Year 8 Science · Unit 1 · Lesson 1
Challenge Worksheet
Learning Goals
Find the errors
This paragraph contains exactly three errors. Underline each one and write the correction below.
A student’s summary
Cells are the basic unit of life. Similar cells group together to form organs. Different tissues work together to make an organ, such as the heart. The heart is the circulatory system, because it moves blood. Plants have cells and tissues, but they do not have organs.
1. Write the three corrections.
| Cell | What is measured | Size (µm) |
|---|---|---|
| Bacterium (a whole organism) | length | 2 |
| Human red blood cell | diameter | 8 |
| Human muscle cell (fibre) | width | 50 |
| Human egg cell | diameter | 100 |
Source: typical textbook values. Muscle cells are long, so only their width is given; many are several centimetres long.
2. How many times wider is the egg cell than the bacterium is long?
Answer times
3. How many red blood cells, lined up edge to edge, would stretch across 1 mm? Show the unit conversion.
Answer cells
4. A student reads Table 1 and says: “The egg cell is the biggest, so it must be the most complex level of organisation.” Explain why size does not decide the level.
Evaluate the claim
Someone claims
“A cell is just a tiny version of an organism. It takes in food, uses energy and gets rid of waste, so there is no real difference. If you made a cell big enough, it would be the same as a whole organism.”
5. Identify one part of the claim that is supported by science. Use an example from Table 1.
6. A red blood cell cannot survive for long outside the body. Use this to explain what the claim leaves out about the cells in a multicellular organism.
7. An amoeba is a single-celled organism. A kangaroo is a multicellular organism with trillions of cells. Compare how the two are organised, and explain what the kangaroo can do because it has tissues, organs and organ systems that an amoeba cannot.
Find the errors
1. (i) Similar cells form tissues, not organs. (ii) The heart is an organ and one part of the circulatory system, not the whole system. (iii) Plants do have organs: roots, stems and leaves. [3 marks: 1 per correction]
Your Turn
2. 100 ÷ 2 = 50 times. [1 mark]
3. 1 mm = 1000 µm; 1000 ÷ 8 = 125 red blood cells. [2 marks: 1 conversion, 1 answer]
4. The egg cell is still only one cell, so it is at the cell level. The level depends on what a structure is made of (one cell, similar cells, different tissues, or several organs), not on how big it is. [2 marks]
Evaluate the claim
5. A single cell can carry out life processes on its own: the bacterium is a whole organism made of one cell. [1 mark]
6. In a multicellular organism, cells are specialised for one job (a red blood cell carries oxygen and has no nucleus). It cannot get its own food or remove its own waste, so it depends on other cells, tissues and organ systems (for example the circulatory system). The claim ignores that these cells only survive as part of the organised levels above them. [3 marks: specialised; depends on other levels; named example]
Show What You Know
7. Marking guide: 1 for the levels (amoeba: one cell only; kangaroo: cells, tissues, organs, organ systems); 1 for specialised cells that depend on each other; 2 for two named kangaroo examples with jobs (for example the heart and circulatory system move blood around a large body; leg muscles and the skeleton let it hop); 1 for an advantage stated as a comparison (the kangaroo can grow large and do many jobs at once, which one cell cannot); 1 for a cost (damage to one organ can harm the whole organism, and it needs far more food). [6 marks]
Common error to watch for: saying the amoeba is “not organised”. It is highly organised inside its one cell; it simply has no tissues or organs.