During the light-dependent reactions, due to electron transport, the thylakoid space becomes

Question

A.  the site of ATP synthesis.
B.  the site of NADPH production.
C.  the site where the Calvin cycle occurs
D.  more acidic than the stroma.

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What type of thinking is required?

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What do you already know about thylakoid membranes and proton gradients? What other information is related to the question?

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Answer 1

D.  more acidic than the stroma.

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What is the key concept addressed by the question?
        · The question asks you to determine what happens to the thylakoid space due to electron transport.
What type of thinking is required?
        · You are being asked to use, or apply, what you know about the light reactions and electron transport to determine what happens to the thylakoid space.

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What do you already know about thylakoid membranes and proton gradients? What other information is related to the question?
        · First, let’s orient you to chloroplast membrane structure and function. The innermost membrane in the chloroplast is the thylakoid membrane, and it is important because it contains the proteins used to produce both ATP and NADPH. The thylakoid membrane maintains an impermeable barrier to protons, the positively charged subatomic particles used to generate the proton gradient needed to produce ATP.
        · Remember that a biological outcome of photosynthesis is to produce carbohydrate. In order to do that, plants and similarly photosynthesizing organisms use the light reactions, where energy from the sun is captured using chlorophyll molecules localized in photosystems I and II to strip electrons from water, excite them to a higher energy level, and use redox reactions in order to produce ATP and NADPH via proton gradients. The reason why thylakoid membranes are important is because this is where the protein complexes used to capture sunlight and produce ATP and NADPH are located, and it is the barrier that establishes and maintains the proton gradient.
        · Remember also that the ATP and NADPH produced from the light reactions are used by the Calvin cycle, located in the chloroplast stroma. Given this information, what happens to the thylakoid space?

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Do you have the information needed to make a determination?
        · Under normal conditions, if both photo systems and their electron transport chains are working properly, a plant will harness light energy, split water and excite electrons to higher potential energy, and produce ATP and NADPH, based partly on the integrity of thylakoid membranes. The light reactions rely on the integrity of the thylakoid membrane, which establishes and maintains separation of protons, to allow an electrochemical gradient to be formed. This gradient is crucial to production of ATP in particular, without which the Calvin cycle that produces sugar cannot function.
        · Electrochemical gradient formed across the thylakoid membrane represents a difference in proton concentration. Another way to look at changes in proton concentration is pH. Thus, any difference in proton concentration across a membrane also represents a pH change. In this case, since there are more protons on the lumen side of the thylakoid membrane than the stroma side, the thylakoid space is in fact more acidic than the stroma.

  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 the light reactions and electron transport to determine what happens to the thylakoid space.
        · If you got a correct answer, nice job! If you got an incorrect answer, did you recall where the light reactions and the Calvin cycle physically take place within the chloroplast? Did you remember that the light reactions and electron transport product ATP via a proton gradient across the thylakoid membrane? Were you able to correlate the difference in proton concentration across the thylakoid membrane with a pH difference as well?