Determining Control Groups and Baseline Conditions

Researchers investigated the effects of ocean acidification and microplastic pollution on the calcification rate (measured in $\text{mg}$ of $\text{CaCO}_3$ per $\text{g}$ of dry weight per day) of the reef-building coral *Porites lutea*.

Four trials were conducted for 30 days in separate tanks containing $10\text{ L}$ of artificial seawater maintained at $26^\circ\text{C}$. The salinity, light intensity, and calcium concentration were identical across all tanks. In each trial, the partial pressure of carbon dioxide ($p\text{CO}_2$), measured in microatmospheres ($\mu\text{atm}$), and the concentration of polyethylene microplastic beads, measured in milligrams per liter ($\text{mg/L}$), were varied as shown in Table 1.

To isolate the specific impact of microplastic contamination on the calcification rate under projected future ocean acidification conditions (elevated $p\text{CO}_2$), researchers should compare the results of which two trials?

To investigate how salinity affects the heart rate of *Daphnia magna* (water fleas), a researcher prepared $4$ beakers containing different water solutions. Beaker $1$ contained pond water with $0\%$ added salt. Beaker $2$ contained pond water with $0.5\%$ added salt. Beaker $3$ contained pond water with $1.0\%$ added salt. Beaker $4$ contained pond water with $1.5\%$ added salt. In each beaker, $10$ *Daphnia magna* were placed, and their average heart rate was recorded after $30$ minutes. Which beaker serves as the control group to establish the baseline heart rate under normal conditions?

A student conducted three experiments to investigate how a new liquid fertilizer affects the stem growth of a certain plant species under various environmental conditions. In each experiment, the plants were grown in identical pots with standard soil:

* Experiment 1 was conducted in a low-light environment ($500\text{ lux}$).
* Experiment 2 was conducted in a high-light environment ($2{}000\text{ lux}$).
* Experiment 3 was conducted in a high-temperature chamber ($30^\circ\text{C}$).

To determine if the liquid fertilizer is effective, the student must compare the fertilizer-treated plants to the correct control groups. Match each experimental setup with the correct control group configuration required to isolate the fertilizer's effect.

To study the effectiveness of a new liquid antimicrobial agent, Agent Y, researchers performed an experiment using agar plates inoculated with *Escherichia coli*. Four plates were prepared under identical conditions:

* Plate 1: Treated with $5\text{ mL}$ of a $20\%$ Agent Y solution dissolved in distilled water.
* Plate 2: Treated with $5\text{ mL}$ of a $10\%$ Agent Y solution dissolved in distilled water.
* Plate 3: Treated with $5\text{ mL}$ of a $5\%$ Agent Y solution dissolved in distilled water.
* Plate 4: Treated with $5\text{ mL}$ of pure distilled water.

All plates were incubated at $37^\circ\text{C}$ for $24\text{ hours}$. Afterward, the diameter of the zone of inhibition (the clear area where bacterial growth was prevented) was measured in millimeters ($\text{mm}$).

Which of the plates serves as the control group to determine the baseline bacterial growth in the absence of the antimicrobial Agent Y?

A researcher investigated the effect of different concentrations of glucose on the fermentation rate of yeast (*Saccharomyces cerevisiae*). Five test tubes were prepared, each containing $10\text{ mL}$ of a yeast suspension and a specific concentration of glucose dissolved in distilled water. The tubes were maintained at $30^\circ\text{C}$ for $30$ minutes. The volume of carbon dioxide ($\text{CO}_2$) gas produced in each tube was measured to determine the rate of fermentation. The experimental setups are shown in the table below:

Which of the following tubes served as the control group to establish the baseline level of gas production in the absence of glucose?

A group of students designed an experiment to investigate the factors that influence the corrosion (rusting) of iron. Identical iron nails were placed in 5 different test tubes under the conditions described below:

* Tube 1: Nail fully submerged in $10\text{ mL}$ of distilled water, with a layer of mineral oil on top to prevent contact with atmospheric oxygen.
* Tube 2: Nail exposed to ambient air only (no liquid).
* Tube 3: Nail fully submerged in $10\text{ mL}$ of distilled water exposed to ambient air.
* Tube 4: Nail fully submerged in $10\text{ mL}$ of a $3\%\text{ NaCl}$ (salt) solution exposed to ambient air.
* Tube 5: Nail fully submerged in $10\text{ mL}$ of a $5\%$ acetic acid (vinegar) solution exposed to ambient air.

Match each test tube to its specific role in the experimental design.

