The light microscope and magnification
Cells are far too small to see with the naked eye, so we use a light microscope. Light shines up through a thin sample on a glass slide, then passes through two lenses: the objective lens (close to the sample) and the eyepiece lens (the one you look down). Each lens bends the light so the image looks bigger.
The total magnification is just the two lens powers multiplied together. If the eyepiece is 10x and the objective is 40x, you are seeing the sample at 400 times its real size.
- Magnification
- How many times bigger the image looks compared with the real object. It has no units, it is just a number of "times".
Rearrange it like any formula: cover the part you want. So actual size = image size ÷ magnification, and image size = actual size × magnification. Keep both sizes in the same units before you divide, or the answer will be wildly out.
A cell in a photo measures 30 mm across. The photo was taken at a magnification of 600. What is the real width of the cell?
Units matter
1 mm = 1000 µm (micrometres). Most cells are measured in micrometres, so learn that jump. Getting the unit wrong is the most common way to lose an easy mark here.
Prokaryotic vs eukaryotic cells
All cells fall into two big families. The difference is simple: where is the DNA kept?
Prokaryotic cells, such as bacteria, are small and have no true nucleus. Their DNA floats loose in the cytoplasm as a single ring, plus small extra rings called plasmids. Eukaryotic cells, such as plant, animal and fungal cells, are larger and keep their DNA neatly packaged inside a membrane-bound nucleus.
| Feature | Prokaryotic (bacteria) | Eukaryotic (plant, animal) |
|---|---|---|
| Nucleus | No, DNA is loose in the cytoplasm | Yes, DNA held in a nucleus |
| Size | Small (about 1 to 5 µm) | Larger (about 10 to 100 µm) |
| Cell wall | Yes, but not made of cellulose | Only plant and fungal cells have one |
| Plasmids | Often present | Absent |
Plant vs animal cells and organelles
An organelle is a tiny structure inside a cell that does a specific job, like a department in a factory. Animal and plant cells share the basic ones, but plant cells carry extra kit for making food and staying rigid.
| Organelle | What it does | Animal | Plant |
|---|---|---|---|
| Nucleus | Holds DNA and controls the cell's activities | Yes | Yes |
| Cytoplasm | Jelly where most chemical reactions happen | Yes | Yes |
| Cell membrane | Controls what enters and leaves the cell | Yes | Yes |
| Mitochondria | Release energy from glucose in respiration | Yes | Yes |
| Ribosomes | Build proteins | Yes | Yes |
| Cell wall | Made of cellulose, gives strength and shape | No | Yes |
| Chloroplast | Contains chlorophyll, does photosynthesis | No | Yes |
| Vacuole | Large sap-filled space that keeps the cell firm | No (or tiny) | Yes, permanent |
Where this is assessed
Naming organelles and their jobs is Criterion A (knowing and understanding). Using a microscope to observe cells safely and record what you see feeds Criterion B (inquiring and designing) and Criterion C (processing and evaluating).
Specialised cells and structure fits function
Most cells are not generic blobs. They are specialised, meaning their shape and contents are tuned for one job. The golden rule of biology shows up again and again: structure fits function.
| Cell | Special feature | Why it helps |
|---|---|---|
| Red blood cell | No nucleus, biconcave (dented) disc shape, packed with haemoglobin | More room and surface for oxygen; the shape squeezes through narrow vessels |
| Root hair cell | Long, thin extension from the surface | Huge surface area to absorb water and minerals from soil |
| Nerve cell (neurone) | Very long and thin with branched ends | Carries electrical signals fast over long distances around the body |
| Sperm cell | A tail (flagellum) and many mitochondria | The tail swims; the mitochondria release energy for the journey to the egg |
| Egg cell | Large with a big food store in the cytoplasm | Feeds the early embryo after fertilisation |
When an exam asks you to "explain" a specialised cell, always link the feature to the job in one sentence: "It has X, which lets it do Y." That link is what earns the mark, not just naming the feature.
Check yourself
1. A microscope has a 10x eyepiece and a 40x objective. What is the total magnification? +
Multiply the two lens powers: 10 × 40 = 400x.
2. Name two structures a plant cell has that an animal cell does not. +
Any two of: a cellulose cell wall, chloroplasts, and a permanent (large) vacuole. Animal cells have none of these.
3. Why does a red blood cell have no nucleus? +
Losing the nucleus frees up space so the cell can hold more haemoglobin and therefore carry more oxygen. This is a clear case of structure fitting function.
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