In photosynthesis, electrons are excited by light energy from the sun (photoactivated).
How do they become excited in human cellular respiration?
I believe it has something to do with NADH and FADH2.
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It may be useful to start with a quotation from the Introduction to Ch. 9 of Berg et al. Biochemistry (6e) that addresses the question, more or less as asked. I can’t find a link to the pages in the 5e online so I will transcribe it from my text:
“A principal difference between oxidative phosphorylation and photophosphorylation is the source of the high energy electrons. In oxidative phosphorylation these electrons come from the oxidation of carbon fuels to carbon dioxide. In photosynthesis these electrons are excited to a higher energy level by the energy from photons.”
It is important to think about one of the ways in which the excited electrons are used in photosynthesis. To quote from earlier in the Introduction:
“In essence, light is used to create reducing potential.”
What this means chemically is that in photosynthesis the electrons have sufficient energy to reduce NAD+ to NADPH. In the biosynthetic (dark) reactions of photosynthesis the NADPH can be used to reduce carbon dioxide to sugars from which glucose can be produced.
So glucose is a reduced compound and can be thought of as ‘embodying’ the excited electrons, as its oxidation to carbon dioxide generates NADH (from NAD+). Hence:
There is no need for any additional excitation of electrons — that was done originally in photosynthesis.