1. The diagram below shows a partial model of photosystems II and I in the light-dependent reactions of photosynthesis.

Transcription

1 1. The diagram below shows a partial model of photosystems II and I in the light-dependent reactions of photosynthesis. Which statement explains the process that occurs immediately after the electrons are excited in photosystem II? (A) The excited electrons are replaced when an enzyme splits a water molecule into two electrons, an oxygen atom, and two hydrogen ions. This answer suggests the student understands that the excited electrons at the reaction center must be replaced by other electrons, that water is reduced so that the missing electrons at the reaction site can be replaced (the site is a very strong oxidizing agent without electrons), and that no further electron excitation can occur at the reaction center until the missing electrons are replaced. (B) The excited electrons release energy in the form of ATP and are then transported back to the reaction center through a series of redox reactions. Page 1 of 9

2 This answer suggests the student may understand that electrons are transported via a series of redox reactions and that energy is released, but does not understand that the next step after excitation is the splitting of water to replace the excited electrons so that photophosphorylation can continue, that energy is released as electrons are transported through the electron transport chain via a series of redox reactions (and generate ATP via a concentration gradient), or that the electrons are not transported back to the reaction center in photosystem II (noncyclic photophosphorylation). (C) The excited electrons are accepted by the terminal electron acceptor NADP + and used to generate ATP. This answer suggests the student may understand that electrons are moved through an electron transport chain and ultimately are taken up by the terminal acceptor NADP +, but does not understand that this process is not directly needed to allow for the continued excitation of electrons at the reaction center, and that the replacement of the excited electrons is crucial for the capture of energy from light. (D) The excited electrons are added to ADP + to produce ATP, which is used to drive the reactions needed to produce sugar from carbon dioxide in the Calvin cycle. This answer suggests the student may understand that ATP is a product of the light reactions, but does not understand that excited electrons at the reaction center must be continually replaced by the oxidation of water molecules in order to capture more energy from photons and provide power for ATP production. Page 2 of 9

3 2. The chloroplasts in a plant cell have a mutation that causes hydrogen ions to diffuse directly through the phospholipid bilayer of the membranes identified in the diagram below. Which statement explains how the mutation most directly affects the chloroplasts? (A) Chlorophyll in the photosystems is not able to use free energy to excite electrons to higher energy levels. This answer suggests the student may understand that chlorophyll uses sunlight to excite electrons, but does not understand that the ability to directly capture energy from sunlight is not affected by this mutation (photosystem II can still occur), and that the mutation does affect the ability to establish an electrochemical gradient, which is necessary to move hydrogen ions into an area of high concentration and allow for ATP production through chemiosmosis. (B) Enzymes in the stroma are not able to use ATP and NADPH to generate 3-carbon sugars in the Calvin cycle. This answer suggests the student may understand that ATP and NADPH are used to generate 3-carbon sugars in the Calvin cycle and that this occurs in the stroma, but does not understand that these processes are not affected directly by the mutation, and that the ability to establish an electrochemical gradient is affected, which will prevent ATP production via chemiosmosis. Page 3 of 9

4 (C) The thylakoid membrane is not able to generate an electrochemical gradient needed for the production of ATP via chemiosmosis. This answer suggests the student understands that the mutation affects the cell s ability to produce ATP because hydrogen ions can move freely through the membrane, which prevents the establishment of an electrochemical gradient needed to produce ATP through chemiosmosis. (D) Enzymes in the thylakoid membrane are not able to split water and generate oxygen through photolysis. This answer suggests the student may understand that water is split by a process that occurs in the thylakoid membrane, but does not understand that the splitting of water is not affected by this mutation and that the ability to establish an electrochemical gradient needed to produce ATP in the light reaction via chemiosmosis is affected. 3. Noncyclic photophosphorylation and cyclic photophosphorylation are the two pathways that generate ATP in the light-dependent reactions. The diagrams below show the light-dependent reactions and the Calvin cycle. Page 4 of 9

5 Based on the diagrams, which statement best explains why there are two different pathways that generate ATP in the light-dependent reactions? (A) Noncyclic photophosphorylation allows the chloroplast to undergo photosynthesis, even when no water is available. This answer suggests the student may understand that noncyclic photophosphorylation requires water to donate electrons and cyclic photophosphorylation does not, but does not understand that some water is required for all chemical reactions in cells. (B) The Calvin cycle requires more ATP than NADPH, and the extra ATP is generated by cyclic photophosphorylation without generating an excess of NADPH. This answer suggests the student understands that more ATP than NADPH is required by the Calvin cycle and that cyclic photophosphorylation only generates ATP, which helps to prevent an excess of NADPH in the Calvin cycle. (C) Cyclic photophosphorylation requires less energy from photons, which allows ATP to be produced more quickly than in noncyclic photophosphorylation. Page 5 of 9

