1.8 Culturing microorganisms
7 exam questions worth 18 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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A bacterium divides every 30 minutes.
How many bacteria will there be after 2 hours, starting from one bacterium?
Choose one answer.
Correct Not quite: the answer is 16
Mark scheme
C (16)
Model answer
C: 16. 2 hours is 4 divisions, and 24 = 16.
Explain why Petri dishes and agar are sterilised before they are used.
Mark scheme
- to kill (unwanted) microorganisms (1)
- so that the culture is not contaminated / only the bacteria you want grow (1)
Model answer
Sterilising kills any microorganisms already on the dish and agar, so the culture is not contaminated by other microorganisms.
An inoculating loop is passed through a flame before it is used to transfer bacteria.
Explain why.
Mark scheme
- to sterilise it / kill microorganisms on it (1)
- so that unwanted microorganisms are not added to the culture (1)
Model answer
The flame kills any microorganisms on the loop, so unwanted microorganisms do not get into the culture.
In a school laboratory, cultures are incubated at 25 °C and not at 37 °C. The lid of the dish is taped on loosely and not sealed all the way round.
Explain why each of these is done.
Mark scheme
- at 37 °C, pathogens (that are harmful to humans) would grow best / be more likely to grow (1)
- at 25 °C they are less likely to grow, so it is safer (1)
- a loose lid lets oxygen in, so that harmful anaerobic bacteria do not grow (1)
Model answer
Human pathogens grow best near body temperature, 37 °C, so a lower temperature of 25 °C makes it less likely that harmful bacteria will grow. A loosely taped lid lets oxygen in, which stops harmful anaerobic bacteria growing.
A bacterium divides every 20 minutes.
Calculate the number of bacteria after 2 hours, starting from one bacterium.
Mark scheme
- 120 ÷ 20 = 6 divisions (1)
- 26 (1)
- = 64 (1)
- allow 64 for 3 marks with no working
Model answer
2 hours is 120 minutes. 120 ÷ 20 = 6 divisions, so the number is 26 = 64 bacteria.
A bacterial colony on an agar plate is circular, with a diameter of 12 mm.
Calculate the area of the colony. Use π = 3.14.
Mark scheme
- radius = 6 (mm) (1)
- area = 3.14 × 62 (1)
- = 113 (mm2) (1) (allow 113.04)
Model answer
radius = 12 ÷ 2 = 6 mm. area = πr2 = 3.14 × 62 = 113 mm2.
Describe how aseptic technique is used to produce an uncontaminated culture of bacteria on an agar plate.
Mark scheme
Any four from:
- sterilise the Petri dish and agar (in an autoclave) (1)
- sterilise the inoculating loop in a Bunsen flame (1)
- only open the lid a little / work near a flame (1)
- tape the lid on (1)
- store the dish upside down (1)
- incubate at 25 °C (1)
Model answer
The Petri dish and agar are sterilised first, in an autoclave. The inoculating loop is sterilised in a Bunsen flame and cooled, and the lid is opened only a little to add the bacteria. The lid is then taped on and the dish is turned upside down and incubated at 25 °C.
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