Cell structure and microscopy
Every living thing is made of cells. You need to know the parts of animal, plant and bacterial cells, what each part does, and how to use the magnification equation with the right units.
Part 1 of 3: Learn it
In short
- Eukaryotic cells (animal and plant) have a nucleus; prokaryotic cells (bacteria) do not.
- Magnification = size of image ÷ size of real object.
- 1 mm = 1000 µm and 1 µm = 1000 nm: convert both sizes to the same unit first.
Where this is in your specification
Spec points: AQA 4.1.1.1 to 4.1.1.5 (required practical 1), Edexcel Topic 1, OCR B1.1
| Board | Topic: Cell biology |
|---|---|
| AQA 8461 | 4.1 (8461); 4.1 (8464) |
| Edexcel 1BI0 | Topics 1 and 2 |
| OCR J247 | B1 and B2 |
Parts of a cell
| Part | Job | Found in |
|---|---|---|
| Nucleus | contains the genetic material and controls the cell | animal and plant cells |
| Cytoplasm | where most chemical reactions happen | all cells |
| Cell membrane | controls what enters and leaves | all cells |
| Mitochondria | where aerobic respiration happens | animal and plant cells |
| Ribosomes | where proteins are made | all cells |
| Cell wall | strengthens the cell (cellulose in plants) | plant cells and bacteria |
| Chloroplasts | absorb light for photosynthesis | plant cells (green parts) |
| Permanent vacuole | filled with cell sap; keeps the cell firm | plant cells |
Bacterial cells
Bacteria are prokaryotes. They are much smaller than animal and plant cells and have no nucleus: their genetic material is a single loop of DNA in the cytoplasm, and many also have small rings of DNA called plasmids. They have a cell wall (not made of cellulose), a cell membrane, cytoplasm and ribosomes, but no mitochondria or chloroplasts.
Magnification
Magnification has no units. Before dividing, put both sizes in the same unit. To go from mm to µm multiply by 1000; from µm to nm multiply by 1000 again.
Light and electron microscopes
A light microscope can show cells and nuclei and can look at living cells. An electron microscope has a much higher magnification and resolution (it can tell apart two points that are very close together), so it shows mitochondria, ribosomes and other small structures in detail. It allowed scientists to see sub-cellular structures for the first time.
Name two structures found in plant cells but not animal cells.
Show the answer
Any two of: cell wall, chloroplasts, permanent vacuole.
Part 2 of 3: See it worked
Worked examples
Example 1
In a drawing, a cell is 25 mm across. The real cell is 50 µm across. What is the magnification?
- Change the image size to µm: 25 mm × 1000 = 25 000 µm
- magnification = 25 000 ÷ 50
- = × 500
Answer: × 500.
Example 2
A mitochondrion appears 12 mm long in a micrograph at × 6000. How long is it really, in µm?
- real size = image size ÷ magnification
- = 12 ÷ 6000 = 0.002 mm
- 0.002 × 1000 = 2 µm
Answer: 2 µm.
Common mistakes
- Dividing sizes in different units.
- Giving magnification a unit such as mm.
- Saying bacteria have a nucleus.
- Saying all plant cells have chloroplasts. Root cells, for example, do not.
Convert 0.5 mm to µm.
Show the answer
500 µm.
Part 3 of 3: Test yourself
Check yourself
Answer each one in your head or on paper first, then open it to check.
Name two structures found in plant cells but not animal cells.
Any two of: cell wall, chloroplasts, permanent vacuole.
Convert 0.5 mm to µm.
500 µm.
Why can an electron microscope show ribosomes when a light microscope cannot?
It has a much higher resolution (and magnification).
Jobs that use this
- Biomedical scientist (se abre en otra pestaña)
- GP (se abre en otra pestaña)
- Laboratory technician (se abre en otra pestaña)
Each link opens the job profile on the National Careers Service (England). In the rest of the UK: My World of Work (Scotland), Careers Wales, nidirect careers (Northern Ireland).
Estos apuntes están en inglés porque siguen los programas de examen del Reino Unido.
Full lessons and marked practice for this course are coming soon to Brainlag Learn. See courses