Question 1
Paper 1A style
Which cell normally has no nucleus when it is mature?
- Skeletal muscle fibre
- Aseptate fungal hypha
- Phloem sieve tube element
- Phloem companion cell
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Answer: C [1]
Question 2
Paper 1A style
Which structure in Paramecium is mainly involved in homeostasis?
- Cilia
- Contractile vacuole
- Food vacuole
- Oral groove
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Answer: B [1]
Question 3
Paper 1B style
A student made a temporary mount of human cheek cells stained with methylene blue. At ×400, 40 eyepiece units lined up exactly with 10 divisions of a stage micrometer. Each stage micrometer division is 10 µm. The student then measured the diameter of five cells. The data are for practice. (a) Calculate the length of one eyepiece unit at ×400. [1] (b) Calculate the mean diameter of the cells in µm. Show your working. [2] (c) Explain why the graticule must be calibrated again if the student changes to a total magnification of ×100. [1] (d) Suggest why the student measured five cells rather than one. [1]
| Cell | Diameter / eyepiece units |
|---|---|
| 1 | 22 |
| 2 | 25 |
| 3 | 24 |
| 4 | 26 |
| 5 | 23 |
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- (a) 100 µm ÷ 40 = 2.5 µm [1]
- (b) mean = (22 + 25 + 24 + 26 + 23) ÷ 5 = 24 eyepiece units [1]
- (b) 24 × 2.5 = 60 µm [1] ECF from (a)
- (c) the eyepiece scale does not change size but the specimen is magnified less, so one eyepiece unit represents a different (larger) length of specimen OWTTE [1] Accept 10 µm at ×100
- (d) cells vary in size, so a mean is more representative / reduces the effect of one unusual cell / random error [1]
Question 4
Paper 2A style
An electron micrograph of a mitochondrion has a scale bar 20 mm long labelled 0.5 µm. The mitochondrion is 60 mm long on the micrograph. (a) Calculate the magnification of the micrograph. [2] (b) Calculate the actual length of the mitochondrion in µm. [1]
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- (a) 20 mm = 20 000 µm [1]
- (a) 20 000 ÷ 0.5 = ×40 000 [1]
- (b) 60 000 µm ÷ 40 000 = 1.5 µm [1] ECF from (a)
Question 5
Paper 2A style
Distinguish between the structure of a Gram-positive bacterium, such as Staphylococcus, and an animal cell. [4]
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- bacterium has no nucleus / DNA in a nucleoid region, whereas the animal cell has a nucleus with a double membrane and pores [1]
- bacterial DNA is naked / not bound to histones, whereas eukaryotic DNA is bound to histones [1]
- bacterial DNA is a single loop / circular, whereas eukaryotic chromosomes are linear [1]
- 70S ribosomes in the bacterium, 80S ribosomes in the animal cell cytoplasm [1]
- bacterium has a cell wall (of peptidoglycan); the animal cell has none [1]
- no membrane-bound organelles in the bacterium; the animal cell has mitochondria / ER / Golgi apparatus / lysosomes [1]
- bacterial cytoplasm is not compartmentalised [1]
- bacterium is smaller [1]
- max 4. Do not accept 'bacteria have no DNA'.
Question 6
Paper 2A style
Outline the advantages of cryogenic electron microscopy and of immunofluorescence for studying cells. [4]
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- cryo-EM: rapid freezing preserves molecules in their natural shape / without ice crystals [1]
- cryo-EM: many images are combined by computer to give a 3D structure [1]
- cryo-EM: near-atomic resolution / proteins need not be crystallised / used for membrane proteins or virus capsids [1]
- immunofluorescence: fluorescently tagged antibodies bind specific proteins / antigens [1]
- immunofluorescence: shows the location of a specific protein within the cell [1]
- immunofluorescence: several proteins can be labelled in different colours at once / used with a light microscope [1]
- max 4, with at least one point for each technique