It was experimental as a weight loss drug. Thery was that by short circuiting oxidative phosphoralation to generate ATP the patient’s metabolism would have to relay on the more inefficient anaerobic pathway, consuming mucho more glucose per unit of ATP produced.
of course, I meant ATP. :smack:
Considering a single-molecule instance of this reaction, is there ever any such thing as it occurring quickly or slowly anyway? Is there ever any theoretical difference in the time it takes to break/establish bonds?
It seems to me that there shouldn’t be - in which case, the variables between combustion and oxidation have got to be things like concentration within a physical space, concentration (as in the number of events occurring) within a period of time, and the capacity of other nearby matter to buffer the heat being produced.
It could occur by different paths, e.g. enzymes or catalysts requiring a lower temperature.
When the reaction is catalysed, certainly.
When the reaction occurs as combustion, the carbon chains are broken to produce a series of free radicals that then go on to react with oxygen. That pretty much has to either occur as rapidly as possible or else the free radicals are react with something other than oxygen and the reaction stops for that molecule
But when it’s catalysed, as in biological oxidation, it can take as long as it likes. Each step in the process produces a little bit of energy and another stable molecule such as acetate or citrate, that can sit around for as long as it takes until it comes into contact with the next catalyst and reactant.
That’s all that things like vinegar and citric acid are essentially. They’re partial products of the biological oxidation of glucose that has stalled because of a lack of reactants or enzymes. If you put them back into a cell, eg by eating them, the oxidation reaction will proceed and they will be fully oxidised. But the time it takes from starting to break the bonds in glucose to complete oxidation may be many decades, and theoretically many millennia.
Now to what degree this actually occurs I don’t know. I know that there are some parts of the TCA cycle where the concentration of a more advanced breakdown product inhibits the enzyme and stalls the reaction, but for how long and how much effect that has on the heat generated I have no idea.
My thinking is along the same lines. You might be able to get the same net heat out of different processes of oxidization, but the catalyst, thermal output over time, non-ideal conditions and environment, the oxygen available and the flux of oxygen to the fuel will make for wildly different results.
The metabolism and oxidization of glucose in our bodies is like a laser guided machine on the molecular level. Lots of energy meticulously picked apart and stored in neat boxes for your cells to use a bit over time.
Using thermal radiation (e.g. fire) to act as a “catalyst” for the oxygen and sugar to combust in the open air is like throwing a grenade into your house by comparison.
If that’s right (and I think it probably is), then talking about the ‘temperature of oxidation’ is pretty meaningless. Combustion might need a certain temperature to be maintained in order to be self-sustaining, but otherwise, temperature (in the conventional sense) is a property of matter, not chemical reactions themselves. I think.
But chemical reactions cause a rise in temperature. that’s what we mean when we talk about the heat of a reaction.
Imagine if we could put single molecule of glucose into sealed vessel in a pure oxygen atmosphere and then ignite it. The temperature of the gas in the vessel will rise. It doesn’t matter how log it takes to rise, the rise will always be the same.
Now imagine if we put all the enzymes necessary to run the TCA cycle + some ATPAse into the same vessel, and then drop in a single molecule of glucose. The temperature of the vessel will rise by exactly the same amount. Once again, the time it takes to rise doesn’t matter, the rise should be exactly the same each time.
I believe in the case of oxidization of glucose in the body, the reaction is exothermic; it releases energy/electrons and drops to a more stable, non-reactive state.
Igniting sugar with fire, is an endothermic process, where the glucose has to absorb enough heat (energy/electrons) from another source to kick off the reaction.
What the “temperature” is, depends on where you’re looking and when.
The two question of the thread are:
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Does the temperature rise by the same amount when it oxidises catalytically as when it does so through combustion. Universal consensus seems to be “Yes”.
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Does the temperature at any stage and during any volume during the catalysed reaction equal that of the the combustion reaction? When citrate is cleaved into acetate (or whatever) does that raise the temperature of the surrounding water molecules above 400oC for even one femtosecond? Intuition says no, but intuition is unreliable at these scales.
No, both reactions are strongly exothermic. Both reactions have an activation energy, but that doesn’t make them any less exothermic. Enzymes lower the activation energy of the biological oxidation and decrease the amount of energy that needs to be absorbed, but the energy still needs to be provided: the reaction still slows beyond the effect of Brownian motion if the temperature is decreased. Both reactions cause a loss of electrons to oxygen and produce the more stable end products of water and carbon dioxide.
I’ve got a feeling that this distinction is fairly arbitrary.
If we were talking about some substances other than Glucose, just putting them together with oxygen causes a reaction energetic enough to start self-sustaining combustion.
We’d not be counting the heat added externally to start off combustion, just as we’d not be counting energy inputs contributed by enzymes, would we? Combustion is oxidation, so if you’ve got the same molar quantity in either case (and assuming the reaction ends with the same products), the same quantity of heat should be produced. (noting that ‘quantity of heat’ is not synonymous with ‘temperature’)
Can the exchange of electrons in the process of one molecular reaction be enough energy to excite an H[sub]2[/sub]O molecule to 400ºC?
Gotchya. At first I was thinking oxidization was strictly an exothermic process, but I couldn’t remember if this was true or not.
It must be, otherwise no chemical reaction would be *able *to raise the temperature of water to 400oC, and we know that isn’t true. Fire itself is a chemical reaction, and we know that fire can readily heat water to those temperatures.So somewhere in the mess of reactions that is fire at least one is pushing water molecules from speeds <400oC to speeds >400oC.
But is this true specifically of oxidization of the glucose molecule? Or doesn’t it matter what chemical reaction/matter we’re talking about?
That’s more or less the question I am asking.
I think what you mean to say is that the Activation energy of the combustion reaction is much higher than the enzyme catalysed oxidation.
The overall reaction is exothermic. It does not matter if it is combustion in a test tube or in vivo.
There is more heat being given off by the combustion reaction only because it takes more energy to get it started. The activation energy input is being given off as heat in the output. The net change is the same for both.
One major purpose of a catalyst is precisely to lower this activation energy.
Ahh, thanks Iggy.