Year 7 Science · Unit 1 · Lesson 9
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Learning Goals
1. What does it mean to say that a cell is specialised?
Match each feature to how it helps
Write the letter of the correct explanation next to each feature. Each letter is used once.
| Feature | Letter | How it helps the cell |
|---|---|---|
| Biconcave shape (red blood cell) | A. Lets one cell carry a signal a long way without handing it on | |
| No nucleus (red blood cell) | B. Traps light energy for photosynthesis | |
| Very long axon (neuron) | C. Gives more surface area for oxygen to pass in and out, and lets the cell bend | |
| Midpiece packed with mitochondria (sperm cell) | D. Pokes between soil particles, giving a big area for taking in water | |
| Packed with chloroplasts (palisade cell) | E. Leaves more room for haemoglobin | |
| Long thin extension (root hair cell) | F. Releases energy to power the tail |
Cause and effect: the cost of losing a nucleus
Complete the chain. Each box is caused by the one before it, and row 2 carries on from the end of row 1.
2. The cells lining your small intestine have hundreds of tiny finger-like folds on the side that faces digested food. Predict the job of these cells and explain how the folds help them do it.
Read the data
Scenario: sickle-shaped red blood cells
In sickle cell disease, a change in haemoglobin makes many red blood cells stiff and curved like a crescent (a sickle) instead of flexible biconcave discs. Stiff sickle cells can get stuck in the narrowest blood vessels and block them.
| Property | Normal red blood cell | Sickle-shaped red blood cell |
|---|---|---|
| Shape | Biconcave disc | Crescent |
| Can it bend? | Yes, it is flexible | No, it is stiff |
| Nucleus | None | None |
| Typical lifespan (days) | About 120 | About 10 to 20 |
Source: approximate values from medical references.
3. A sickle cell survives for 15 days. How many times longer does a normal red blood cell last?
Answer times longer
4. Use Table 1 and the idea “structure fits function” to explain why a person with sickle cell disease can have pain in parts of the body and often feels tired.
5. A blood stem cell in the bone marrow is round, keeps its nucleus and has no haemoglobin. Jordan says: “A blood stem cell is just a badly built red blood cell.” Evaluate Jordan’s claim, and explain why a body cannot be made only of specialised cells.
Question 1
It has a special shape or features that suit one particular job (1). [1 mark]
Match each feature to how it helps
Biconcave shape (red blood cell) → C; No nucleus (red blood cell) → E; Very long axon (neuron) → A; Midpiece packed with mitochondria (sperm cell) → F; Packed with chloroplasts (palisade cell) → B; Long thin extension (root hair cell) → D.
Cause and effect: the cost of losing a nucleus
Row 1 blank: without a nucleus it cannot divide or repair itself. Row 2 blank: so it wears out and dies after about 120 days. “More room for haemoglobin” is true but does not lead on to row 2.
Question 2
They absorb digested food (nutrients) into the body (1). The folds give a much larger surface area, so more food is taken in, and faster (1). [2 marks]
Question 3
120 ÷ 15 = 8 times longer (1). [1 mark]
Question 4
The stiff crescent cannot bend through narrow vessels as a biconcave disc can (1). Blocked vessels starve the tissue beyond of oxygen, causing pain (1). Sickle cells die after 10 to 20 days, not about 120, so there are fewer red blood cells, less oxygen is carried, and the person tires (1). [3 marks]
Question 5
Marking guide: 1 position (Jordan is wrong); 1 the stem cell is unspecialised, and that is its job; 1 its nucleus lets it keep dividing; 1 its daughter cells can become red blood cells, white blood cells or platelet-making cells; 1 the red blood cell gave up its nucleus for haemoglobin, so it cannot repair itself or divide and lives only about 120 days; 1 specialising has a cost, so a body needs unspecialised cells to replace the specialists. [6 marks]
Model answer: Jordan is wrong. The stem cell is not a failed red blood cell; it is unspecialised on purpose. Keeping its nucleus means it can keep dividing, and its daughter cells can become red blood cells, white blood cells or platelet-making cells. A red blood cell threw its nucleus away to make room for haemoglobin, so it cannot repair itself and dies after about 120 days. The body needs both.
Common error: treating specialisation as all gain. Ask students what the cell has given up.