Unit 3
Acetylcholinesterase (AChE) is a protein that catalyzes the conversion of acetylcholine to acetate and choline. When the concentration of AChE in an aqueous solution is held constant, the rate of the reaction catalyzed by AChE increases with increasing concentrations of substrate. At low concentrations of acetylcholine, a small increase in the substrate concentration results in a large increase in the reaction rate. At high concentrations of acetylcholine, however, a large increase in the substrate concentration results in only a small increase in the reaction rate. Which of the following statements correctly explains the observed effect of the acetylcholine concentration on the rate of the enzyme-catalyzed reaction? A The active site of AChEAChE is specific for acetylcholine, and only one substrate molecule can occupy the active site at a time. B AChEAChE begins converting product into substrate as the acetylcholine concentration changes from low to high. C The AChEAChE protein becomes denatured as the acetylcholine concentration changes from low to high. D The substrate specificity of AChEAChE changes as the acetylcholine concentration changes from low to high.
Answer A Correct. Acetylcholine is the substrate for AChEAChE, and since the shape and charge of the active site is specific for acetylcholine, only one substrate molecule can occupy the active site at a time. Therefore, at high concentrations of acetylcholine, AChEAChE becomes saturated with substrate. When an enzyme is saturated with substrate, adding more substrate to the solution will not lead to an observable increase in the reaction rate.
Researchers investigated the influence of environmental pH on the activity of peroxidase, an enzyme that catalyzes the conversion of hydrogen peroxide to water and oxygen gas. In an experiment, the researchers added a hydrogen peroxide solution containing guaiacol to several identical test tubes and adjusted the solution in each test tube to a different pH. The researchers included the guaiacol because it caused the solutions to change color as the reactions proceeded, which the researchers relied on for measuring reaction rates. Finally, the researchers added the same amount of peroxidase to each test tube and measured the rate of each reaction at 23°C. The results of the experiment are represented in Figure 1. Figure 1. The effect of pH on peroxidase activity Based on Figure 1, which of the following statements best predicts the effect that a change from a moderately acidic environment (pH near 6) to a basic environment will have on peroxidase activity? A Peroxidase activity will decrease. B Peroxidase activity will increase. C Peroxidase activity will stay the same. D Peroxidase activity will increase at first and then decrease.
Answer A Correct. Based on Figure 1, peroxidase's optimal pHpH is 5. A change from a moderately acidic environment (pHpH near 6) to a basic environment (pHpH above 7) will result in a decrease in peroxidase activity.
Researchers investigated the effect of urea on the three-dimensional structure of a certain enzyme. The researchers dissolved the enzyme in an aqueous buffer solution and added urea to the solution. The enzyme did not appear to have a secondary or tertiary structure. The researchers carefully removed the urea from the solution and determined that the enzyme had the original secondary and tertiary structure again. Based on the results of the experiment, which of the following statements best predicts the effect of urea on the enzyme's function? A Function will be disrupted by adding the urea and regained by removing the urea. B Function will be disrupted by adding the urea, but it will not be regained by removing the urea. C Function will be gained by adding the urea and disrupted by removing the urea. D Function will be unaffected by the addition and removal of the urea.
Answer A Correct. Based on the information, adding the urea denatures the enzyme, which will disrupt the enzyme's function by disrupting the active site. In contrast, removing the urea causes the enzyme to refold, which will restore the active site and the enzyme's function.
Catalase is an enzyme found in yeast cells that facilitates the chemical breakdown of hydrogen peroxide to water and oxygen gas. An experiment was conducted to determine the effect of pH on catalase function. Five buffer solutions of varying pH (2, 4, 6, 8, and 10) were prepared and added to separate test tubes. Hydrogen peroxide (H2O2) was added to each test tube. Yeast was added, and the reactions were timed. After 1 minute the amount of oxygen gas released was determined by measuring the foam layer produced in each test tube. Figure 1 illustrates the experimental setup. Figure 1. Illustration of experimental procedure A set of five additional test tubes were prepared and used as controls. Which of the following best describes the contents expected to be contained in one of the five control test tubes? A pHpH 4 buffer solution and hydrogen peroxide only B All five pHpH buffer solutions combined and hydrogen peroxide only C Water, hydrogen peroxide, and yeast D Water and yeast only
Answer A Correct. Since the experiment is studying the effect of pHpH on catalase in yeast, the control set should determine what would happen without yeast present. Each test tube in the control set should contain hydrogen peroxide and one of the pHpH solutions.