A science class designed four different experiments to study the effects of various independent variables. Match each experimental setup with the correct control group or baseline condition required to validate the results.

An investigator conducted an experiment to evaluate how different concentrations of a newly synthesized chemical compound, Compound Y, affect the rate of starch hydrolysis by the enzyme amylase. The experiment was conducted at $37^\circ\text{C}$ and a neutral pH of $7.0$. Four trials were prepared with the compositions shown in the table below:

The rate of starch hydrolysis was determined for each trial by measuring the concentration of maltose produced after $10\text{ minutes}$.

To determine the effect of Compound Y on amylase activity, the investigator compared the rate of starch hydrolysis in the trials containing Compound Y to the rate in a control group that established the baseline activity of amylase alone. Which trial served as this control group?

Researchers investigated the effects of ocean warming and acidification on the calcification rates of the coccolithophore *Emiliania huxleyi*. They grew cultures of the marine microalgae under various combinations of temperature, partial pressure of carbon dioxide ($pCO_2$), and salinity for 14 days. All other growth parameters, including light-dark cycles and nutrient concentrations, were held constant. The conditions for Trials 1–5 are shown in the table below:

In Trial 5, salinity was lowered to 30 psu to simulate the influx of fresh water from glacier melting, which is associated with warming oceans. To isolate and determine the specific effect of this reduced salinity on the calcification rate under projected future conditions of ocean warming and acidification, which of the other trials should be used as the control?

An investigator conducted a study to evaluate how different types of dissolved organic matter (DOM) influence the rate of photochemical degradation of a synthetic pesticide, Pesticide $X$, in natural sunlight. Pesticide $X$ degrades when exposed to ultraviolet (UV) light, but dissolved organic matter can either shield the pesticide from light (attenuation) or sensitize it by generating reactive oxygen species (sensitization).

*Experiment 1*
Four identical quartz tubes were prepared, each containing a $10\text{ mg/L}$ aqueous solution of Pesticide $X$. To three of the tubes, a different type of DOM (humic acid, fulvic acid, or amino acids) was added at a concentration of $5\text{ mg/L}$. The fourth tube received no DOM. All four tubes were exposed to natural sunlight for $24\text{ hours}$. The percentage of Pesticide $X$ degraded in each tube was measured. The results are shown in Table 1.

*Experiment 2*
To determine whether the degradation was driven specifically by UV light rather than thermal decomposition (since the sun also heats the samples), the investigator prepared two additional quartz tubes. Each tube contained a $10\text{ mg/L}$ aqueous solution of Pesticide $X$ and $5\text{ mg/L}$ of humic acid. One tube was exposed to sunlight (Tube 5), while the other tube was wrapped in aluminum foil to block all light and placed adjacent to the first tube in the same outdoor environment (Tube 6). After $24\text{ hours}$, the percentage of Pesticide $X$ degraded was measured. The results are shown in Table 2.

Based on the design of Experiments 1 and 2, which of the following options correctly identifies the control group for the presence of DOM in Experiment 1, and the control group for light exposure in Experiment 2, along with the correct justification for their selection?

To investigate the factors affecting the rate of a chemical reaction, researchers conducted two experiments measuring the rate of decomposition of nitrogen dioxide ($NO_2$) into nitrogen monoxide ($NO$) and oxygen ($O_2$):

Experiment 1
Researchers introduced $1.0\text{ mol}$ of $NO_2$ gas into four separate rigid 10-liter containers at different temperatures. No other gases were initially present. The reaction rate was measured at the start of the reaction (initial rate). The results are shown in Table 1.

Experiment 2
Using Container 1 ($25^\circ\text{C}$), researchers repeated the reaction but added different amounts of helium ($He$), an inert gas that does not participate in the reaction, to test whether the total pressure of the container affects the reaction rate. The initial concentration of $NO_2$ was kept at $0.10\text{ mol/L}$ ($1.0\text{ mol}$ in the 10-liter container) in all trials. The results are shown in Table 2.

Based on the results of Experiments 1 and 2, which of the following setups serves as the control group to determine the effect of adding helium gas on the initial reaction rate in Experiment 2?

A biochemist investigated the catalytic activity of amylase extracted from *Geobacillus stearothermophilus* at $70^\circ\text{C}$ in the presence of various divalent metal ions ($\text{Mg}^{2+}$, $\text{Ca}^{2+}$, $\text{Zn}^{2+}$, and $\text{Cu}^{2+}$). Five test tubes were prepared as shown in the table below. Each tube contained $1.0\text{ mL}$ of a $1\%$ starch solution, $1.0\text{ mL}$ of buffer solution ($\text{pH } 7.0$), and $0.1\text{ mL}$ of purified amylase enzyme.

