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# Why are cells different shapes?
Year 8 Biology · From cells to organs
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## Today
**We are learning to** almost all your cells have the same DNA, but they specialise: each develops a shape and contents that suit its job, like a red blood cell packed with haemoglobin or a nerve cell with a long axon.
- I can describe two features of a specialised cell and explain how each helps.
- I can explain that cells with the same DNA can specialise for different jobs.
- I can link a cell's contents (such as mitochondria) to its job.
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## Why it matters
> A nerve cell in your leg can be a metre long. A red blood cell is a tiny squashed disc with no nucleus. Same DNA, same body. Why so different?
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💬 Discussion
## Engage: predict
**A nerve cell and a muscle cell from the same person. How do you predict their DNA compares?**
- The same DNA, but each cell uses different parts of it
- Only nerve cells have DNA
- Completely different DNA
- Muscle cells have more DNA
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✍️ Activity
## Explore it
{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fbiology-8%2Fspecialised%2Fstep-2&code=widget%3A+cell%0Ashow%3A+redblood%2C+nerve%0A&fresh=1}}
[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/biology-8/specialised/?full=step-2#step-2)
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✅ Check
## What happened?
* The same DNA. Each cell switches on the instructions it needs, so it grows the shape and parts that suit its job.
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✅ Check
## Feature and function
**Match each feature to how it helps.**
| | |
|---|---|
| `Red blood cell: no nucleus` | A. Carries signals a long way |
| `Nerve cell: long axon` | B. Large surface for absorbing water |
| `Muscle cell: many mitochondria` | C. Releases lots of energy for contracting |
| `Root hair cell: long extension` | D. More room for haemoglobin to carry oxygen |
* Answer: Red blood cell: no nucleus → D. More room for haemoglobin to carry oxygen; Nerve cell: long axon → A. Carries signals a long way; Muscle cell: many mitochondria → C. Releases lots of energy for contracting; Root hair cell: long extension → B. Large surface for absorbing water
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✅ Check
## Root hairs
**Why do root hair cells have no chloroplasts?**
- They are underground, where there is no light for photosynthesis
- Roots take in their food from the soil, so they don't need to make any
- Only leaves are allowed chloroplasts
* Answer: They are underground, where there is no light for photosynthesis
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✅ Check
## Specialised
**What does 'specialised' mean for a cell?**
- It has different DNA
- Its shape and parts suit one particular job
- It is bigger than other cells
* Answer: Its shape and parts suit one particular job
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✅ Check
## Elaborate: design a cell
**Cells lining your small intestine absorb digested food. Which feature would you expect them to have?**
- A long tail to swim
- Tiny folds (microvilli) on the surface, for a much larger area
- No membrane, so food gets in easily
* Answer: Tiny folds (microvilli) on the surface, for a much larger area
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✅ Check
## Elaborate: energy-hungry cells
**Sperm cells and muscle cells both have many mitochondria. What do the two jobs have in common?**
- Both make proteins
- Both store food
- Both need lots of energy for movement
* Answer: Both need lots of energy for movement
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## Evaluate: look back
Write one sentence: "At first I thought… Now I think… because…"
**Revision summary**
- Nearly all your cells have the **same DNA**; each **specialised** cell uses different instructions.
- A specialised cell's **shape and parts suit its job**: red blood cells (no nucleus, haemoglobin), nerve cells (long axon), muscle cells (many mitochondria), root hair cells (long extension, no chloroplasts).
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✅ Check
## Exit ticket
**Give one feature of a nerve cell and explain how it helps.**
* A long axon, so it can carry signals a long way through the body.