A researcher designs an experiment to investigate whether soil bacteria trigger the synthesis of defense enzymes in plant roots. The design of the experiment is presented in Table 1. For each group in the experiment, the researcher will determine the average rate of change in the amount of defense enzymes in the roots of the seedlings. Table 1. An experiment to investigate the effect of soil bacteria on plant defenses GroupNumber of SeedlingsType of SoilTreatment Solution110Sterile potting soilContains actively reproducing soil bacteria210Sterile potting soilContains heat-killed soil bacteria310Sterile potting soilContains no soil bacteria Which of the following statements best helps justify the inclusion of group 2 as one of the controls in the experiment? A It will show whether the changes observed in group 1 depend on the metabolic activity of soil bacteria. B It will show whether the changes observed in group 1 depend on the type of plants used in the experiment. C It will show the average growth rate of seedlings that are maintained in a nonsterile environment. D It will show the changes that occur in the roots of seedlings following an infection by soil bacteria.
Answer A Correct. The seedlings in group 1 will be treated with a solution that contains actively reproducing bacteria, which will be metabolically active. In contrast, the seedlings in group 2 will be treated with a solution that contains heat-killed bacteria, which will be metabolically inactive. The researcher will be able to compare the results for group 1 with those for group 2. The comparison will help the researcher determine whether the changes observed in group 1 depend on the metabolic activity of soil bacteria.
European flycatchers feed caterpillars to their hatchlings. Graph 1 shows the average dates of hatching and fledging (leaving the nest), and the biomass of the caterpillars between early May (when flycatcher young hatch) and June (when fledging of young occurs). Graph 1. Comparison of European flycatcher hatching and fledging dates and caterpillar biomass Based on the data, scientists claim that the reproductive behavior of European flycatchers is influenced by the availability of energy sources. Which of the following statements best justifies this claim? A Young European flycatchers hatch from eggs when caterpillar biomass is available for the young birds to consume and convert into energy for growth. B European flycatcher hatchlings begin to need energy to leave the nest only after the caterpillars have turned into pupae. C Female European flycatchers require energy to lay eggs, so they lay their eggs when the caterpillar biomass is maximal. D The energy requirements for hatching European flycatchers and caterpillars are proportional to each other.
Answer A Correct: The date of European flycatchers hatching matches the date when the peak biomass of caterpillars is available, ensuring better odds of survival for young flycatchers.
A researcher claims that spinach leaves capture the most energy from light waves in the range of 500 nm to 600 nm. To test the claim, the researcher will place spinach leaves in separate chambers and expose the leaves to different wavelengths of light. For each chamber, the researcher will measure the amount of oxygen gas (O2) that is produced in one hour. Which of the following graphs best represents data from the experiment that will support the researcher's claim? A B C D
Answer B Correct. The high levels of O2O2 produced at wavelengths between 500 nmnm and 600 nmnm directly support the researcher's claim. Because oxygen is produced as a by-product of photosynthesis, a greater level of oxygen production indicates a greater level of energy-capture from light.
A researcher claims that a certain herbicide suppresses plant growth by inhibiting chloroplast function. To test the claim, the researcher treats isolated chloroplasts with increasing concentrations of the herbicide. The data from the experiment are presented in Table 1. Table 1. The effect of an herbicide on the function of isolated chloroplasts Herbicide Concentration (μg/L)pH Difference across Thylakoid MembraneATP Production03.5Very high22.5Moderate80.5Very low320Very low Which of the following statements best clarifies how the data support the researcher's claim? A The thylakoid membrane is more permeable to carbon dioxide than to polar molecules. B ATPATP synthase activity depends on a proton gradient across the thylakoid membrane. C Some enzymes embedded in the thylakoid membrane catalyze the hydrolysis of ATPATP. D Carbon fixation in the Calvin-Benson cycle takes place in the stroma of chloroplasts.
