1. 1
   Researchers compared mitochondria isolated from yeast cells with free-living aerobic bacteria. Mitochondria are about 1 µm long, are surrounded by two membranes, contain their own small circular DNA molecules that are not associated with histones, and contain ribosomes that sediment as 70S particles. New mitochondria arise only by the division of existing mitochondria. When the antibiotic chloramphenicol, which binds to 70S ribosomes, was added to the yeast, protein synthesis inside the mitochondria stopped, but protein synthesis on the ribosomes in the yeast cytoplasm continued normally.

   (a)

   Which observation provides evidence that mitochondria evolved by endosymbiosis?

   [1 mark]

   • AMitochondria are the site of aerobic respiration
   • BMitochondria contain naked circular DNA and 70S ribosomes
   • CMost mitochondrial proteins are coded for by genes in the nucleus
   • DMitochondria are larger than ribosomes

   (b)

   Which sequence of events is proposed by the theory of the origin of eukaryotic cells?

   [1 mark]

   • AChloroplasts were gained first; then a nucleus formed; mitochondria were gained last
   • BMitochondria and chloroplasts were gained at the same time by all eukaryotes before a nucleus evolved
   • CEach group of eukaryotes evolved from a different prokaryote without any endosymbiosis
   • DA unicellular ancestor with a nucleus and sexual reproduction evolved; mitochondria were then gained by endosymbiosis; chloroplasts were later gained in some lineages

   (c)

   Explain how the effect of chloramphenicol supports the endosymbiotic origin of mitochondria.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A transmission electron micrograph of a palisade mesophyll cell from a leaf is printed at a magnification of ×12 000. On the print, one chloroplast is 60 mm long and one mitochondrion is 18 mm long. The cell has a thick outer wall, a large central vacuole and a nucleus.

   (a)

   What is the actual length of the chloroplast?

   [1 mark]

   • A200 µm
   • B50 µm
   • C5 µm
   • D0.5 µm

   (b)

   Which structure in this cell would NOT be present in an animal cell?

   [1 mark]

   • ACellulose cell wall
   • B80S ribosomes
   • CGolgi apparatus
   • DMitochondria

   (c)

   A scale bar representing 2 µm is to be added to the micrograph. Calculate the length of the scale bar on the print, giving the unit.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Volvocine green algae live in fresh water. Chlamydomonas reinhardtii is unicellular, about 10 µm across, and each cell both swims and reproduces. Gonium forms colonies of about 16 similar cells, each of which can divide to form a new colony. Volvox carteri forms a hollow sphere of about 2000 small flagellated somatic cells, which cannot divide, and about 16 large reproductive cells, which do not swim. All three have very similar sets of genes. In Volvox, a gene called regA is switched on in somatic cells only. Evidence from DNA suggests this group became multicellular about 200 million years ago, independently of animals, plants and fungi.

   (a)

   Explain how the somatic and reproductive cells of Volvox can be so different even though they contain the same genes.

   [3 marks]

   (b)

   Using the data, discuss the advantages and disadvantages of multicellularity in Volvox compared with Chlamydomonas.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Paramecium is a unicellular eukaryote found in ponds. Its surface is covered in beating cilia. It feeds on bacteria, which are enclosed in food vacuoles and digested by enzymes from lysosomes. Water entering by osmosis is expelled by contractile vacuoles. When it bumps into an obstacle it reverses the beating of its cilia and swims away. It usually reproduces by dividing in two. One species, Paramecium bursaria, contains several hundred cells of the green alga Chlorella in its cytoplasm. The algae photosynthesise and pass sugars to their host, and the host can be grown without the algae if it is fed.

   (a)

   Explain how Paramecium carries out the functions of life within a single cell.

   [6 marks]

   (b)

   Discuss the extent to which the relationship between Paramecium bursaria and Chlorella supports the theory that chloroplasts have an endosymbiotic origin.

   [6 marks]

   Total for question 4: 12 marks

End of questions