Madonna is positing the chemical energy of CO2 and water (the products of respiration) as "degraded energy".
I found support for Madonna's idea in Campbell:
The products store less free energy per mole than the reactants; that fits our colloquial definition of "degraded"=not useful.
It also fits with Rachel's idea of degraded=less of a gradient, I think--because the products store less free energy, there is less of a gradient between them and the products of any future metabolic reaction that might involve them. Glucose is non-degraded--and useful--because there is a big difference in free energy between it and the products of respiration, a metabolic reaction that the cell can carry out. The cell CANNOT carry out a reaction with a similar release of free energy from CO2. But if it could...then the chemical energy in CO2 would be useful.
I got this idea from Campbell, that debunks the biologist's shorthand that "energy is stored in bonds". "It is important to realize that the breaking of bonds does not release energy," as biology teachers, yours truly included, often tell their students. The "stored energy" we refer to, says Campbell, is actually "potential energy that can be released when new bonds are formed after the original bonds break, as long as the products are of lower free energy than the reactants."
If the energy "can be released", then why is it not appropriate to call it stored?
With the phrase "potential energy that can be released when new bonds are formed", is Campbell reiterating the potential energy=possible energy misconception?
Can I help myself here by renaming potential energy as gradient energy or difference energy? In that case, the potential energy is not in the reactants, but between the reactants and products. This jives with something I already know about gravitational potential energy, which is that the amount you have is relative to where the "bottom" is.
[00:00:00.00](Steve: Is there any other kind of degraded energy besides heat?
Madonna: In respiration, the energy contained in the CO2 bonds...)
and your water at the end are not necessarily useful
[00:00:05.10]they're degraded to you, they weren't utilized
[00:00:08.04]you're using your ATP to do work for your body
[00:00:12.23]but the CO2 and the water that is given off
Debra: They're lost.
[00:00:18.00]I don't know--is that degraded energy?
[00:00:21.20]There's still energy in the chemical bonds...
[00:00:28.18]And the same thing with a battery.
[00:00:30.04]In a battery you have movement of those electrons due to a difference in electrical potential or reduction potential
[00:00:41.06]but it's degraded once...
Debra: Well it's the concentration difference that drives it
[00:00:49.01]Madonna: In a copper battery, once that copper has received its electrons and becomes a solid, it's no longer...
[00:00:57.29]Steve: So those are degraded matter. I'm trying to think...
[00:01:00.21]if you've got the carbon dioxide when you're done
[00:01:04.25]it's had the transfer of energy by breaking its bonds
[00:01:12.11]no, reforming more stable, tighter bonds
[00:01:15.10]so they've given up that energy
[00:01:18.07]so now you've got a matter leftover
[00:01:21.14]Madonna: that contains energy
[00:01:24.01]Steve: But does it?
Madonna: Yeah! The carbon to oxygen bonds have energy
[00:01:30.07]Steve: But it bonds at a lower energy than it started
[00:01:33.21](Don makes a "downhill" gesture) It's been degraded.
[00:01:36.12]It's a much lower potential
[00:01:41.01]Brad: The energy density in sugar is higher
than the energy concentration in CO2 and water
[00:01:48.15]Debra: So then it IS coming down a gradient
[00:01:54.10]Madonna: it would be waste energy that you're putting off
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