NADH is used mainly in cellular respiration and the NAD+ to NADH ratio inside the cell is kept high, but NADPH is mainly used in photosynthesis and the NADPH to NADP+ ratio is kept low in animal cells.
Why is this?
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$\begingroup$ Welcome to Biology.SE. This is not simply an answer site, but instead a site that promotes self-learning with some expert help. Consequently, questions that show little or no prior research effort are off-topic on this site as are "homework" questions unless you have shown your attempt at an answer. Please edit your question and tell us where you've looked for answers, what you do know about the topic, and where exactly you still have questions. Please take the tour and consult the help center starting with How to Ask for details. $\endgroup$– tyersomeCommented Feb 20, 2022 at 1:55
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1$\begingroup$ Does this answer your question? NADH vs. NADPH: Where is each one used and why that instead of the other?. ——— Please note that one part of the expected prior research on this site is that you look for previous questions and answers.. $\endgroup$– tyersomeCommented Feb 20, 2022 at 1:56
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$\begingroup$ [1] For a very interesting perspective on a slightly different question, "why are there two almost identical cofactors (NAD and NADP) anyway?" see Why NADP by B.T. Kaufman, who argues that phosphorylation on the 2'-position of the ribose may be protective "from the drastic effects of the ribosylation reactions" and other non-redox reactions that NAD is involved in. ... $\endgroup$– user338907Commented Feb 20, 2022 at 13:11
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$\begingroup$ [2] He points out that (only?) NADP and DNA are modified on the 2'-position of ribose, but Coenzyme-A, cyclic-AMP and PAPS are all modified at the 3'-position $\endgroup$– user338907Commented Feb 20, 2022 at 13:11
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NADPH is first generated in the light reactions of photosynthesis before it is used in the reductive synthesis of sugars. But this would seem irrelevant to mammalian systems. NADPH is used in mammalian cells for reductive synthetic reactions, e.g. fatty acid and steroid synthesis.
Although it is merely an opinion, it would seem to me advantageous to separate pools of reducing power for energy generation and synthesis so as to allow regulatory prioritization of the former over the latter. And, incidentally, the generation of NADPH in mammalian cells is independent of that of NADH, being primarily in the pentose phosphate shunt, a pathway found particularly in tissues like adipose tissue, mammary gland, liver and steroid-synthesizing tissues.
Although I now see that a similar question has been asked before the most voted answer repeats the somewhat unsatisfactory statement of the question (to be found in Albers et al.):
“The ratio of NAD+ to NADH inside the cell is high, while the ratio of NADP+ to NADPH is kept low.”
The reason I consider this unsatisfactory is, as just stated, some tissues are specialized for reductive synthesis of fatty acids and steroids, whereas others are not. Their need for NADPH will obviously differ, as will the redox ratio. Remarks about “the cell” are unfortunately typical of texts generalizing metabolism from bacteria.
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$\begingroup$ @user338907 — Yes, of course you are right. I'm afraid I always tend to focus on the actual energy generation aspects of photosynthesis (the light reactions) — the dark reactions being of less fundamental interest to me. I'll correct my answer. $\endgroup$– DavidCommented Feb 20, 2022 at 15:01