Which statement best explains the results observed in the table?

Transcription

1 1. The effect of light on the photosynthetic rate of chloroplasts is tested during a controlled experiment. Two chloroplast suspensions are mixed with DPIP, an electron acceptor that changes from blue to clear when it is reduced and takes the place of NADP +. One test tube is exposed to light, while the other is covered with aluminum foil to provide a dark environment. The two samples are placed in a machine that measures the percent of light transmittance, and the percent transmittance is measured and recorded every five minutes for 20 minutes. The results are shown in the table below. Which statement best explains the results observed in the table? (A) The suspension in the dark environment shows a decrease in free energy because no free energy is available to excite the DPIP molecules and cause the solution to change color. This answer suggests the student may understand that the free energy in light is required to reduce the DPIP molecules, but does not understand that the data shows no change or a very slight overall increase in free energy of the dark environment as evidenced by a relatively constant percent transmittance, which indicates that DPIP molecules are not being reduced and free energy capture via photosynthesis is not occurring. (B) The suspension exposed to light shows a decrease in free energy because more DPIP molecules are reduced, which causes the solution to change color from blue to clear, using free energy in the process. Page 1 of 7

2 This answer suggests the student may understand that molecules of DPIP are reduced during the light reaction and that the solution changes from blue to clear, but does not understand that this indicates an increase in free energy, not a decrease, because the color change indicates that energy is being captured from light during photosynthesis. (C) The amount of free energy in both suspensions does not change because energy cannot increase or decrease within a system and can only be converted from one form to another. This answer suggests the student may understand that energy can be transferred within a system, but does not understand that free energy must be continuously supplied to a system because free energy is continually being used by organisms and that most of the free energy captured is released as thermal energy and is no longer available as a source of energy. (D) The suspension exposed to light shows an increase in free energy because the color of the solution changes from blue to clear, which indicates that DPIP molecules are reduced. This answer suggests the student understands that the light transmittance of the suspension exposed to light increases over time because the color of the solution changes from blue to clear as the chloroplasts capture free energy from light and DPIP molecules are reduced during photosynthesis, and this indicates that the free energy of the solution is increasing. Page 2 of 7

3 2. The hydrolysis of ATP to ADP and phosphate is performed both inside and outside of living cells. The table below shows the change in free energy in each environment. Which statement best explains the data? (A) Both samples are endergonic, but sample A shows a greater increase in entropy because G is more negative than in sample B. This answer suggests the student may understand that entropy is increasing in these reactions, but does not understand that the reactions are exergonic because energy is released, as indicated by the negative G. (B) Both samples are energy-capturing reactions, but the total energy in sample A is greater because G is more negative than in sample B. This answer suggests the student may understand that G is more negative in sample A, but confuses enthalpy (total energy) with entropy (disorder) and does not understand that a negative G indicates that free energy is being released, not gained. (C) Both samples are exergonic, but sample A shows a greater decrease in entropy because G is more negative than in sample B. This answer suggests the student may understand that the reactions are exergonic, but does not understand that there is an increase in entropy (disorder) when energy is released, not a decrease. Page 3 of 7

4 (D) Both samples are energy-releasing reactions, but the entropy in sample A is greater because G is more negative than in sample B. This answer suggests the student understands that the hydrolysis of ATP involves the release of energy and that a more negative change in free energy ( G) results in an increase in entropy (disorder). 3. Biochemical processes are influenced by changes in free energy. The graph below represents a process that involves a change in free energy. Which process does the graph most likely model? (A) Cellular respiration, because energy from the reactants is transferred to the environment and G is negative This answer suggests the student understands that cellular respiration involves the release of free energy from energy-rich molecules and that G is negative because free energy is released during cellular respiration. Page 4 of 7

5 (B) Dehydration synthesis, because energy is stored in the products and G is positive This answer suggests the student may understand that the reaction involves a transfer of energy, but does not understand that free energy in this reaction is transferred to the environment and not stored in the products, and that G for this reaction is negative. (C) Photosynthesis, because energy is transferred to the products and G is positive This answer suggests the student may understand that energy is transferred through light reactions and that light reactions have a positive G, but does not understand that light reactions require free energy and that the reaction shown has a negative G since free energy is released. (D) Active transport, because energy is used to move products from a higher concentration to a lower concentration and G is negative This answer suggests the student may understand that free energy is involved in active transport and that the diagram shows a negative G, but does not understand that this reaction shows a transfer of energy from the reactants to the environment and does not show products moving from a higher concentration to a lower concentration. Page 5 of 7

6 4. Yeast cells are able to undergo both aerobic respiration and fermentation. The table below shows the conditions in two flasks before yeast cells are added. Once the yeast cells are added, the concentrations of sucrose and oxygen as well as the temperature of each flask are monitored for 60 minutes. Which statement explains the changes that will most likely be observed in the two flasks? (A) The available free energy in both flasks will increase because both flasks will have more sucrose, no oxygen, and an increase in temperature due to cellular respiration. This answer suggests the student may understand that some free energy is transformed into heat energy during cellular respiration and fermentation, but does not understand that oxygen is not a source of free energy and will not increase in flask 2, that sucrose is consumed during cellular respiration, that no oxygen will be consumed during fermentation in flask 1, and that the available free energy in both flasks decreases during cellular respiration and fermentation. (B) The available free energy in flask 1 will not change because there is not enough oxygen for cellular respiration, but the free energy in flask 2 will decrease because there will be less sucrose and less oxygen due to cellular respiration. This answer suggests the student may understand that oxygen and sucrose are consumed during cellular respiration and that free energy will decrease, but does not understand that sucrose will be used during fermentation in flask 2 due to the low oxygen concentration, and that the available free energy in both flasks will decrease. Page 6 of 7

7 (C) The available free energy in both flasks will decrease because both flasks will have less sucrose and an increase in temperature, but the yeast cells in each flask will generate different waste products. This answer suggests the student understands that cellular respiration consumes oxygen and generates low-energy waste products, that fermentation will occur in flask 1 due to low oxygen, that sucrose is consumed as the free energy source in both cellular respiration and fermentation, and that the available free energy in both flasks decreases as a result. (D) The available free energy in neither flask will change because neither flask will have an increase in temperature, since the yeast cells are heterotrophic and will consume all available sucrose and oxygen. This answer suggests the student may understand that yeast cells consume sucrose, but does not understand that cellular respiration occurs in the presence of oxygen in flask 2 and that fermentation occurs in the absence of oxygen in flask 1, that the temperature will increase in both flasks, and that the available free energy in both flasks decreases as a result. Page 7 of 7