Year 7 Science · Unit 1 · Lesson 8
Challenge Worksheet
Learning Goals
1. What is an organelle? Give two examples found in an animal cell, with the job of each.
Find the mistakes
A student wrote this answer
“Animal cells are basically the same as plant cells except they don’t have chloroplasts. They have a cell wall instead of a cell membrane to protect the cell, and both types have a large vacuole for water storage. Animal cells are more complex because they have more organelles.”
2. The answer contains four scientific errors. Quote each error and write a correct version of it.
Cause and effect: what if?
Imagine a white blood cell suddenly grew a thick cellulose cell wall. Complete the chain. Each box explains what happens next.
Read the data
Scenario: counting powerhouses
Table 1 shows roughly how many mitochondria are found in four kinds of human cell, and what each cell does.
| Cell | What it does | Mitochondria per cell (about) |
|---|---|---|
| Liver cell | Builds and breaks down many chemicals | 1500 |
| Mature red blood cell | Carries oxygen around the whole body for about four months | 0 |
| Heart muscle cell | Contracts about once a second, for life | 5000 |
| Fat-storage cell | Holds a large store of the body’s fuel | 50 |
Source: rounded values for this worksheet; the fat-cell figure is made up to show the pattern.
3. Explain why the heart muscle cell has the most mitochondria in Table 1.
4. Jack says: “A cell’s mitochondria count shows how busy it is. The red blood cell travels round the body non-stop, so Table 1 must be wrong.” Use the fat-storage cell and the red blood cell to evaluate Jack’s idea. What does the number of mitochondria really tell you?
5. A science article claims: “Animal cells are less advanced than plant cells because they are missing key structures like the cell wall and chloroplasts.” Do you agree? Justify your answer using the idea that structure fits function.
Question 1
A specialised part inside a cell that has its own job (1). Any two, 1 each with a correct job: nucleus, control centre that holds the DNA; mitochondria, release energy from food; small vacuoles, store tiny amounts of substances. [3 marks]
Find the mistakes
Question 2 (1 each): “the same except chloroplasts”: animal cells also lack a wall and a large central vacuole. “A cell wall instead of a cell membrane”: animal cells have a membrane and no wall; plant cells have both. “Both types have a large vacuole”: only plant cells do; animal cells have small vacuoles, if any. “More organelles”: animal cells lack three parts plant cells have, so they have fewer. [4 marks]
Cause and effect: what if?
Box 2: the cell is locked into one rigid shape and can no longer bend or change shape (1). Box 3: it cannot squeeze out through a gap in the blood-vessel wall or wrap itself around the bacterium (1). [2 marks]
Question 3
Heart muscle contracts non-stop, which needs a lot of energy (1). Mitochondria release energy from glucose, so a cell with high energy needs has many of them (1). [2 marks]
Question 4
Jack is wrong (1). Holding fuel costs the fat cell almost no energy, so it has few (1). The red blood cell has none because its oxygen is for the rest of the body; burning it would use up its own delivery (1). The count shows how much energy the cell needs for itself, not how busy it looks (1). [4 marks]
Question 5
Marking guide: 1 position (disagree); 1 animals eat food, so chloroplasts would be useless; 1 mitochondria release energy from that food; 1 no wall lets cells change shape, with an example; 1 wall and chloroplasts suit a plant’s life; 1 “structure fits function”: judge a cell by how well its parts suit its job, not by how many it has. [6 marks]
Model answer: I disagree. Animals eat food, so chloroplasts would be useless; mitochondria release the food’s energy. No wall is an advantage: a white blood cell must squeeze between cells and wrap around bacteria, which no cell in a rigid box can do. Plants stand still and make food, so walls and chloroplasts suit them. Structure fits function, so each cell suits its own life.
Common error: treating “fewer parts” as “less advanced”. The missing parts match things an animal does not need to do.