### Passage To study the biodegradation of polyethylene terephthalate (PET) by a newly eng… — ACT

### Passage

To study the biodegradation of polyethylene terephthalate (PET) by a newly engineered strain of *Pseudomonas putida*, researchers set up several liquid culture vessels containing a baseline mineral medium ( $M9$ ), a fixed concentration of PET film ( $0.5\%\text{ w/v}$ ), and varying combinations of additional factors. The strain normally requires a co-substrate, such as succinate, to support growth during PET degradation. The researchers tested the effects of adding $10\text{ mM}$ succinate, adding $50\text{ }\mu\text{M}$ of cadmium ( $Cd^{2+}$ , a heavy metal inhibitor), and incubating at different temperatures. Each vessel was inoculated with $10^6\text{ cells/mL}$ of *P. putida* and incubated for 7 days.

The setups for the groups are described in the table below:

Group	Mineral Medium	PET Film	Succinate ( $10\text{ mM}$ )	Cadmium ( $50\text{ }\mu\text{M}$ )	Temperature (°C)
1	Yes	Yes	No	No	30
2	Yes	Yes	Yes	No	30
3	Yes	Yes	Yes	Yes	30
4	Yes	Yes	Yes	No	37
5	Yes	No	Yes	No	30
6	Yes	Yes	No	Yes	30

The researchers measured the dry weight loss percentage of the PET film after 7 days to evaluate PET-degrading activity.

To isolate and determine the specific inhibitory effect of cadmium ( $Cd^{2+}$ ) on PET biodegradation at the standard growth temperature of $30\text{ °C}$ , the researchers must compare the PET weight loss of Group 3 to the PET weight loss of which of the following groups?

A
Group 1, because it lacks both succinate and cadmium, providing a baseline for a system with no additives.
Group 2, because it is identical to Group 3 in all conditions except it lacks cadmium, isolating cadmium as the single variable.Answer
C
Group 5, because it lacks PET film, acting as a negative control to prove that PET weight loss is mediated by biological activity.
D
Group 6, because it contains cadmium but lacks succinate, demonstrating the baseline effect of cadmium without metabolic activation.

Answer

Group 2, because it is identical to Group 3 in all conditions except it lacks cadmium, isolating cadmium as the single variable.

To isolate the effect of cadmium (

Cd^{2+}

) on PET biodegradation at

30\text{ °C}

, the researchers must compare the experimental group containing cadmium to a control group that is identical in every way except for the presence of cadmium. Group 3 contains mineral medium, PET, succinate, cadmium, and is incubated at

30\text{ °C}

. The group that matches all of these conditions but lacks cadmium is Group 2. Therefore, comparing Group 3 to Group 2 isolates cadmium as the single independent variable.

Step-by-Step Solution

Identify the treatment group of interest and its parameters.

Group 3 contains mineral medium, PET film, succinate, and cadmium, incubated at

30\text{ °C}

This establishes the active experimental group containing the independent variable (cadmium) whose effect we want to isolate.

Determine the single independent variable being investigated.

The target variable to isolate is the presence of cadmium (

Cd^{2+}

) at a temperature of

30\text{ °C}

A controlled experiment requires keeping all other variables constant to ensure any change in the dependent variable is due to the single manipulated variable.

Scan the table for a group that is identical to Group 3 in all parameters (medium, PET, succinate, temperature) except for the presence of cadmium.

Group 2 matches Group 3 in all conditions (incubation at

30\text{ °C}

with mineral medium, PET, and succinate) but does not contain cadmium.

Comparing Group 3 directly to Group 2 isolates cadmium as the only variable that differs, establishing Group 2 as the proper control group.

Key Concept

Determining Control Groups and Baseline Conditions

Estimated Time:1m 30s

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Group	Mineral Medium	PET Film	Succinate ( $10\text{ mM}$ )	Cadmium ( $50\text{ }\mu\text{M}$ )	Temperature (°C)
1	Yes	Yes	No	No	30
2	Yes	Yes	Yes	No	30
3	Yes	Yes	Yes	Yes	30
4	Yes	Yes	Yes	No	37
5	Yes	No	Yes	No	30
6	Yes	Yes	No	Yes	30

Group	Mineral Medium	PET Film	Succinate ( $10\text{ mM}$ )	Cadmium ( $50\text{ }\mu\text{M}$ )	Temperature (°C)
1	Yes	Yes	No	No	30
2	Yes	Yes	Yes	No	30
3	Yes	Yes	Yes	Yes	30
4	Yes	Yes	Yes	No	37
5	Yes	No	Yes	No	30
6	Yes	Yes	No	Yes	30

Group	Mineral Medium	PET Film	Succinate ( $10\text{ mM}$ )	Cadmium ( $50\text{ }\mu\text{M}$ )	Temperature (°C)
1	Yes	Yes	No	No	30
2	Yes	Yes	Yes	No	30
3	Yes	Yes	Yes	Yes	30
4	Yes	Yes	Yes	No	37
5	Yes	No	Yes	No	30
6	Yes	Yes	No	Yes	30