Cells, viruses and reproduction of living thingsEdexcel A-Level Biology B: Topic test

20 questions, 54 marks

Edexcel A-Level Biology B

Cells, viruses and reproduction of living things topic test

Total 54 marks

Name

Class

Date

1
A student compares electron micrographs of a bacterium, Escherichia coli, and of a liver cell from a mouse. She is asked to identify the structures that distinguish the two types of cell.
(a)
Which row correctly states the size of the cytoplasmic ribosomes in each cell?
[1 mark]
ABoth cells have 70S ribosomes
BE. coli has 70S ribosomes and the liver cell has 80S ribosomes
CE. coli has 80S ribosomes and the liver cell has 70S ribosomes
DBoth cells have 80S ribosomes
(b)
Which structure would be seen in the cell of E. coli but not in the liver cell?
[1 mark]
ANucleolus
BLysosome
CNucleoid
DGolgi apparatus
(c)
Ribosomes can be seen in the electron micrographs but cannot be seen with a light microscope. Explain why.
[2 marks]
Total for question 1: 4 marks
2
Herpes simplex virus (HSV) causes cold sores. After the first infection, the virus can remain in nerve cells without producing new virus particles, and cold sores return when the virus becomes active again. The antiviral drug aciclovir stops the virus copying its DNA. Antibiotics have no effect on HSV.
(a)
Why are antibiotics ineffective against HSV?
[1 mark]
AViruses replicate inside host cells and lack the structures that antibiotics target
BViruses are too small for antibiotics to reach
CViruses have a thicker cell wall than bacteria
DViruses do not contain any nucleic acid
(b)
What is the term for the period during which the virus is present in nerve cells without producing new virus particles?
[1 mark]
AThe lytic cycle
BReplication
CTranscription
DLatency
(c)
Suggest why antiviral drugs that stop viral replication are more likely to harm the patient's own cells than antibiotics are.
[2 marks]
Total for question 2: 4 marks
3
Cells in the lining of the small intestine of a mouse divide constantly to replace cells that are lost. A student examines a stained section containing 250 of these cells under a light microscope and finds that 25 of them are in one of the stages of mitosis. The complete cell cycle for these cells takes 22 hours.
(a)
Calculate the mitotic index and use it to estimate the time these cells spend in mitosis.
[3 marks]
(b)
Describe what happens to the chromosomes from prophase to anaphase of mitosis, and explain why this is important in the intestinal lining.
[4 marks]
Total for question 3: 7 marks
4
Turner's syndrome is a condition in which a girl is born with only one X chromosome and 45 chromosomes in each body cell. Her ovaries do not develop normally and she is usually infertile.
(a)
Explain how an error during meiosis, followed by fertilisation, can lead to a baby with 45 chromosomes.
[6 marks]
(b)
Explain how meiosis and fertilisation in humans produce genetically different zygotes, and describe the events that follow fertilisation up to the blastocyst stage.
[6 marks]
Total for question 4: 12 marks
5
In maize (Zea mays), pollen grains from the anthers land on the long silks, which are the styles and stigmas of the female flowers. Each grain of maize is a seed that has developed from one ovule and has a large store of starch in its endosperm.
(a)
Which statement describes fertilisation that leads to the formation of the endosperm?
[1 mark]
AOne male nucleus fuses with the egg cell nucleus
BTwo male nuclei fuse with the egg cell nucleus
COne male nucleus fuses with the two polar nuclei
DOne male nucleus fuses with the tube nucleus
(b)
What is the role of the tube nucleus in a pollen grain?
[1 mark]
AIt controls the growth of the pollen tube
BIt fuses with the egg cell nucleus
CIt divides to form the two male gametes
DIt forms the triploid endosperm
(c)
Describe how the male nuclei reach the embryo sac of a maize flower after a pollen grain lands on a silk.
[2 marks]
Total for question 5: 4 marks
6
Sperm are made in the testes of a mammal, in tubes called seminiferous tubules. A mature sperm cell has an acrosome at its head, many mitochondria in its mid-piece and a long flagellum.
(a)
Which type of cell division produces secondary spermatocytes from primary spermatocytes?
[1 mark]
AMitosis
BMeiosis II
CCytokinesis without nuclear division
DMeiosis I
(b)
Which part of the sperm cell releases enzymes that digest the zona pellucida?
[1 mark]
AMid-piece
BAcrosome
CFlagellum
DNucleus
(c)
Explain why the mid-piece of a sperm cell contains many mitochondria.
[2 marks]
Total for question 6: 4 marks
7
The antibiotic vancomycin acts on the cell wall of bacteria by stopping the cross-linking of peptidoglycan. It is effective against Gram-positive bacteria but much less effective against Gram-negative bacteria. Electron micrographs of a Gram-negative bacterium were used in a study of its structure.
(a)
Explain why vancomycin is less effective against Gram-negative bacteria than against Gram-positive bacteria.
[3 marks]
(b)
In one micrograph the image of a bacterium is 45 mm long and the magnification is ×30 000. Calculate the actual length of the bacterium in µm. Then explain why specimens are stained before they are viewed.
[4 marks]
Total for question 7: 7 marks
8
Bacteriophage therapy is being trialled for patients with chest infections caused by Pseudomonas aeruginosa that is resistant to antibiotics. The phage used infects only this species of bacterium. In one trial the phage was given to a patient who had no other treatment option and whose condition was worsening.
(a)
Explain how the phage destroys a population of P. aeruginosa without harming the patient's own cells.
[6 marks]
(b)
Evaluate the use of bacteriophage therapy compared with antibiotics for treating bacterial infections, including the ethics of using it in the trial described.
[6 marks]
Total for question 8: 12 marks

End of questions

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).