A plant is treated with a chemical that blocks the flow of electrons between photosystem II and photosystem I, such that protons are not transported from the stroma into the thylakoid compartment. What is the effect of this chemical on photosynthesis?

Question

A.  the increased number of protons will be directly used in the thylakoid to produce glucose
B.  an increased proton gradient will provide the energy needed to produce ATP from the light reactions
C.  a decreased proton gradient will cause less ATP to be produced from the light reactions
D.  the increased proton gradient will be used to make NADPH for the Calvin cycle

Clarify Question
What is the key concept addressed by the question?
What type of thinking is required?

  Gather Content
What do you already know about photo systems II and I and electron transport? What other information is related to the question?

  Choose Answer
Do you have the necessary information to solve the problem?

  Reflect on Process
Did your problem-solving process lead you to the correct answer? If not, where did the process break down or lead you astray? How can you revise your approach to produce a more desirable result?

Answer 1

C.  a decreased proton gradient will cause less ATP to be produced from the light reactions

Clarify Question
What is the key concept addressed by the question?
        · The question asks you to determine what would happen to a plant that was treated with a chemical that blocks electron flow between photosystems II and I.
What type of thinking is required?
        · You are being asked to use what you know about electron transport and proton gradients and use, or apply, that knowledge to predict what would happen to a plant treated with a chemical that blocks electron transport between photo systems II and I.

  Gather Content
What do you already know about photo systems II and I and electron transport? What other information is related to the question?
        · The first phase of photosynthesis includes the light reactions, where energy from the sun is captured using chlorophyll molecules localized in Photosystems II and I to remove electrons from water, excite them to a higher energy level, and to use redox reactions and electron transport in order to eventually produce ATP and NADPH.
        · Remember that the light reactions, which include photosystems II and I, absorb light, make oxygen by splitting water and produce ATP and NADPH as major products. Electrons focused and amplified by photosystems I and II undergo a series of oxidation-reduction reactions as part of their electron transport chains. What is done with the energy released during electron transport?

  Choose Answer
Do you have the necessary information to solve the problem?
        · The light reactions of photosynthesis capture photons from sunlight using chlorophyll embedded in photosystems II and I. Under normal conditions, if both photosystems and their electron transport chains are working properly, the plant will harness light energy, split water and excite electrons to higher potential energy, and produce ATP and NADPH, with oxygen as a byproduct.
        · Two electron transport chains occur during the light reactions. Energy given off during the passage of electrons is used to to move protons across the thylakoid membrane in order to generate the proton gradient used to produce ATP via ATP synthase. Therefore, if electron transport between photosystem II and I were to be disrupted by the chemical, the direct effect would be a decreased proton gradient that would lead to less production of ATP.

  Reflect on Process
Did your problem-solving process lead you to the correct answer? If not, where did the process break down or lead you astray? How can you revise your approach to produce a more desirable result?
        · Answering this question correctly depended on your ability to apply what you know about electron transport and proton gradients to determine what happens when chemical treatment of a plant disrupts electron transport between photosystem II and I.
        · If you got a correct answer, nice job! If you got an incorrect answer, did you recall that the light reactions rely on redox reactions during electron transport? Did you remember that photosystems and electron transport work together during the light reactions to produce a proton gradient? Were you able to determine that a chemical that blocks electron transport would decrease the proton gradient and reduce ATP production?