Exam (elaborations)
OCR A Level Biology A May-June 2024 QuestionPaper 3 (H420-03)
OCR A Level Biology A May-June 2024 QuestionPaper 3 (H420-03)
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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, estimate 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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