8. Phenolphthalein is an indicator - Edexcel - GCSE Biology - Question 9 - 2016 - Paper 1

From Figure 10, it is evident that the rate of lipase activity increases with temperature up to an optimal point, around 40°C. Beyond this temperature, the activity declines. This suggests that the enzyme operates most effectively at this optimal temperature, but higher temperatures may denature the enzyme, reducing its activity.

Above 40°C, the activity of lipase decreases due to denaturation. The heat alters the enzyme's structure, disrupting the active site which is crucial for substrate binding. This conformational change prevents the enzyme from catalyzing the reaction effectively.

To calculate the rate of amylase activity, first find the average time taken from the provided results. The times are 120, 125, 110, 115, and 118 seconds. The average time is:

$ext{Average time} = \frac{120 + 125 + 110 + 115 + 118}{5} = \frac{588}{5} = 117.6 ext{ seconds}$

The rate of enzyme activity is then calculated as the inverse of the average time:

$\text{Rate of activity} = \frac{1}{\text{Average time}} = \frac{1}{117.6} \approx 0.0085 ext{ units/second}$

One important variable that should be controlled is the pH of the starch solution, as different pH levels can significantly affect enzyme activity.

Enzymes are highly specific due to their unique active sites, which are shaped to fit particular substrate molecules. This specificity is often described by the 'lock and key' model, where only the correct 'key' (substrate) can fit into the 'lock' (enzyme). Consequently, each enzyme is tailored to catalyse its specific reaction without affecting other processes.