Microscopy and cell fractionationAQA A-Level Biology: Subtopic test

1. 1
   A student examines a transmission electron micrograph of a liver cell. On the micrograph one mitochondrion is 45 mm long, and the magnification of the micrograph is ×30 000. The same magnification is later used to photograph a chloroplast from a leaf cell, which is 6 μm long in life.

   (a)

   What is the actual length of the mitochondrion?

   [1 mark]

   • A0.15 μm
   • B15 μm
   • C1.5 μm
   • D150 μm

   (b)

   Which statement correctly describes resolution?

   [1 mark]

   • AThe number of times larger an image is than the specimen
   • BThe size of the image on the micrograph
   • CThe ability to make the image larger without limit
   • DThe ability to distinguish between two points that are close together as separate points

   (c)

   Calculate the length of the image of the chloroplast on the micrograph at the same magnification. Give your answer in mm.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student measures the length of an onion epidermal cell using an optical microscope fitted with an eyepiece graticule. The student places a stage micrometer on the stage. Ten divisions on the stage micrometer, which total 100 μm, line up exactly with 40 divisions on the eyepiece graticule. The onion cell is then measured as 18 eyepiece divisions long, using the same lenses.

   (a)

   What is the actual length represented by one division of the eyepiece graticule at this magnification?

   [1 mark]

   • A2.5 μm
   • B4.0 μm
   • C0.4 μm
   • D250 μm

   (b)

   What is the actual length of the onion cell?

   [1 mark]

   • A7.2 μm
   • B45 μm
   • C450 μm
   • D4.5 μm

   (c)

   The student changes to an objective lens of higher power. Explain why the eyepiece graticule must be calibrated again before further measurements are made.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A research team wants to study three specimens: the surface of a pollen grain, the internal structure of a mitochondrion, and living single-celled algae in pond water.

   (a)

   For each specimen, name the most suitable type of microscope and give a reason for your choice.

   [3 marks]

   (b)

   Explain why a transmission electron microscope can show more detail than an optical microscope, and describe two limitations of its use.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Cell fractionation is used to isolate organelles from tissue so that their functions can be studied. When transmission electron microscopes were first used in the 1940s and 1950s, scientists saw structures in cells that were later shown not to be real organelles, and there was a considerable period before the scientific community could tell artefacts from organelles.

   (a)

   Describe how cell fractionation and ultracentrifugation could be used to isolate mitochondria from a sample of liver tissue.

   [6 marks]

   (b)

   Explain what is meant by an artefact, why artefacts can appear in electron micrographs, and how scientists could decide whether a structure is a real organelle or an artefact.

   [6 marks]

   Total for question 4: 12 marks

End of questions