Structures and Functions in Living Organisms · 6 question types
Past paper frequency (2018 to 2024)
This topic accounts for approximately 18% of your exam marks.
Cell structure and organelle function appear on nearly every paper; one of the highest-frequency topics.
Most cells are far too small to see with the naked eye. A human red blood cell is only about 8 μm across; a bacterium is around 2 μm. A microscope uses lenses (or a beam of electrons) to magnify the image so structures invisibly small can be seen.
| Light microscope | Electron microscope | |
|---|---|---|
| Source used to see the sample | Visible light through glass lenses | A beam of electrons through magnetic lenses |
| Maximum useful magnification | About ×1500 | Up to ×500 000 or more |
| Resolution (smallest detail visible) | About 200 nm | Down to 0.2 nm |
| Living specimens? | Yes, can view living cells in colour | No, the sample must be in a vacuum and is killed |
| Cost and size | Cheap and benchtop-sized | Very expensive, room-sized |
| What you can see | Cell wall, cell membrane, nucleus, chloroplasts, large vacuole | Also small organelles such as ribosomes and mitochondria in detail |
A light microscope is what you use in a school biology lab. An electron microscope is needed to see the smallest organelles (ribosomes, the internal structure of mitochondria, individual viruses).
The relationship between the actual size of an object, the size of its image, and the magnification of the microscope is:
magnification = image size ÷ actual size
Rearranged forms:
actual size = image size ÷ magnification
image size = actual size × magnification
Always make sure both sizes are in the same unit before dividing. Useful conversions:
1 mm = 1000 μm
1 μm = 1000 nm
1 mm = 1 000 000 nm
Example — A student looks at an onion cell down a microscope. The cell measures 12 mm long in the eyepiece image. The cell's actual length is 0.06 mm. What is the magnification?
Example — A bacterium is photographed under an electron microscope. The image shows the cell at 50 mm long, and the magnification is ×25 000. What is the bacterium's actual length in micrometres?
Example — An animal cell is 20 μm wide. What size will it appear in an image at a magnification of ×400?