All five tubes were incubated at $70^\circ\text{C}$ for 15 minutes, after which the rate of starch hydrolysis was determined for each tube.

Which of the following test tubes served as the control group to establish the baseline rate of starch hydrolysis in the absence of added metal ions?

### Passage
An agricultural biologist investigated the physiological stress responses of the green alga *Chlorella vulgaris* exposed to common components of agricultural runoff. The study focused on four environmental variables: nitrate ($NO_3^-$) enrichment, phosphate ($PO_4^{3-}$) enrichment, atrazine (a widely used herbicide) exposure, and elevated temperature.

The biologist set up 5 culture flasks with identical initial densities of *C. vulgaris*. Each flask was subjected to a specific combination of nutrient concentrations, atrazine concentration, and temperature for 7 days. The experimental conditions for each flask are detailed in Table 1.

To evaluate the specific, independent contribution of each variable or combination of variables to algal stress, the biologist must compare the growth rates of algae in the experimental flasks against their appropriate control groups or baseline conditions.

### Matching Task
Match each of the following experimental objectives with the specific Flask that serves as its primary control group or baseline condition.

### Passage I

Researchers investigated the regulation of the electron transport chain (ETC) in isolated mammalian mitochondria by measuring the rate of oxygen ($O_2$) consumption (in $\text{nmol } O_2/\text{min}/\text{mg protein}$) under different biochemical conditions.

In all trials, a fixed concentration of isolated mitochondria was suspended in a reaction chamber containing a physiological buffer solution at $37^\circ\text{C}$. The baseline concentration of dissolved $O_2$ in the buffer was monitored. Respiration substrates, adenylates, and metabolic inhibitors were added sequentially or in combination to study their effects on the rate of mitochondrial $O_2$ consumption. The components added to each trial are shown in Table 1.

### Table 1

An investigator wants to establish a baseline condition to prove that the decrease in dissolved $O_2$ in Trials 1–5 is due to the biological activity of the electron transport chain in the mitochondria, rather than abiotic chemical reactions or gas leakage from the reaction chamber. Which of the trials listed in Table 1 serves as the most appropriate control group for this purpose?

Researchers investigated the leaching of calcium ions ($Ca^{2+}$) from sandy loam soil under simulated rainfall conditions. Six soil columns, each containing $500\text{ g}$ of soil, were prepared. The simulated rainfall rate was held constant at $50\text{ mL/hr}$ for $10\text{ hours}$. The leachate was collected and analyzed for total dissolved $Ca^{2+}$ concentration (in $\text{mg/L}$). The composition of each column and the pH of the simulated rainfall applied are summarized in the table below:

Which columns must be compared to address each research objective while isolating the single variable of interest? Match each research objective on the left with the correct column comparison on the right.

### Passage

To study the biodegradation of a synthetic polymer, polyethylene terephthalate (PET), by a newly isolated bacterium (*Thermobacillus petrolei*), researchers conducted two experiments.

In Experiment 1, five test tubes were prepared with $10\text{ mL}$ of a liquid nutrient medium, $50\text{ mg}$ of PET film, and $1.0\text{ mL}$ of an active *T. petrolei* suspension. The pH and temperature were kept constant. A different concentration of a synthetic surfactant, Surfactant X, was added to Tubes 1–4. Tube 5 did not receive Surfactant X. After 7 days, the remaining mass of PET was measured to evaluate degradation.

In Experiment 2, to determine whether the PET degradation observed in Experiment 1 was due to active bacterial metabolic activity rather than non-biological chemical hydrolysis or passive physical absorption onto the bacterial biomass, the researchers prepared four additional test tubes (Tubes 6–9) under the same environmental conditions as Experiment 1, but modified the tube contents as shown below:

To demonstrate that the degradation of PET film in Experiment 1 was specifically caused by the active metabolism of living *T. petrolei* cells rather than passive physical absorption of the polymer by the bacterial biomass, which of the following tubes from Experiment 2 serves as the most appropriate control when compared to Tube 5?

### 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:

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?

### Passage

An environmental scientist investigated the efficiency of sunflower plants (*Helianthus annuus*) in phytoremediation—the process of using living plants to remove heavy metals from contaminated soil. Sunflowers were grown in pots containing soil with a baseline lead ($Pb^{2+}$) concentration of $500\text{ mg/kg}$ under controlled greenhouse conditions. The scientist set up several experimental groups to evaluate how two soil amendments—biochar and arbuscular mycorrhizal fungi (AMF)—affect the rate of $Pb^{2+}$ uptake by the plants.

