Question 1
Paper 1A style
Which human cell normally has the largest diameter?
- Red blood cell
- Lymphocyte
- Egg cell
- Head of a sperm cell
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Answer: C – the egg is about 110 µm across; red blood cells are about 7–8 µm and a sperm head about 5 µm [1]
Question 2
Paper 1B style
Agar cubes containing an indicator were placed in acid. The indicator changes colour as acid diffuses into the agar. After 4 minutes the percentage of each cube's volume that had changed colour was estimated. The data are for practice. (a) Calculate the surface area-to-volume ratio of the 2 cm cube. [1] (b) Describe the relationship between SA:V and the percentage of volume that changed colour. [1] (c) Explain what these results suggest about the constraints on cell size. [2] (d) State one way in which agar cubes are a simplified model of cells. [1]
| Side length / cm | Surface area / cm² | Volume / cm³ | SA:V | Volume changed colour after 4 min / % |
|---|---|---|---|---|
| 1 | 6 | 1 | 6 : 1 | 94 |
| 2 | 24 | 8 | ? | 66 |
| 3 | 54 | 27 | 2 : 1 | 49 |
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- (a) 24 ÷ 8 = 3 : 1 / 3 cm⁻¹ [1]
- (b) the higher the SA:V, the greater the percentage of volume that changed colour / positive correlation [1]
- (c) exchange depends on surface area but the need for exchange depends on volume [1]
- (c) large cells have a smaller SA:V, so materials cannot be exchanged / reach the centre fast enough [1]
- (c) so cells must stay small / divide before they become too large [1]
- (c) max 2
- (d) any one: cells are not cubes / agar is not living and has no metabolism / agar has no plasma membrane or membrane proteins [1]
Question 3
Paper 2A style
(a) State the two properties of stem cells. [2] (b) Outline the role of the stem cell niche in adult bone marrow. [2]
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- (a) can divide endlessly / repeatedly [1]
- (a) can differentiate along different pathways / into different cell types [1]
- (b) bone marrow contains stem cells that produce blood cells [1]
- (b) the niche maintains the stem cells (undifferentiated) [1]
- (b) the niche promotes proliferation and differentiation when more blood cells are needed [1]
- (b) max 2
Question 4
Paper 2A style
Distinguish between totipotent, pluripotent and multipotent stem cells, giving an example of each.
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- totipotent: can form any cell type, including placenta / a whole organism, for example the zygote / very early embryo cells [1]
- pluripotent: can form any body cell but not a whole organism, for example embryonic stem cells [1]
- multipotent: can form a limited range of related cell types, for example bone marrow stem cells [1]
- An example is needed for each mark