Black pepper (Piper nigrum) is a plant grown for its fruits. These fruits are dried and
crushed to produce black pepper. Black pepper is used in many recipes and is also
thought to have medicinal properties.
The fruits contain alkaloids and phenols that have antimicrobial properties.
An investigation was carried out in a research laboratory.
Two extracts from black pepper were produced: one containing alkaloids and
one containing phenols. Each extract was used to prepare a solution containing
0.4 mg dm–3 of the chemicals.
The resulting solutions were then used to investigate the antimicrobial properties of
these two chemicals on a range of bacteria, using the following method.
• Each solution was pipetted onto discs of filter paper.
• All the discs were dried.
• Four plates were prepared, using agar, containing the following bacteria:
○ Escherichia coli
○ Staphylococcus aureus
○ Streptococcus agalactiae
○ Streptococcus pyogenes.
• A disc containing each chemical was placed on the agar in each of the four
agar plates, as shown in the diagram below.
The agar plates were incubated.
The size of each zone of inhibition was then measured after 48 hours.
Name the dependent variable in this investigation and explain how it could
be measured accurately.
Dependent variable
How it could be measured accurately