Answer B Correct. The results of the experiment indicate that ATPATP production in the isolated chloroplasts decreases with increasing concentrations of the herbicide. The data also show that the decrease in ATPATP production is associated with a decrease in the pHpH difference across the thylakoid membrane. The pHpH difference is created by a proton gradient, which ATPATP synthase relies on to produce ATP
Brown fat is a type of fat tissue found in hibernating mammals. Inside the mitochondria of these fat tissue cells, these mammals have an uncoupling protein embedded in the inner mitochondrial membrane. This uncoupling protein allows hydrogen ions to leak from the intermembrane space back into the mitochondrial matrix. Figure 1 shows details of the processes in the inner mitochondrial membrane. Figure 1. Processes in the inner mitochondrial membrane Which of the following statements provides reasoning that supports the claim that brown fatty tissue keeps an animal warm? A The uncoupling protein in this tissue increases the production of ATPATP and causes more body heat to be produced to warm the animal. B The uncoupling protein in this tissue reduces the proton gradient across the membrane and thus produces heat to warm the animal without ATPATP production. C The uncoupling protein in this tissue causes an increase in the proton gradient, which causes more ATPATP to be produced that helps to warm the animal. D The uncoupling protein in this tissue reduces the production of ATPATP and creates an increase in the proton gradient that allows more heat energy to be produced to warm the animal.
Answer B Correct. The uncoupling protein causes a reduction in the proton gradient across the inner mitochondrial membrane because protons constantly leak into the matrix. Heat energy is released as the protons move, but less ATPATP is made because of the decreased concentration of hydrogen ions available to move across ATPATP synthase.
Researchers investigated the influence of environmental pH on the activity of peroxidase, an enzyme that catalyzes the conversion of hydrogen peroxide to water and oxygen gas. In an experiment, the researchers added a hydrogen peroxide solution containing guaiacol to several identical test tubes and adjusted the solution in each test tube to a different pH. The researchers included the guaiacol because it caused the solutions to change color as the reactions proceeded, which the researchers relied on for measuring reaction rates. Finally, the researchers added the same amount of peroxidase to each test tube and measured the rate of each reaction at 23°C. The results of the experiment are represented in Figure 1. Figure 1. The effect of pH on peroxidase activity One of the researchers proposes using oxygen gas production to measure reaction rates. Which of the following statements best justifies the use of the proposed modification as a way of creating an appropriate control for the investigation? A The experiment can be repeated without hydrogen peroxide, which will help eliminate an uncontrolled variable. B The experiment can be repeated without peroxidase, which will introduce a second independent variable. C The experiment can be repeated without guaiacol, which will reveal the effect of guaiacol on the reaction rates. D The experiment can be repeated without water, which will reveal whether the reaction can occur inside a living cell.
Answer C Correct. In the original experiment, the researchers used guaiacol as a reaction indicator. By using oxygen gas production to measure reaction rates, the researchers no longer need the guaiacol and can repeat the experiment without it, which will reveal the effect of guaiacol on the reaction rates.
Pectinase is a protein that catalyzes the breakdown of pectic polysaccharides in plant cell walls. A researcher designs an experiment to investigate the effect of salinity on the ability of pectinase to lower the activation energy of the reaction involved. The design of the experiment is presented in Table 1. For each test tube, the researcher will measure the amount of product formed over 20 minutes. Table 1. An experiment to investigate the effect of salinity on pectinase function Test TubeSodium Chloride Concentration (molar)Temperature (degrees C)Substrate AddedPectinase Added1023YesNo2023YesYes30.523YesNo40.523YesYes51.023YesNo61.023YesYes71.523YesNo81.523YesYes Which of the following statements best helps justify the inclusion of test tube 5 in the experiment? A It will act as a control for test tube 4 by showing the effect of the presence or absence of the substrate. B It will act as a control for test tube 4 by showing the effect of a change in environmental temperature. C It will act as a control for test tube 6 by showing the effect of the presence or absence of the enzyme. D It will act as a control for test tube 6 by showing the effect of a change in sodium chloride concentration.
Answer C Correct. Test tube 5 will act as a control for test tube 6. The solutions in test tubes 5 and 6 are nearly identical except that the researcher will add pectinase to test tube 6 but not to test tube 5. A comparison of the data for test tubes 5 and 6 will show the effect of the presence or absence of the enzyme.
A researcher claims that budding yeast are able to survive in different environments because they produce enzymes that allow them to use different molecules as sources of matter and energy. Which of the following statements best helps justify the researcher's claim by providing a relevant example? A Yeast cells produce protein kinases, which are enzymes that catalyze the transfer of phosphate groups from ATPATP to protein substrates. B Yeast cells produce DNADNA polymerases, which are enzymes that catalyze the conversion of free nucleotides into strands of DNADNA. C Yeast cells produce invertase, which is an enzyme that catalyzes the conversion of the disaccharide sucrose into glucose and fructose. D Yeast cells produce catalase, which is an enzyme that catalyzes the conversion of hydrogen peroxide into water and oxygen gas.
