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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. Rationale: 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.
Distractor Rationale: 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. Distractor Rationale: 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. Distractor Rationale: 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.
Aligned to: LO 2.5 CA 2.5: Explain Energy Use, Storage & Capture
(C) The thylakoid membrane is not able to generate an electrochemical gradient needed for the
production of ATP via chemiosmosis. Rationale: 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. Distractor Rationale: 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.
Aligned to: LO 2.5 CA 2.5: Explain Energy Use, Storage & Capture
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.
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. Distractor Rationale: 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. Rationale: 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.
(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. Distractor Rationale: 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. Distractor Rationale: 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.
Aligned to: LO 2.5 CA 2.5: Explain Energy Use, Storage & Capture
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.
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. Distractor Rationale: 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. Rationale: 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.
