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Light-dependent Reaction Questions


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Light-dependent Reaction Questions

Postby vertciel » Sat Oct 25, 2008 3:17 pm

Hello everyone,

I'd like to clarify the following questions so that I understand the light reaction completely.

Thank you!


1. My teacher says that there are three ways in which a [H+] gradient is maintained: photolysis, translocation of hydrons across the thylakoid membrane, and formation of NADPH.

I am not certain why the formation of NADPH would maintain a hydron gradient, but here is my educated guess.
Since (NADP+) + (2H+) --> NADPH + (H+), would the decrease in mols of (H+) after this redox reaction contribute to the lower concentration of H+ in the stroma?

2. Is the electron from the P700 chlorophyll a molecule furthered into the electron transport chain (passing through Fd and NADP+ reductase) the same electron from photosystem II? In other words, does the electron from PS II become energised and continue after PS I; or does the electron from PS II simply replace the P700 chlorophyll molecules now deficient in one electron?

3. Chemically, what is different about the special chlorophyll a molecules?
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Postby Yali » Tue Oct 28, 2008 3:04 pm

i have some ideas about Queastion1.
i think fomation of NADPH may be related to Mehler raction. Mehler raction can maintain eletrons trsport from PSII to PSI, so, if the eletron trsport is acitive, the hydron gradient is maintain.
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Postby Jonty » Sat Dec 06, 2008 10:08 am

Can anyone offer an thoughts' on question 2 and 3?
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Postby MrMistery » Sat Dec 06, 2008 3:18 pm

2. Yes, it is the electron from photosystem I(P700) that is used to reduce NADP+.
3. There is nothing different about the chl a molecules. Chl a is chl a, and they all have the same structure. However, because they are surrounded by different proteins, their absorption properties change. This is a widespread phenomenon in biology: if you've ever taken human physiology, you will know that the cones in the eye have three different proteins for red, blue and red, but all three are bound to the same retinal molecule. because the retinal is in a different environemnt(surrounded by different proteins) it can absorb different wavelengths of light. The exact same thing occurs with chlorophyll.
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