# Cellular respiration weird question.

1. Nov 12, 2007

### alexwaylo2008

Humans oxidize glucose in the presence of oxygen. For each mole of glucose, 686 Kcal are realeased. This is true whether the glucose is oxidized by cellular respiration or burned in air. So why don't humans spontaneusly combust?

2. Nov 12, 2007

### NoTime

How long do you think it takes for the human body to metabolize a mole of glucose?

3. Nov 12, 2007

### ganstaman

Just a guess that will hopefully be corrected if wrong: If I put glucose and oxygen together in a jar, what will happen?

Better yet, we know that matches are combustible. If I put a match in a jar with oxygen, will it spontaneously combust? Well, even better yet, how long would you have to wait for it to spontaneously combust, on average?

Glucose and matches aren't so different, I suspect, besides the obvious differences...

I'm thinking of delta G, perhaps.

4. Nov 13, 2007

### Jekertee

Oxidation needs electron separation and transfer between atoms, there thus need to be some conditions for that to happen. Oxygen is not automatically combustible or else we are burning our lungs, methinks.

5. Nov 13, 2007

### Staff: Mentor

686 kCal is the equivalent of walking several miles at a very brisk pace.

The rate of aerobic oxidation of glucose is slowed by a large number of factors, the primary one is the rate of oxygen uptake. What is the volume of a mole of oxygen at STP? Now account for the fact that the atmosphere is about 20% $$O_{2}$$ and normal tidal volume (breath volume is less than 700ml), the exchange rate of $$O_{2}$$ between alveolar air and hemoglobin is not 100%, and diffusion of oxygen is required to move oxygen into alveolar chambers.
http://faculty.washington.edu/kepeter/118/notes/pdf-set4/118gas-exchange-06.htm [Broken]

In short - we can't get enough oxygen into our bodies to support a decent flame.
Plus oxidation is going on all at once in cells located all over the place in the body, not inside our stomach. The "fuel" is so spread out so far (diluted) it cannot burn anyway.

Last edited by a moderator: May 3, 2017
6. Dec 2, 2007

### Spirochete

I'm dredging up an old post, so I won't feel bad about giving this guy his homework answer. I think I might disagree with your explanation to this question, not to discount the truth in that article.

This is a synopsis of the standard answer a text will give about this. I'm not sure if lack of oxygen would be a contributing factor if this other stuff weren't true, but then we'd all be really screwed. . .

It's not the lack of oxygen that prevents the reduced carbon molecules in our bodies, including glucose of course, from spontaneously combusting to a more stable, oxidized state. It's the activation energy for such a reaction that prevents it from occuring. Reaction kinetics prevents a thermodynamically favorable reaction from happening. Heat raises the energy of a molecule, which is one way to cross the energy barrier.

Also as was hinted at previously, our cells have developed a system for slowly extracting the energy from food molecules, the electron transport chain. In addition, the body carefully regulates how much energy it wants to generate, storing the rest for later as fat and glycogen. The body doesn't use heat to cross the energy barrier, it uses enzymes which actually lower the energy of activation. This is different from heat, which raises the energy of the reactant. But even if we had evolved to more rapidly oxidize food molecules, oxidizing a single molecule of glucose all at once isn't going to blow somebody up.

Last edited by a moderator: May 3, 2017
7. Jan 26, 2008

### rimewire

Drink gasoline and swallow fire. If you don't burn, tell us, okay?

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