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Difficulty: MediumDetermining Control Groups and Baseline Conditions

A biochemist investigated the catalytic activity of amylase extracted from *Geobacillus stearothermophilus* at 70∘C70^\circ\text{C} in the presence of various divalent metal ions (Mg2+\text{Mg}^{2+}, Ca2+\text{Ca}^{2+}, Zn2+\text{Zn}^{2+}, and Cu2+\text{Cu}^{2+}). Five test tubes were prepared as shown in the table below. Each tube contained 1.0 mL1.0\text{ mL} of a 1%1\% starch solution, 1.0 mL1.0\text{ mL} of buffer solution (pH 7.0\text{pH } 7.0), and 0.1 mL0.1\text{ mL} of purified amylase enzyme.

TubeAdded Solution (0.5 mL0.5\text{ mL})
Tube 1Distilled water
Tube 210 mM MgCl210\text{ mM } \text{MgCl}_2
Tube 310 mM CaCl210\text{ mM } \text{CaCl}_2
Tube 410 mM ZnCl210\text{ mM } \text{ZnCl}_2
Tube 510 mM CuCl210\text{ mM } \text{CuCl}_2

All five tubes were incubated at 70∘C70^\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?

  1. Tube 1, because it contained all reaction components except the tested metal ions.Answer
  2. B
    Tube 2, because magnesium ions are naturally present in bacterial cells and provide standard physiological conditions.
  3. C
    Tube 3, because calcium ions stabilize thermophilic enzymes to ensure maximum baseline activity.
  4. D
    Tube 5, because copper ions completely halt enzyme activity, setting the lower boundary for the experiment.

Answer

Tube 1 served as the control group because it contained distilled water in place of metal ion solutions, establishing a baseline rate of starch hydrolysis with no added metal ions.
The correct answer identifies Tube 1 as the control group. A control group provides a baseline measurement by keeping all experimental factors identical except for the independent variable being tested. Since Tube 1 contains distilled water in place of metal ion solutions, it establishes the baseline rate of starch hydrolysis under standard reaction conditions without metal ion intervention.

Step-by-Step Solution

1
Identify the independent variable tested across the experimental setups.
The independent variable is the type of added divalent metal ion (Mg2+\text{Mg}^{2+}, Ca2+\text{Ca}^{2+}, Zn2+\text{Zn}^{2+}, or Cu2+\text{Cu}^{2+}).
Control groups require isolating the independent variable by withholding it or substituting it with a neutral substance.
2
Examine the composition of each test tube to find the setup where the independent variable is omitted.
Tube 1 receives 0.5 mL0.5\text{ mL} of distilled water instead of a metal ion solution, keeping all other reaction conditions (enzyme, substrate, buffer, volume, temperature) constant.
Distilled water acts as a neutral solvent control to measure baseline enzyme kinetics.
3
Select the option that correctly identifies Tube 1 and explains its role as a baseline control.
The option stating Tube 1 is the control group because it contains all components except the tested metal ions is correct.
A true control group isolate changes caused solely by the experimental treatments.

Key Concept

Identifying Negative Control Groups and Baseline Conditions
Estimated Time:1m 0s
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Topics

Determining Control Groups and Baseline Conditions

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