NAD+ and NADPH?

Question

In the mitochondrial electon transport chain, electrons flow energetically downhill from NADH to oxygen and reduce it to water. In the cholroplasts' electron trnsport chains, electrons that come from the splitting of water flow energetically downhill and reduce NADP+ to NADPH. Being that the elctrons that reduce NAD+ and NADPH have aprox. the same reducing power. How is this possible?

Answer 1

That's a good question. Let me try and answer this-

You have put forward a very valid question that how can the energy of a bond (in water molecule) be strong enough to hold together the molecule (NADH) which in another reaction, breaks down to release enough energy to not only form that bond but also give energy to numerous other reactions?

In other words, bond energy of NADPH is greater than Water's. Then, how can the electron and energy released from water molecule's hydrolysis reduce NADP+ to NADPH?

The answer, as I see it, is-- it can't. Water molecule's bond energy alone is fairly incapable of reducing NADP+.

If you observe the Electron transport chain in photophosphorylation in the chloroplast, you'll see that the energy for reducing NADP+ to NADPH2 is provided by LIGHT and not Water.

The electrons released from water's oxidation (Photolysis) are only used to keep the reaction in Photosystem 1 going. When in PS 2, (hehe.. PS means PhotoSystem here, not PlayStation 2).... Yeah, as I was saying--

When electrons are released in PS 2 by the photolysis of water, they via a few chain reactions, reach PS 1 which is already short of electrons. They just keep coming to PS 1 and replenishing the rapidly depleting electrons there.

Now, you might ask-- By whom are the electrons being snached from PS1 and where the heck are they going?

The answer is, yup! You guessed it right! Sunlight!! (It's ok if you didn't guess it, most don't either.)

The PS1 is constantly excited by absorbing sunlight and keeps releasing electrons. The electrons from PS2 just keep coming filling up the empty space.

Where are the energised electrons going? Yup! They are going to provide NADP with lots of energy and electrons to form the High energy, power packed molecule of NADPH2.

I hope you read and liked my answer.

God bless!

Answer 2

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In one case you have a downhill reaction (NADH->NAD(+) + H(+) +e and the energy released turns eventually into chemical energy in ATP.
In the other you have an uphill reaction (NADP(+) + H(+) +e -> NADPH) and the required energy comes from the photons through Photosystems I and II.