Which other type of organism causes decay? Tick (✓) one box - AQA - GCSE Biology - Question 5 - 2023 - Paper 2

The dependent variable in the investigation is The pH of the milk. This measures the extent of decay caused by the bacteria.

To maintain the milk at 30°C for the duration of the experiment, the correct action is to put the beaker in a water bath. This method ensures a stable temperature.

The type of acid produced during the digestion of fats in the milk is Fatty acid. Fatty acids are the products of lipid breakdown.

The reason the pH did not fall below 4.5 at 20°C is that all the fat had been digested. Once the fats are exhausted, digestion slows down.

The rate of digestion is faster at 30°C because the enzyme activity is higher. At higher temperatures, enzymatic reactions occur more rapidly due to increased kinetic energy.

Using Figure 9, at day 2 the pH at 30°C is 6.0, and at day 3, the pH is 4.7. Therefore, the fall in pH from day 2 to day 3 is:

$ext{Fall} = 6.0 - 4.7 = 1.3 ext{ pH units}$

The rate of digestion is:

rac{1.3 ext{ pH units}}{1 ext{ day}} = 1.3 ext{ pH units/day}

To find out how many times faster the rate of digestion at 30°C is compared to 5°C, we compute:

rac{1.3 ext{ pH units/day (at 30°C)}}{0.1 ext{ pH units/day (at 5°C)}} = 13 ext{ times faster}