DP Biology (last assessment 2024)

Question 18M.3.SL.TZ2.8

Date	May 2018	Marks available	[Maximum mark: 10]	Reference code	18M.3.SL.TZ2.8
Level	SL	Paper	3	Time zone	TZ2
Command term	Describe, Distinguish, List, Outline, Predict	Question number	8	Adapted from	N/A

[Maximum mark: 10]

18M.3.SL.TZ2.8

Vinegar is a water-based solution of ethanoic acid. Acetobacter aceti is used in the production of vinegar. This bacterium has the ability to convert ethyl alcohol, C₂H₅OH, into ethanoic acid, CH₃COOH.

(a.i)

A. aceti is a Gram-negative bacterium. If a Gram staining procedure were carried out on a sample of A. aceti, predict the result that would be observed after decolourization.

[1]

Markscheme

the bacteria would be clear/colourless

Examiners report

(a.ii)

A. aceti is a Gram-negative bacterium. If a Gram staining procedure were carried out on a sample of A. aceti, predict the result that would be observed after counterstaining.

[1]

Markscheme

would stain pink

Do not accept “violet” or “lilac”.

Examiners report

In the generator method of ethanoic acid fermentation, Acetobacter aceti are grown over wood shavings in a fermenter to the point where they form a biofilm.

At the base of the fermenter oxygen is bubbled in, which then rises through the wood shavings. Ethanol drips through the wood shavings. At a suitable temperature the ethanol is converted to ethanoic acid, which is withdrawn from the bottom of the fermenter, and new ethanol is added at the top.

(b.i)

List two abiotic variables that would need to be monitored during this fermentation process.

[2]

Markscheme

a. oxygen

b. temperature

c. pH levels

d. CO₂

Examiners report

(b.ii)

Describe one way in which microorganisms in this fermenter could be limited by their own activities.

[2]