Exam (elaborations)
ACTUAL OCR 2024 OCR A Level Biology A May-June 2024 Question Paper 3 WITH MARK SCHEME
ACTUAL OCR 2024 OCR A Level Biology A May-June 2024 Question Paper 3 WITH MARK SCHEME
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1 Escherichia coli is a bacterium that is used widely in scientific experiments and in biotechnology.
(a ) This is a transmission electron micrograph of E. coli.
A
(i) Name the structure labelled A.
A .................................................................................................................................................. [ 1 ]
(ii) Based on your knowledge, e s t i m a t e the diameter of the E. coli cell.
Give your answer in µm.
Diameter = .................................................. µm [ 1 ]
(b ) A student carries out a serial dilution of an E. coli culture.
This is the method the student uses:
• Transfer 10 cm3 of E. coli culture to a sterilised test tube from an original culture that has a
volume of 50 cm3.
• Carry out four 10-fold serial dilutions. Each dilution involves transferring 1 cm3 of culture from
one test tube to another test tube containing 9 cm3 of distilled water.
• Transfer 1 cm3 of the final 10 cm3 diluted culture to an agar plate. Evenly spread 1 cm3 of
liquid across the plate using a sterilised spreader.
• Use a micropipette for each transfer.
• Incubate the agar plates for 24 h.
• Count the number of colonies that develop on the plate. Each colony is assumed to develop
from a single bacterium.
• Estimate the E. coli population in the original 50 cm3 culture.
© OCR 2024
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(i) Describe t w o improvements to the student’s method, ot h er t h a n using different equipment, that
could improve the accuracy of their population estimate.
1 .......................................................................................................................................................
..........................................................................................................................................................
2 .......................................................................................................................................................
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[2 ]
(ii) The student counts 22 colonies on the agar plate.
Calculate the total number of E. coli cells in the original 50 cm3 culture.
Give your answer in standard form.
Total number of E. coli cells in 50 cm3 = ......................................................... [ 3 ]
(c ) E. coli can be genetically modified to produce useful proteins.
State t w o other reasons why E. coli is a suitable microorganism to use in biotechnology.
1 .......................................................................................................................................................
..........................................................................................................................................................
2 .......................................................................................................................................................
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[2 ]
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