MicroscopyEdexcel International A Level Biology: Subtopic test

1. 1
   A scientist photographs a human liver cell using a transmission electron microscope (TEM). The micrograph is printed at a magnification of ×15 000. On the micrograph the diameter of the cell measures 300 mm and the length of one mitochondrion measures 45 mm.

   (a)

   What is the actual diameter of the cell?

   [1 mark]

   • A2 µm
   • B20 µm
   • C200 µm
   • D4.5 × 10⁶ mm

   (b)

   Which statement about a transmission electron microscope is correct?

   [1 mark]

   • ALiving specimens can be viewed in air to give a colour image
   • BIt produces a three-dimensional image of the surface of the specimen
   • CIt has a lower resolution than a light microscope because electrons have a longer wavelength
   • DVery thin sections of a dead, stained specimen are viewed in a vacuum to give a two-dimensional image

   (c)

   Calculate the actual length of the mitochondrion. Give your answer in micrometres (µm).

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student prepares temporary mounts of onion epidermis and of potato tissue to view with a light microscope. In an unstained mount the cells are almost transparent and few structures can be seen.

   (a)

   Which stain would be most suitable for showing starch grains in the potato cells?

   [1 mark]

   • AEosin
   • BMethylene blue
   • CIodine in potassium iodide solution
   • DSudan III

   (b)

   The student increases the magnification beyond the useful limit of the microscope and finds that the image is larger but blurred. Why?

   [1 mark]

   • AThe resolution limit of the light microscope has been reached, so a bigger image shows no extra detail
   • BThe stain has faded under the brighter light
   • CThe wavelength of light becomes longer as magnification is increased
   • DThe resolution has increased but the contrast has been lost

   (c)

   Explain why a stain is added to the onion epidermis before it is viewed.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student calibrates an eyepiece graticule using a stage micrometer, which is a slide marked in divisions of 10 µm. With the ×40 objective lens, 80 divisions on the eyepiece graticule line up exactly with 40 divisions on the stage micrometer. The student then measures the length of a stained onion epidermis cell as 14 graticule divisions.

   (a)

   Calculate the actual length of the onion cell. Show your working.

   [3 marks]

   (b)

   The student now changes to the ×10 objective lens. Describe how the student should obtain a reliable mean cell length with this lens.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A school biology department plans to use light microscopes to observe living yeast cells budding and to make scale drawings of stained plant root-tip cells, measuring them with an eyepiece graticule. A university laboratory wants to study the cristae of mitochondria, which are about 20 nm apart, using an electron microscope.

   (a)

   Evaluate the suitability of a light microscope and a transmission electron microscope for these investigations.

   [6 marks]

   (b)

   Describe how the students should produce a scale drawing of the stained root-tip cells and use the graticule to find the actual size of one cell.

   [6 marks]

   Total for question 4: 12 marks

End of questions