1. 1
   A student makes a temporary mount of cheek cells, stains them with methylene blue and views them with a light microscope at a total magnification of ×400. She has calibrated her eyepiece graticule with a stage micrometer: at this magnification one eyepiece unit equals 2.5 µm. One cheek cell measures 24 eyepiece units across and its nucleus measures 3 eyepiece units. She then photographs the same cell, and on the printed photograph the cell is 48 mm across.

   (a)

   What is the actual diameter of the cheek cell?

   [1 mark]

   • A9.6 µm
   • B60 µm
   • C600 µm
   • D6.0 µm

   (b)

   What is the magnification of the cell in the printed photograph?

   [1 mark]

   • A×80
   • B×0.8
   • C×800
   • D×8000

   (c)

   Explain why the eyepiece graticule must be recalibrated if the student changes to a ×100 objective lens.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Microbiologists sampling a hot spring find a previously unknown organism. Under the light microscope it appears as rod-shaped units about 2 µm long. It stains purple with the Gram stain, showing that it has a thick cell wall. Electron micrographs show a plasma membrane, cytoplasm with many ribosomes, and a region of cytoplasm containing a loop of DNA with no membrane around it. Ribosomes isolated from it sediment as 70S particles.

   (a)

   Which structure would NOT be expected in this organism?

   [1 mark]

   • APlasma membrane
   • B70S ribosomes
   • CCell wall
   • DMitochondrion

   (b)

   Before examining the organism, the microbiologists predicted that it would consist of one or more cells. What type of reasoning is this?

   [1 mark]

   • ADeductive reasoning from a theory
   • BInductive reasoning from repeated observations
   • CFalsification of a hypothesis
   • DQuantitative observation using an instrument

   (c)

   Explain why this organism, like all cells, must have DNA and a plasma membrane.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A biologist counts nuclei in four types of cell. A single skeletal muscle fibre from a leg muscle, 30 mm long, contains several hundred nuclei. Mature red blood cells in human blood contain no nucleus and survive for about 120 days. Mature phloem sieve tube elements in a stem contain no nucleus, but each is joined by many plasmodesmata to a companion cell that has one nucleus. Hyphae of the bread mould Rhizopus have no cross walls and contain many nuclei in one continuous cytoplasm.

   (a)

   Suggest how the absence of a nucleus is related to the functions of red blood cells and sieve tube elements.

   [3 marks]

   (b)

   A student claims that these observations disprove cell theory. Evaluate this claim.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A research team is comparing a Gram-positive bacterium, Staphylococcus, with a unicellular fungus, baker's yeast (Saccharomyces cerevisiae). They use a light microscope with a resolution of about 200 nm, including fluorescent stains and immunofluorescence with antibodies against a membrane protein. They also use transmission electron microscopy (resolution better than 1 nm), freeze-fracture electron microscopy and cryogenic electron microscopy.

   (a)

   Compare and contrast the cell structure of Staphylococcus and yeast.

   [6 marks]

   (b)

   Evaluate the contribution of the microscopy techniques used by the team to understanding cell structure.

   [6 marks]

   Total for question 4: 12 marks

End of questions