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ocr A Level Biology A:H420/03 Unified biology-june 2024 Question paper
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ocr A Level Biology A:H420/03 Unified biology-june 2024 Question paper
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ocrWednesday 19 June 2024 – Morning
A Level Biology A
H420/03 Unified biology
Time allowed: 1 hour 30 minutes
You can use:
a scientific or graphical calculator
a ruler (cm/mm)
Please write clearly in black ink. Do not write in the barcodes.
Centre number Candidate number
First name(s)
Last name
INSTRUCTIONS
• Use black ink. You can use an HB pencil, but only for graphs and diagrams.
• Write your answer to each question in the space provided. If you need extra space use
the lined pages at the end of this booklet. The question numbers must be clearly shown.
• Answer all the questions.
• Where appropriate, your answer should be supported with working. Marks might be
given for using a correct method, even if your answer is wrong.
INFORMATION
• The total mark for this paper is 70.
• The marks for each question are shown in brackets [ ].
• Quality of extended response will be assessed in questions marked with an asterisk (*).
• This document has 24 pages.
ADVICE
• Read each question carefully before you start your answer.
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, Turn over
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, 2
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.
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