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12. The substance produced by the light-reaction and used in the light-independent reaction is
A. water
B. oxygen
C. hydrogen
D. carbon dioxide

hi there so I belive that the answer is b or d but i am leaning more toward b is that correct?


Your question is really not phrased right. Maybe whoever wrote it is confused (not unusual). You see, CO2 is not really produced by the light reaction It's taken directly from the atmosphere through the stomata. It is however acted on by NADPH to make glucose in the light independent reactions (dark reactions). Read the following carefully (analyze the last sentence several times)

Light energy entering the plant splits the water into hydrogen and oxygen:

H2O + light energy ¡æ ¨ö O2 + 2H+ + 2e-

These electrons travel through the membrane much like the electrons in oxidative phosphorylation, using their energy to pump protons through the membrane. The proton gradient thus established can be used to synthesize ATP.

More importantly, that same electron reduces NADP+ to NADPH. This molecule plays the same role in synthesis as does NAD+ in the respiratory pathway, as a carrier of reductive power. This store of power serves to reduce carbon dioxide to the more complex carbon structure of glucose, the building block of life.

The reactions leading to the production of ATP and reduction of NADP+ are called the light reactions because they are initiated by the splitting of water by light energy. The reduction of carbon dioxide to glucose, using the NADPH produced by the light reactions, is governed by the dark reactions.
Source:
http://www.netlexikon.akademie.de/Photosynthesis.html



I agree with lance....confusing. However....in the light reaction H2O is split, and O2 released, and the H is saved for use in the dark reaction to make carbohydrates. Answer C fits this bill.


The hydrogen is removed from NADPH in the dark reactions. True it came from the light reactions, but it is not really just hydrogen as the answer suggests. If one of the answeres had been NADPH, then that would have been the correct choice. Additionally a proton (what results from the light reactions) is not hydrogen either; not until it gets an electron back, before then it is a hydrogen ion. That's why I think who ever wrote the question was/is confused.

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