetre cubed = 1 cm3
1 decimetre cubed = 1 dm3
1 second = 1 s
1 minute = 1 min
1 hour = 1 h
concentration is measured in mol dm−3
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