6 This answer suggests the student may understand that cyclic photophosphorylation requires energy from photons to generate ATP, but does not understand that the Calvin cycle uses more ATP than NADPH, and that cyclic photophosphorylation is used to produce ATP without producing NADPH in order to prevent the excess buildup of NADPH. (D) The Calvin cycle generates NADP +, which is converted into NADPH more quickly if two pathways are utilized rather than one. This answer suggests the student may understand that NADP + is produced during the Calvin cycle, but does not understand that the primary reason for having two cyclic pathways is to generate extra ATP without generating an excess of NADPH for the Calvin cycle because the Calvin cycle requires more ATP than NADPH. 4. The cells of a plant are exposed to a chemical that binds to and inactivates proteins that serve as proton pumps on cell membranes. Which statement explains how this chemical will most likely disrupt the process of photosynthesis? (A) The plant cells will be unable to produce ATP because they will be unable to generate the electrochemical gradient across the thylakoid membrane. This answer suggests the student understands that proton pumps are required to generate an electrochemical gradient across the thylakoid membrane, which is used to generate ATP in the light-dependent reaction. (B) The plant cells will be unable to use active transport and will use simple diffusion to move hydrogen ions across the thylakoid membrane, which will reduce the rate at which sugars are produced. This answer suggests the student may understand that proton pumps can increase movement of substances across membranes via active transport, but does not understand that hydrogen ions and other charged substances cannot move across a membrane by simple diffusion without the use of proton pumps. Page 6 of 9

7 (C) The plant cells will be unable to reduce NADP + to NADPH because the necessary hydrogen ions will be unable to move across the thylakoid membrane. This answer suggests the student may understand that NADP + picks up a hydrogen ion in addition to electrons when it is reduced, but does not understand that the production of NADPH does not require an electrochemical gradient because it is driven by excited electrons in photophosphorylation, and not by the proton pumps in the thylakoid membrane. (D) The plant cells will be unable to move carbon dioxide into the stroma, which will result in the inability to produce sugars in the Calvin cycle. This answer suggests the student may understand that carbon dioxide is fixed into sugars in the Calvin cycle, but does not understand that the use of proton pumps or the generation of an electrochemical gradient is not required for the Calvin cycle because these are only needed during the light-dependent reactions to produce ATP. 5. An absorption spectrum is a graph that plots a pigment s degree of light absorption at different wavelengths of light. The graph below shows the relationship between the absorption spectrum for chlorophyll a (solid line) and the overall rate of photosynthesis (dotted line) in an alga exposed to different wavelengths of light. Page 7 of 9

8 Which statement best explains the data shown in the graph? (A) Chlorophyll a must serve a minor role in the absorption of light during photosynthesis because the absorption spectrum of chlorophyll a does not match the rate of photosynthesis. This answer suggests the student understands that the absorption of chlorophyll a is not identical to the rate of photosynthesis, but does not understand that photosynthetic rates peak in the same wavelengths in which chlorophyll a absorbs light, so chlorophyll a must play a major role in absorbing light for photosynthesis. (B) Other pigments must absorb light in other wavelengths, which increases the range of visible light that can be utilized by the alga for photosynthesis. This answer suggests the student understands that chlorophyll a absorbs more light from the violet-blue and orange-red wavelengths and that the rate of photosynthesis does not exactly match this pattern, so other pigments must be utilizing light in other wavelengths and broadening the spectrum of light that can be utilized for photosynthesis. (C) Photosynthesis will not occur if the alga is exposed to light that is in the green-yellow wavelength because chlorophyll a absorbs only the violet-blue and orange-red wavelengths of light. Page 8 of 9

9 This answer suggests the student may understand that chlorophyll a absorbs light from the violet-blue and orange-red wavelengths, but does not understand that the rate of photosynthesis does not stop in the green-yellow wavelengths, so chlorophyll a and other pigments must be absorbing some light in these wavelengths that is used to drive photosynthesis. (D) The alga is only able to use a small percentage of the total energy received from the sun for photosynthesis because chlorophyll a does not absorb light in the yellow wavelength and most of the light produced by the sun is in the yellow wavelength. This answer suggests the student may understand that chlorophyll a does not absorb much light in the yellow wavelengths, but does not understand that sunlight is composed of all wavelengths of light, and the alga is able to use a broad spectrum of light for photosynthesis. Page 9 of 9