Answer C Correct. The disaccharide sucrose is not transported efficiently into yeast cells. By producing the enzyme invertase, budding yeast can use the sucrose, glucose, and fructose in the surrounding environment as sources of matter and energy. As such, the production of invertase by budding yeast is a relevant example that will help justify the researcher's claim.
In an experiment, a researcher prepares a reaction mixture by dissolving a substance in a buffered solution. The substance is the substrate of a certain enzyme. The researcher adds a small amount of the enzyme to the reaction mixture and measures the amount of product that is formed over time. The data are represented in Figure 1. Figure 1. The amount of product formed by an enzyme-catalyzed reaction over time Which of the following best predicts the immediate result of adding more substrate to the reaction mixture at the point indicated by the arrow in Figure 1? A The amount of product will decrease until the reaction rate goes to zero. B The amount of product will decrease until the reaction reaches its equilibrium point or until the enzyme is been used up by the reaction. C The amount of product will increase until the reaction reaches its equilibrium point or until the substrate is used up by the reaction. D The amount of product will increase without stopping because the enzyme will be unchanged by the reaction.
Answer C Correct. The graph shown in Figure 1 indicates that the amount of product became constant as a consequence of the reaction reaching its equilibrium point or as a consequence of the substrate being used up. Adding more substrate will likely result in an increase in the amount of product because the reaction catalyzed by the enzyme will convert the added substrate into product.
Aminolevulinate dehydratase (ALAD) is an enzyme that relies on zinc as a coenzyme. A zinc ion binds to the ALAD active site, where it forms favorable interactions with the side chains of three amino acids. Researchers have found that substituting a lead ion for a zinc ion in the ALAD active site causes inhibition of ALAD. Which of the following statements best helps explain how the lead ion causes inhibition of ALAD? A It changes the shape and charge of the substrate so that it becomes more compatible with ALADALAD's active site. B It changes the amino acid sequence of the ALADALAD protein so that the enzyme catalyzes a different reaction. C It changes the three-dimensional structure of the active site so that ALADALAD is no longer compatible with its substrate. D It changes the enzyme-substrate complex so that the transition state is more stable and the reaction proceeds at a faster rate.
Answer C Correct. The substitution of a lead ion for a zinc ion most likely changes the three-dimensional structure of the ALADALAD active site. The three-dimensional structure of the active site plays an important role in the compatibility between an enzyme and its substrate. Changing the structure of the active site will likely interfere with enzyme function by disrupting the enzyme-substrate interaction.
A student designs an experiment to investigate the influence of temperature on enzyme function. The student's plan is presented in Table 1. Table 1. An experiment to investigate the influence of temperature on enzyme function Test TubeSubstrate AddedEnzyme AddedTemperature (°C)1YesNo102YesYes103YesNo204YesYes205YesNo306YesYes307YesNo408YesYes40 Which test tubes are controls in the experiment? A Test tubes 1 and 2 only B Test tubes 5 and 6 only C Test tubes 1, 3, 5, and 7 D Test tubes 2, 4, 6, and 8
Answer C Correct. The test tubes that do not contain the enzyme are negative controls in the experiment. The negative controls help confirm that the results of the experiment are related to enzyme function and not some other factor.
Researchers investigated the influence of environmental pH on the activity of peroxidase, an enzyme that catalyzes the conversion of hydrogen peroxide to water and oxygen gas. In an experiment, the researchers added a hydrogen peroxide solution containing guaiacol to several identical test tubes and adjusted the solution in each test tube to a different pH. The researchers included the guaiacol because it caused the solutions to change color as the reactions proceeded, which the researchers relied on for measuring reaction rates. Finally, the researchers added the same amount of peroxidase to each test tube and measured the rate of each reaction at 23°C. The results of the experiment are represented in Figure 1. Figure 1. The effect of pH on peroxidase activity Which of the following actions will provide a negative control for the investigation? A Repeating the experiment at 25°C25°C B Repeating the experiment using twice the amount of hydrogen peroxide C Repeating the experiment using twice the amount of peroxidase D Repeating the experiment using heat-denatured peroxidase
Answer D Correct. A negative control is a group in which no response is expected. It is the opposite of the positive control, in which a known response is expected. Heat-denatured peroxidase will be inactive. Repeating the experiment using heat-denatured peroxidase will allow the researcher to measure reaction rates in the presence of inactive peroxidase.