The experimental groups were designed as follows:
* Group A: Soil with $0\text{ mg/kg}$ of $Pb^{2+}$, no biochar, no AMF.
* Group B: Soil with $500\text{ mg/kg}$ of $Pb^{2+}$, no biochar, no AMF.
* Group C: Soil with $500\text{ mg/kg}$ of $Pb^{2+}$, biochar added ($5\%\text{ w/w}$), no AMF.
* Group D: Soil with $500\text{ mg/kg}$ of $Pb^{2+}$, no biochar, AMF added.
* Group E: Soil with $500\text{ mg/kg}$ of $Pb^{2+}$, biochar added ($5\%\text{ w/w}$), AMF added.

All groups were watered daily with $100\text{ mL}$ of distilled water and kept at a constant temperature of $24^\circ\text{C}$ under a $14\text{-hour}$ light/$10\text{-hour}$ dark cycle. After $6\text{ weeks}$, the dry biomass of the plants and the concentration of $Pb^{2+}$ in the plant tissues were measured.

To evaluate the specific influence of each experimental variable, the scientist must compare the results of a test group to a corresponding control or baseline group. Match each of the following experimental objectives with the group that serves as the most appropriate control or baseline.

### Passage
A team of geobiologists investigated the role of the soil bacterium *Bacillus subtilis* and soil humic acids on the dissolution rate of calcite ($CaCO_3$), a common carbonate mineral. Five experimental trials were conducted at $25^\circ\text{C}$ for 14 days. In each trial, 10.0 grams of calcite was placed in a reaction vessel containing 500 mL of an aqueous solution. The dissolution rate was determined by measuring the cumulative concentration of calcium ions ($Ca^{2+}$) released into the solution. The experimental setups are detailed below:

* Trial 1: Sterile deionized water (pH $7.0$), no bacteria.
* Trial 2: Aqueous solution containing $10\text{ mg/L}$ of humic acids (pH $5.5$), no bacteria.
* Trial 3: Sterile deionized water (pH $7.0$) inoculated with $10^6\text{ cells/mL}$ of living *B. subtilis*.
* Trial 4: Aqueous solution containing $10\text{ mg/L}$ of humic acids (pH $5.5$) inoculated with $10^6\text{ cells/mL}$ of living *B. subtilis*.
* Trial 5: Aqueous solution containing $10\text{ mg/L}$ of humic acids (pH $5.5$) inoculated with $10^6\text{ cells/mL}$ of heat-killed *B. subtilis*.

To determine if the enhancement of calcite dissolution in the presence of humic acids is specifically driven by the active metabolic processes of living *B. subtilis* rather than the passive physical presence of bacterial cell structures, researchers must compare Trial 4 to a control group. Which trial serves as the most appropriate control group for this comparison, and why?

### Passage

Astrophysicists and astrobiologists simulated Martian surface environments to evaluate the survival and methane ($\text{CH}_4$) production of the methanogenic archaeon *Methanosarcina barkeri*. Under optimal laboratory conditions, *M. barkeri* is cultured anaerobically in a liquid medium under an atmosphere of $80\%\ \text{H}_2$ and $20\%\ \text{CO}_2$ at $37^\circ\text{C}$ (Standard Growth Condition).

In the Martian simulation experiments, the researchers varied three main environmental variables:
1. Atmosphere: Standard Growth atmosphere vs. Simulated Martian Atmosphere (SMA: $95.3\%\ \text{CO}_2$, $2.7\%\ \text{N}_2$, $1.6\%\ \text{Ar}$, and $0.13\%\ \text{O}_2$).
2. Substrate: No substrate (liquid medium only) vs. Inert quartz sand vs. Simulated Martian Regolith (SMR) containing $1.0\%\ \text{Mg(ClO}_4)_2$ (perchlorate salt, a strong oxidizing agent).
3. Radiation: Shielded (no UV exposure) vs. UV-irradiated (exposure to $200\text{--}400\text{ nm}$ UV flux).

To evaluate the specific effect of each environmental variable on the growth rate and $\text{CH}_4$ production of *M. barkeri*, the researchers prepared multiple experimental setups. To validate their conclusions, each test setup must be compared against a specific control or baseline setup that isolates the variable of interest.

Match each research goal with the appropriate control or baseline setup needed to isolate the variable of interest.