1.3.3Additional lessons
Microscopes — IB Biology SL Revision Notes & Topical Questions
5 min read 6 parts Lesson 2 of 142
Part 1 of 6Magnification and Resolution
Magnification makes an image bigger, but only resolution makes extra detail visible.
Magnification and Resolution
Magnification
- How many times larger the image is than the real object, found using image size divided by actual size.
Resolution
- The ability to see two close objects as separate; a higher resolution shows more detail in the image.
Limit for light
- Light waves cannot separate objects closer than about half their wavelength, giving a limit of roughly 0.2 µm.
Why electrons are better
- Electrons have a much shorter wavelength than light, so electron microscopes can resolve far smaller structures.
Part 2 of 6Light Microscope
Image formation
- Light passes through a thin specimen and through lenses to form a coloured image.
Resolution
- About 0.2 µm, which is enough to see nuclei, and possibly mitochondria and chloroplasts.
Useful magnification
- Up to about ×1500, because enlarging further only blurs the image without adding detail.
Strength
- It is small and cheap, uses simple preparation and can be used to observe living cells.
Limit
- Ribosomes, lysosomes and the endoplasmic reticulum are too small to be seen with it.
Part 3 of 6Electron Microscopes
TEM
- Electromagnets focus a beam of electrons through a very thin section, and denser regions absorb more and look darker.
TEM detail
- It has the highest resolution and shows internal structures of cells and organelles such as ribosomes.
SEM
- A beam of electrons scans across the surface and reflected electrons are detected to build a three-dimensional image.
SEM limits
- It shows only surfaces and gives a lower resolution than a TEM, but it can examine thick specimens.
Shared limits
- The specimen is in a vacuum, so it must be dead, and the image is black and white.
| Feature | Light microscope | Electron microscope |
|---|---|---|
| Radiation used | Visible light | Beam of electrons |
| Approximate resolution | 0.2 µm | Below 1 nm |
| Living specimens | Possible | Not possible |
| Image colour | Colour | Black and white |
| Preparation | Simple | Long and complex, may create artefacts |
Part 4 of 6Artefacts and Developments
Artefacts and Developments
Artefacts
- Structures that appear in the image because of preparation, such as fixing and staining, but are not present in life.
Fluorescent stains
- Dyes that bind specific structures and glow under ultraviolet light, which lets particular molecules be located in cells.
Immunofluorescence
- Antibodies attached to fluorescent dyes bind to target molecules, so that molecules such as viral proteins can be detected.
Impact
- Each improvement in microscopes has revealed more about cell structure and has refined cell theory.
Part 5 of 6Worked Example
Question
- A TEM micrograph of a mitochondrion is 36 mm long and the stated magnification is ×12 000.
- Calculate the actual length in micrometres.
- Use the formulaactual size = image size ÷ magnification.
- Actual length = 36 mm ÷ 12 000 = 0.003 mm.
- Convert to micrometres0.003 mm × 1000 = 3 µm.
Answer The mitochondrion is 3 µm long.
Part 6 of 6Exam Checklist & Tips
1.3.3 – Key Points for the Exam
- Resolution The minimum distance at which two points are seen as separate.
- TEM Transmission electron microscope, which passes electrons through a thin section.
- SEM Scanning electron microscope, which shows surface structure in three dimensions.
- Artefact A feature of an image that results from preparation and is not in the living cell.
- Micrograph A photograph or digital image taken through a microscope.
Common Mistakes
- Using magnification and resolution as if they mean the same;
- a blurred image can be large but show little detail.
- Saying that electron microscopes can examine living cells; the vacuum and preparation kill the specimen.
- Thinking that an SEM shows internal structure; it shows the surface, while a TEM shows the interior.
- Mixing units, such as comparing 0.2 µm with 0.2 nm without converting them.
Last updated:
Cambridge 9700 AS Biology
Core Formula
Magnification Triangle & Unit Conversion
Formula: M = I / A ⇔ A = I / M ⇔ I = M × A
Units: 1 mm = 1,000 μm = 1,000,000 nm.
Rule: Convert Image size (ruler mm) to μm by ×1,000 before dividing by Actual size.