Lesson focus: Cell structures form coordinated systems, and microscopy provides scaled evidence rather than decorative pictures.
Relate organelleA tiny part inside a cell that does a particular job, such as the nucleus. structure to function, compare plant, animal and bacterial cells, and calculate magnificationHow many times bigger an image is than the real object. from biological images.
Build on earlier learning
Observing cellsYear 7 introduced cell structures. This bridge lesson adds ribosomes, bacterial comparison and quantitative microscopy.
Practical
Observing cells
- Peel a thin layer of onion epidermis and lay it flat on a slide.
- Add a drop of iodine to stain the nuclei.
- Lower a coverslip and view at low then high power.
Record: A labelled drawing with the magnification.
You will be successful when you can…
- Identify major cell structures.
- Compare plant, animal and bacterial cells.
- Explain organelle functions precisely.
- Use magnification = image size ÷ actual size.
Words that carry the science
- organelle
- A specialised structure within a cell that performs a particular function.
- ribosome
- A small cellular structure where proteins are synthesised.
- mitochondrion
- An organelle where most aerobic respiration reactions occur.
- cell-surface membrane
- A selectively permeable boundary controlling movement between cell and surroundings.
- magnification
- Image size divided by actual size, expressed without units.
- resolution
- The ability to distinguish two close points as separate.
Organelles divide cellular work
01Organelles divide cellular work
The nucleusThe part of a cell that contains the chromosomes, made of DNA, which carry the genes that control the cell's activities. contains most genetic material and influences cell activities. ribosomeA tiny part of a cell where proteins are made. synthesise proteins. mitochondrionA tiny part of a cell where aerobic respiration happens, releasing energy from glucose. are sites of most aerobic respiration reactions, while cytoplasm contains many other enzyme-controlled reactions.
The cell-surface membraneThe partially permeable layer around a cell that controls which substances enter and leave. controls exchange. Plant cells additionally have a cellulose wall for support, a permanent vacuole containing cell sap, and chloroplasts in photosynthetic cells. Not every plant cell contains chloroplasts.
02Bacterial cells use a different organisation
Bacteria have cytoplasm, a membrane, ribosomeA tiny part of a cell where proteins are made. and usually a wall, but no nucleus or membrane-bound mitochondrionA tiny part of a cell where aerobic respiration happens, releasing energy from glucose.. Their main chromosome is circular DNA in the cytoplasm; smaller DNA rings called plasmidA small ring of DNA found in bacteria, separate from the main DNA. may also occur.
Bacterial ribosomes still make proteins and their membranes support important reactions. Lacking mitochondria does not mean bacteria cannot respire.
03Microscopy links images to real size
Total light-microscope magnificationHow many times bigger an image is than the real object. is eyepiece magnification multiplied by objective magnification. For a measured image, magnification = image size ÷ actual size, after converting both measurements to the same unit.
Magnification makes an image larger; resolutionHow well a microscope can show two things that are very close together as separate. determines whether close detail can be distinguished. A large blurry image has high magnification but poor useful detail.
Cells and their organelles
Cells are built from parts with different jobs. Plant and animal cells are eukaryotic: their DNA is inside a nucleus. Bacteria are prokaryotic: they are much smaller and have no nucleus. Play to tour each part, or pick one to compare the three cells.
The highlighted part is outlined in red. Sizes are typical: real cells vary.
Controls
| Part | Animal | Plant | Bacterium |
|---|---|---|---|
| Nucleus | Yes | Yes | No |
| Cytoplasm | Yes | Yes | Yes |
| Cell membrane | Yes | Yes | Yes |
| Mitochondria | Yes | Yes | No |
| Ribosomes | Yes | Yes | Yes |
| Cell wall | No | Yes | Yes |
| Chloroplasts | No | Yes | No |
| Vacuole | No | Yes | No |
| DNA loop | No | No | Yes |
| Plasmids | No | No | Yes |
Calculate an actual cell size
A cell image is 48 mm long at ×1200 magnification.
- Write actual size = image size ÷ magnification.
- Substitute: 48 mm ÷ 1200 = 0.040 mm.
- Convert millimetres to micrometres: 0.040 × 1000 = 40 µm.
- Check that the unit and scale are realistic for a eukaryotic cell.
How to make the reasoning visible: The actual cell length is 40 µm. Magnification has no unit because it is a ratio of like measurements.
Microscopy investigation
Use these image measurements to identify cells and audit the scale calculations.
| Image | Image size / mm | Magnification | Actual size / µm |
|---|---|---|---|
| cheek cell | 54 | ×900 | 60 |
| onion cell | 72 | ×600 | 120 |
| bacterium | 20 | ×10,000 | 2 |
| palisade cell | 45 | ×750 | 60 |
Show how the bacterium value is obtained.
Why would adding more magnification not necessarily reveal ribosomes?
Check your understanding
Answer from memory, then check against the suggested answer.
Where are proteins synthesised?
Which organelle contains most eukaryotic genetic material?
What is the magnification equation?
A root hair cell has mitochondria but no chloroplasts. Explain both observations.
Why is “mitochondria make energy” an incorrect statement?
Exam Question Practice
Exam Question Practice
Mark points shown after marking
Palisade mesophyll cells carry out most of the photosynthesis in a leaf.
Explain how a palisade cell is adapted to absorb light.
(2)Explain how the cell exchanges substances with the rest of the leaf.
(3)Total for this question: 5 marks
Command word: For explainA command word: say how or why something happens, giving reasons with words like "because" or "so"., link every named structure to a specific process or advantage.