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
Question 5
Paper 1A style
HL only (what this means)
HL only: additional Higher Level content, only for HL students. SL students can skip it. What the labels meanWhich adaptation increases the surface area of the membrane of proximal convoluted tubule cells that faces the filtrate?
- Biconcave shape
- Microvilli
- Invagination of the basal membrane
- Lamellar bodies
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Answer: B – microvilli form a brush border facing the filtrate; invagination is on the basal side, facing the blood [1]
Question 6
Paper 2A style
HL only (what this means)
HL only: additional Higher Level content, only for HL students. SL students can skip it. What the labels meanCompare and contrast the adaptations of type I and type II pneumocytes.
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- both are cells in the wall / epithelium of the alveolus [1]
- type I are extremely thin / flattened, whereas type II are thicker / rounded [1]
- type I give a short diffusion distance for O₂ and CO₂ [1]
- type II contain many secretory vesicles / lamellar bodies [1]
- type II discharge surfactant into the alveolar lumen (by exocytosis) [1]
- type I cover most (about 95%) of the alveolar surface, whereas type II cover only a small part of it [1]
- Do not accept 'type II cells are fewer in number'
- At least one similarity and one difference are needed for full marks
- max 4
Question 7
Paper 2B style
HL only (what this means)
HL only: additional Higher Level content, only for HL students. SL students can skip it. What the labels meanCompare cardiac muscle cells with striated muscle fibres, suggest reasons for the differences, and discuss whether a striated muscle fibre should be considered a cell.
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- both contain contractile myofibrils / are striated [1]
- cardiac cells are branched, whereas striated fibres are unbranched [1]
- cardiac cells are short, whereas striated fibres are very long [1]
- cardiac cells usually have one nucleus, whereas striated fibres have many nuclei [1]
- branching / intercalated discs let impulses spread quickly through the heart wall, so it contracts as a unit [1]
- many nuclei are needed to supply mRNA / proteins for a large volume of cytoplasm [1]
- for being a cell: one continuous plasma membrane surrounds the fibre [1]
- against: formed by fusion of many cells / multinucleate, so not a typical cell [1]
- conclusion with a reason, for example an atypical cell / exception to the cell theory [1]
- max 7
Question 8
Paper 2A style
HL only (what this means)
HL only: additional Higher Level content, only for HL students. SL students can skip it. What the labels meanExplain how human sperm and egg cells are adapted to their functions, giving two adaptations of each.
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- sperm: acrosome contains enzymes to digest a path through the zona pellucida [1]
- sperm: midpiece has many mitochondria to provide ATP for movement [1]
- sperm: flagellum for swimming [1]
- sperm: small / streamlined with little cytoplasm [1]
- egg: large, with cytoplasm / nutrients for the early embryo [1]
- egg: zona pellucida for sperm binding / protection [1]
- egg: cortical granules release contents (by exocytosis) to prevent polyspermy [1]
- Award [2] max for sperm and [2] max for egg
- max 4