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Biological molecules

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Question 20

A student investigated the activity of the enzyme polyphenol oxidase (PPO) in avocado tissue.

When avocado tissue is exposed to air, polyphenol oxidase catalyses a reaction between colourless phenolic compounds in avocado tissue and atmospheric oxygen to form dark quinones (melanin pigments), which turn the tissue brown.

phenolic compounds+oxygen→polyphenol oxidasedark pigment (melanin) \text{phenolic compounds} + \text{oxygen} \xrightarrow{\text{polyphenol oxidase}} \text{dark pigment (melanin)} phenolic compounds+oxygenpolyphenol oxidase​dark pigment (melanin)

The student measured the time taken for a standard dark brown discoloration to appear on the surface of avocado tissue from two different varieties (H\text{H}H and F\text{F}F).

Method

  1. Cut a 1.5 cm1.5\text{ cm}1.5 cm cube of avocado tissue from variety H\text{H}H.
  2. Place the cube on a clean watch glass and leave the watch glass at 22 ∘C22\text{ }^\circ\text{C}22 ∘C.
  3. Measure the time taken for the standard brown discoloration to appear across the surface.
  4. Repeat steps 1 to 3 two more times.
  5. Repeat steps 1 to 4 with avocado tissue from variety F\text{F}F.

The student obtained the following results but did not record them in a results table:

  • Variety H\text{H}H: 14 min 20 s14\text{ min } 20\text{ s}14 min 20 s, 15 min 50 s15\text{ min } 50\text{ s}15 min 50 s, 13 min 50 s13\text{ min } 50\text{ s}13 min 50 s
  • Variety F\text{F}F: 6 min 20 s6\text{ min } 20\text{ s}6 min 20 s, 7 min 40 s7\text{ min } 40\text{ s}7 min 40 s, 7 min 00 s7\text{ min } 00\text{ s}7 min 00 s
1.

Design a suitable results table in your answer, entering the results and their mean values calculated to 1 decimal place.

[3]
2.

Suggest and explain why the results for variety H\text{H}H are different from the results for variety F\text{F}F.

[2]
3.

The student repeated the investigation but made one change to the method used to prepare the avocado tissue, observing shorter times for the pigment to appear. Suggest this change (do not suggest using a different volume or mass) and explain why the dark pigment appeared in a shorter time.

[3]
4.

Which change in procedure will provide a successful measure of polyphenol oxidase activity?

  • Option A: Measure the increase in oxygen concentration in the air surrounding the intact avocado tissue on the watch glass.
  • Option B: Measure the depth of brown colour by comparing the avocado tissue against a standardized colour chart representing known concentrations of melanin pigment.
  • Option C: Measure the intensity of colour produced from the brown avocado tissue in a biuret test.
  • Option D: Measure the percentage of light transmitted through a whole solid cube of brown avocado tissue using a colorimeter.
[1]

Biological molecules Questions

  1. A Level
  2. /Biology
  3. /Biological molecules