The key idea
Animal, plant and bacterial cells contain different structures.In exams, link each structure to its function rather than listing names.
magnification = image size / real size
The bit that matters
Learn the process in clean chunks. If a sentence explains a cause, make sure you can say the effect too.
Eukaryotic and prokaryotic cells
All cells are either eukaryotic or prokaryotic.Eukaryotic cells (animal and plant cells) have a true nucleus that encloses their DNA.Prokaryotic cells (bacteria) are much smaller and have no nucleus — their DNA floats freely in the cytoplasm as a single loop, sometimes with extra small rings called plasmids.
Animal cell structures
Animal cells contain a nucleus (controls the cell and holds DNA), cytoplasm (where reactions happen), a cell membrane (controls what enters and leaves), mitochondria (site of aerobic respiration) and ribosomes (site of protein synthesis).In the exam, always pair each structure with its function.
Plant cell structures
Plant cells have everything an animal cell has, plus three extra structures: a cell wall made of cellulose (provides strength and support), chloroplasts (contain chlorophyll and are the site of photosynthesis) and a permanent vacuole (filled with cell sap to keep the cell turgid).
Magnification and microscopy
Magnification = image size ÷ real size. Always convert both measurements to the same unit first (1 mm = 1000 µm).Light microscopes magnify up to about 2000x; electron microscopes have far higher magnification and resolution, revealing sub-cellular structures like ribosomes.
Definitions to learn
Eukaryotic cell
A cell with a nucleus enclosing its DNA (animal and plant cells).
Prokaryotic cell
A cell with no nucleus; DNA is a single loop in the cytoplasm (bacteria).
Magnification
How many times larger an image is than the real object.
Resolution
The smallest distance between two points that can still be seen as separate.
Organelle
A specialised structure inside a cell with a specific function.
A cell image is 24 mm wide. The real cell is 0.06 mm wide. Find the magnification.
Convert both measurements to the same unit. 24 mm = 24 mm and 0.06 mm is already in mm, so no conversion needed here.
Write the equation: magnification = image size ÷ real size.
Substitute: magnification = 24 ÷ 0.06.
Calculate: 24 ÷ 0.06 = 400.
Magnification = 400x (no units — magnification is a ratio).
Always convert units before applying magnification = image size ÷ real size. Mixing mm and µm gives the wrong answer.
Convert units before calculating. Mixing mm and micrometres gives the wrong answer.
How to score full marks
- 1Always link each structure to its function — naming the structure alone rarely scores full marks.
- 2Convert mm and µm to the same unit before any magnification calculation. 1 mm = 1000 µm.
- 3Magnification has no units — it is a ratio, so never write 'x cm' or 'x mm' after it.
Test yourself
Pick an answer — you'll see instantly if it's right.
Which structure is found in plant cells but NOT animal cells?
Magnification = 500×. Image size = 5 mm. What is the real size?
Which organelle is the site of aerobic respiration?
Why do electron microscopes reveal more detail than light microscopes?
What does a prokaryotic cell lack that a eukaryotic cell has?
Try these yourself
Open each answer only after you have explained the full biological process.
1Name two structures found in plant cells but not animal cells.[2 marks]
- 1.Recall the structures that support photosynthesis and rigidity.
2State one function of the cell membrane.[1 mark]
- 1.Think about what enters and leaves the cell.
3Describe the difference between a eukaryotic and a prokaryotic cell.[2 marks]
- 1.Compare where the genetic material is found.
4Explain why sperm cells contain many mitochondria.[2 marks]
- 1.Mitochondria release energy by aerobic respiration.
- 2.Link the energy to the cell's job.
5An image is 15 mm long at 300x magnification. Find the real length.[2 marks]
- 1.Rearrange magnification = image ÷ real to make real the subject.
- 2.Substitute and divide.
6A bacterial cell is 2 µm long. An image of it is 8 mm long. Calculate the magnification.[2 marks]
- 1.Convert both to the same unit first.
- 2.8 mm = 8000 µm.
- 3.Use magnification = image ÷ real.
7Describe how to use a light microscope to observe onion cells.[3 marks]
- 1.Order the steps from slide preparation to focusing.
8Explain why an electron microscope allows scientists to see sub-cellular structures that a light microscope cannot.[2 marks]
- 1.Compare resolution and magnification of the two instruments.