[John Chapman (OP)]

Without giving it a great deal of thought, I have always assumed that digestion, decomposition and combustion are all the same process, the only difference being the speed of the reactions involved and the environment. Is this tue?

My Grandad once told me that migratory birds disappear in winter because they turn into fish. Please tell me that I'm not turning into my Grandad and that assumimng digestion and combustion are chemically the same isn't as ridiculous!

[RD]

My Grandad once told me that migratory birds disappear in winter because they turn into fish.

Barnacle geese ?

The natural history of barnacle goose was long surrounded with a legend claiming that they were born of driftwood

This belief may be related to the fact that these geese were never seen in summer, when they were supposedly developing underwater (they were actually breeding in remote Arctic regions).

Based on this legend — indeed, the legend may have been invented for this purpose — some Irish clerics considered barnacle goose flesh to be acceptable fast day food,

http://en.wikipedia.org/wiki/Barnacle_Goose#Folklore

[Chemistry4me]

Digestion is breaking up large molecules into smaller ones so that they can be absorbed into the bloodstream and used by the body. Decomposition: to break down organic matter from a complex to a simpler form, mainly through the action of fungi and bacteria, or be broken down in this way. In terms of chemistry, decomposition is to separate into constituent parts, or cause something to separate into its constituent parts. Combustion is a chemical process in which a substance reacts with oxygen to produce heat and light.
 
 

[John Chapman (OP)]

Thanks for that Chemistry4me

I know what each of these things are and what the differences are. What I was wondering about is what they each have in common.

It seems to me that since both digestion and decomposition are in part driven by bacteria breaking large organic molecules into smaller ones then the process might be exactly the same, differing only in the location that the reaction occurs. As chemical bonds are broken energy is released and exploited by the bacteria (and the products are exploited by the host in the case of digestion and mainly flora in the case of decomposition). With a bit of encouragement the bonds can be encouraged to break so fast that the energy is released largely as heat which encourages rapid oxidation breaking more bonds, releasing more heat and so on. Light energy is also created and we have combustion. I can see now that this process is a little different from the other two but the end result seems much the same. In all three cases the reaction is fuelled by oxygen (taken in as part of respiration in digestion) and similarly in decomposition (at least aerobic decomposition) and also to fuel combustion.

I once read an article in which it was suggested that the similarity between digestion and combustion might explain the phenomenon of Spontaneous Human Combustion which, it was alleged, start in the mid region and burn in both directions often leaving arms, head and legs untouched. If such a thing exists, that is.

Finally, I once had a go at building an anaerobic digester to rot a mixture of food waste and cow poo and produce methane. The attempt failed and my prototype, made from sweet jars, exploded filling the kitchen (yes, I know. But I'm a man) with sh1t. The significance of this story is that it was called a digester, not a decomposer. It seems that the two words may to some extent be exchangeable.

What do you think?

[Chemistry4me]

Digestion is helped by mainly enzymes. That is all that I can think of at the moment, of course, there will be similarities... as for the anaerobic digester  [xx(][xx(][xx(]...

[lancenti]

Decomposition and Digestion are, technically, both driven by enzymes and not bacteria.

When we have a piece of meat buried under the earth decomposing, or just rotting (we assume the insects aren't around), bacteria will release their digestive enzymes to cause the decomposition of the larger protein molecules into smaller, more manageable chunks that are ready to be absorbed. Similarly, we have multiple enzymes in our body specialized for digesting proteins, fats, and carbohydrates. These are found in our saliva, as well as the intestinal juices in the small intestines.

While it does seem like we could say that the reaction is fuelled by oxygen as the hydrolysis of peptide bonds or amide linkages (between amino-acids) among other things depend highly on the nucleophilic character of water, fuelled is not the correct term. The hydrolysis itself doesn't use O₂ and while we could see it as ATP being used to open the channels and absorb the processes, as far as the reaction itself is concerned oxygen doesn't fuel it.

I'm not sure why this machine of yours was called a digester, but it seems more like forcing the bacteria responsible to undergo anaerobic respiration so that they would not oxidize whatever the product was supposed to be.

While I think it's perfectly fine to say that a decomposing piece of food is being digested by bacteria, "decomposing" alone doesn't exactly work as inorganic molecules such as MgCO₃ decompose under heat by the following equation:

MgCO₃ → MgO + CO₂

And I don't think you can call that digesting.

Combustion, on the other hand, always involves oxygen being burned... now I'm going to have to think a bit more on whether oxidation can be considered a combustion reaction, but I'm leaning on the side of no.