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A2.2 Cell size - written task

This structured extension question is in the style of the IB exam question on these Application of Skills. 

  • Use a light microscope and eye piece graticule.
  • Calculate measures of central tendency: mean, median and mode.
  • Use approximation and estimation.

  • Calculate scales of magnification.

It also requires an understanding of the methodology of collecting sufficient data with precision in a scientific way.

Johannes wanted to measure the length of the cells of the epidermis of a sunflower petal.

Read the account of this investigation to measure the size of sunflower petal epidermis cells (shown in the image) under the microscope and then answer the questions.

Method

  • He carefully peeled a small sample of the epidermis from one petal,
  • He placed the epidermis in a drop of distilled water on a microscope slide and covered it with a cover slip.
  • He found that x100 magnification allowed him to see approximately 4 cells across the centre of the field of view.
  • The image above shows this view through the microscope (right image).
  • He then counted how many cells fitted across the diameter of the field of view in thee different views of the epidermis.

Analysis

  • He calculated the mean number of cell lengths that could be seen along the diameter of the field of view.
    This was 3.6 cell lengths.
  • He replaced the slide with a ruler and measured the width across the centre of the field of view.
    The width was 0.4 mm.
  • He used this formula to calculate the actual length of the cells: \(size\ of \ cell = {width\ of\ field\ of \ view \over mean \ number \ of \ cells}\)
  • This is the calculation he carried out,  Size of a cell = 0.4 / 3.6 = 0.11 mm

 

What features of the epidermal cells mean that the calculation should be given as an estimate of size rather than an actual size. 

(2 marks)

3 lines
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Johannes expressed the estimate of the mean length of the cells as 0.11mm.
Deduce the mean length of the cells in μm. 

(1 mark)

1 line
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Suggest the effect that using distilled water as the mounting liquid could have had on the size of the cells. 

(1 marks)

3 lines
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How could Johannes have measured the field of view more accurately than using a ruler?

(1 marks)

3 lines
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Explain two improvements to the methodology, not the equipment, that would give a more accurate or more reliable estimate of the width of the cells. 

(2 marks)

3 lines
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Teachers note: One mark for each improvement with its explanation. Do not give a mark for "use a lower magnification" in part 4 as it would decrease the accuracy of measurement, even though it might increase the reliability of the mean calculation.

 

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