GCSE Biology Cell Biology Notes
Cells are the building blocks of life. Knowing each organelle's job and how to compare animal, plant and bacterial cells is the foundation of biology.
Key concepts
- Eukaryotic cell
- A cell with a nucleus and membrane-bound organelles — animal and plant cells are eukaryotic.
- Prokaryotic cell
- A smaller cell with no nucleus; DNA floats freely and there may be plasmids — bacteria are prokaryotic.
- Organelle
- A specialised structure inside a cell, e.g. mitochondria (respiration) or ribosomes (protein synthesis).
- Magnification
- How many times larger an image is than the real object: magnification = image size ÷ real size.
- All cells have a cell membrane, cytoplasm and ribosomes.
- Plant cells additionally have a cell wall, chloroplasts and a permanent vacuole.
- Mitochondria are the site of aerobic respiration, releasing energy.
Key formulas
Magnification
magnification = image size ÷ real size
Rearrange to find real size or image size; keep units consistent.
Required practicals
- Use a light microscope to observe, draw and label cells, calculating magnification.
Worked example
A cell is 50 µm wide. Under the microscope the image is 10 mm wide. Find the magnification.
- 1Convert to the same units: 10 mm = 10 000 µm.
- 2Use magnification = image size ÷ real size.
- 3Substitute: 10 000 ÷ 50.
- 4Calculate: magnification = ×200.
Answer: ×200
Common mistakes
✗ Forgetting to convert units before using the magnification formula.
✓ Convert both measurements to the same unit (1 mm = 1000 µm) first.
✗ Saying all plant cells have chloroplasts.
✓ Only cells that photosynthesise (e.g. leaf cells) have chloroplasts; root cells do not.
✗ Confusing the cell wall with the cell membrane.
✓ The cell wall (cellulose) is rigid and only in plants; the membrane controls what enters and leaves all cells.
Quick check
Try these, then reveal the answers.
Which organelle is the site of respiration?Reveal answer
The mitochondria.
Name two structures found in plant cells but not animal cells.Reveal answer
Cell wall and chloroplasts (also a permanent vacuole).
What is the magnification if the image is 30 mm and the real size is 0.1 mm?Reveal answer
×300
Exam tips
- Command words: 'name' needs one word, 'describe' needs detail, 'compare' needs similarities and differences.
- Always show unit conversions in magnification calculations to secure method marks.
- Learn organelles as 'structure → function' pairs so you can answer either way round.
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Frequently asked questions
What is the difference between a eukaryotic and a prokaryotic cell?+
Eukaryotic cells have a nucleus and membrane-bound organelles; prokaryotic cells (bacteria) have no nucleus and their DNA floats freely in the cytoplasm.
How do you calculate magnification?+
Magnification = image size ÷ real size, with both measurements in the same units.
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