A researcher claims that the initial rise of oxygen in Earth's early atmosphere, which occurred approximately 2.3 billion years ago, resulted from the metabolic activity of prokaryotic organisms. The claim is based on an interpretation of the geochemical and fossil evidence represented in Figure 1. Figure 1. Selected events in geologic time based on geochemical and fossil evidence Which of the following types of evidence will best support the researcher's claim? A Evidence that some of the earliest eukaryotes used oxygen to produce ATPATP by cellular respiration B Evidence that the earliest plants produced oxygen as a by-product of photosynthesis C Evidence that some of the earliest organisms carried out photosynthesis without producing oxygen D Evidence that the cyanobacteria produced oxygen as a by-product of photosynthesis
Answer D Correct. Based on Figure 1, cyanobacteria appear in the fossil record before the rise of oxygen in Earth's early atmosphere. The researcher's claim will be supported by evidence that the cyanobacteria that were on Earth more 2.3 billion years ago produced oxygen as a by-product of photosynthesis. Also, cyanobacteria are prokaryotes, which is consistent with the researcher's claim.
A researcher proposes a model to explain how enzyme-substrate interactions determine enzyme specificity. The model is based on the idea that substrate molecules form favorable interactions with the amino acid side chains in an enzyme's active site. Based on the model, which of the following statements best explains an enzyme's specificity for a particular substrate molecule? A A hydrophilic molecule interacts with nonpolar side chains in the enzyme's active site. B A hydrophobic molecule interacts with polar side chains in the enzyme's active site. C A molecule with positive charges interacts with positively charged side chains in the enzyme's active site. D A molecule with negative charges interacts with positively charged side chains in the enzyme's active site.
Answer D Correct. Because opposite charges attract, a molecule with negative charges will form favorable interactions with positively charged side chains in an enzyme's active site. Based on the model, the favorable interactions helps explain the enzyme's specificity for the molecule as a substrate.
Phycobiliproteins are a complex of accessory pigments and proteins found in cyanobacteria but not in green algae. A researcher claims that the phycobiliprotein pigments in cyanobacteria allow the cyanobacteria to survive in certain aquatic niches better than green algae can. Which of the following statements best justifies the researcher's claim? A The additional pigments allow the cyanobacteria to store light energy so that it can be used at night to continue photosynthesis. B The additional pigments block light and prevent it from reaching photosynthetic organisms at greater depths, so no photosynthetic organisms can live below the surface waters containing cyanobacteria. C The additional pigments require energy and cellular resources to produce, so they can be used as an energy source during times of insufficient light. D The additional pigments absorb light at wavelengths that green algae cannot absorb; this may allow cyanobacteria to capture more light energy for photosynthesis than green algae can in certain areas.
Answer D Correct. Since the phycobiliprotein pigments absorb light wavelengths not absorbed by green algae, cyanobacteria are able to better perform photosynthesis and survive in areas where there is insufficient light at the needed frequencies for green algae to survive.
A researcher claims that genetic variation provides organisms with the ability to survive and reproduce in different environments. To support the claim, the researcher makes the following observation: bacteria that contain plasmids (small DNA molecules) are resistant to a wider range of antibiotics than are bacteria that contain no plasmids. Which of the following statements best establishes a connection between the observation and the researcher's claim? A Some antibiotics inhibit the synthesis of bacterial cell walls. B Some antibiotics inhibit protein synthesis in bacteria cells. C Some plasmids cannot exist in a bacterial cell with certain other plasmids. D Some plasmids contain antibiotic resistance genes.
Answer D Correct. Because some plasmids contain antibiotic resistance genes, bacteria that contain plasmids are more likely to be resistant to antibiotics than are bacteria that do not contain plasmids. The observation is evidence that genetic variation in the form of plasmids provides bacteria with the ability to survive and reproduce in different environments.
A researcher claims that the incorporation of carbon dioxide into organic molecules during photosynthesis does not violate the second law of thermodynamics. Which of the following statements best helps justify the researcher's claim? A Organisms contain enzymes that lower the activation energies of specific chemical reactions. B An ecosystem is formed by the interaction of a community of organisms with their surrounding environment. C Photosynthetic organisms use the organic molecules produced during photosynthesis for growth and repair. D The total system that includes photosynthetic organisms and the Sun becomes less ordered over time.
Answer D Correct. Because the total system that includes photosynthetic organisms and the Sun becomes less ordered over time, the incorporation of carbon dioxide molecules into organic molecules that occurs during photosynthesis does not violate the second law of thermodynamics.