Why dont humans spontanously combust?

Question

Humans oxidize gluecose in the presence of oxygen. Foreach mole of glucose oxidized, about 686kcal of energy (E) is released. A calorie is the amount of E required to raisethe temperature of 1g of water by 1 degree C; 686kcal=686,000cal. The avg human requires 2,000kcal of E per day, which is equivalent to about 3 glucose mol per day. Given this, why dont we combust?

Answer 1

Look again at the math:

A typical diet could be, as you say, 2000 kcal per day. Or two million real calories. But there are two factors that change what might be the expected result - size and time.

A typical person weighs around 70 kgs. Let's make the (incorrect) assumption that a person is made entirely of water. If you added two million calories to seventy thousand grams of water, it will only change the temperature of the water by 28 degrees. A difference of 28 degrees doesn't seem like enough to combust much of anything!

Now I happen to know that a hand grenade has about a megajoule of energy, which works out to only 239,000 calories. But here's the big difference: the energy from a hand grenade is released ALL AT ONCE, in a fraction of a second.

So yes, if all the energy we consumed on a daily basis were delivered our bodies in a fraction of a second, then you are right: you would see about eight hand-grenades worth of carnage. Fortunately for all of us, the digestive system doesn't work that way!

Hope that helps!

Answer 2

Burning a human being is a favourable reaction, but yet we just don't go on fire. The main reason is that, even though the reaction releases energy, it needs some energy input first before combustion can take place (in technical jargon: the combustion of humans has a very high activation energy). The same reason underlies the observation that a match just doesn't catch on fire spontaneously; it needs someone to provide sufficient initial heat energy to it first by striking it.

Humans oxidise glucose in a controlled manner--largely because of enzymes. Yes, the energy released by a human in a day is sufficient to overcome the activation energy of combustion, but this energy is released in a slow and gradual manner. Given the high surface area of a human body, the heat can dissipate into the surrounding very easily, so does not accumulate and start combustion. So even though a large amount of energy is released, it only warms up a normal human body to about 37C, the normal body temperature. To start combustion, a much higher instantaneous temperature is required.

Answer 3

Way too much water, and out of the 2,000kcal ingested per day, most is expelled as waste, not combusted. Even if 50% were combusted, that's 1500 kcal or 1,500,000 calories, enough to raise 1500 grams 1000 degrees, or 15,000 grams 100 degrees or 150,000 grams (around 330 lbs) of water 10 degrees over a period of 24 hours. Or half of that, 165 lbs 20 degrees over a period of a day. It's a better question to ask how we're able to maintain homeostasis and keep a temp of 98.6F on so little energy consumed. It's not near enough to get to boil, let alone to make fire.

Answer 4

whenever a single glucose molecule is oxidized (through many steps), the energy difference is captured in the phosphate bonds of ATP. Thus, the energy produced by oxidation is used to form an energy currency (ATP) that can be used wherever required in a controlled manner and is not released in the cell to increase its temperature and combust the poor fellow!

Answer 5

Because we don't turn the chemical energy of the things we eat into heat (or not all of it), but to kinetic energy for our movements, and electrical-magnetic energy for our nerve signals.

A big part of the 2000 kcal is turned into heat, but we have a big area of skin where we give off heat to the air, and we need to heat ourselves enough to keep our body temperature at 37 C.

At any given moment, the body frees only as much energy as it needs, the rest doesn't get used, but it is stored as chemical energy in fat molecules (that is the reason we get fat if we eat too much). Were we to convert all 2000 kcal for the day to thermal energy in a single moment, we could really combust, but the body just knows what's good for itself :)

Answer 6

There is some evidence that we can indeed combust. although the scientific jury is still out on this.

Search Yahoo or google and you will find a lot of stuff on this topic

Answer 7

Because we are made mostly of water.

Answer 8

I think because we don't want to. Maybe you should try really really really hard and see what happens.

Answer 9

i guess because we have a lot of water in us

Answer 10

too much water or fat