[robbiep]

* > 2) We don't "burn" any calories whatsoever.*

- It seems like you would know this but we very literally do "Burn" all of our calories that go into heat production -

The Mitochondrial pathways very literally burn/react carbon with oxygen to produce H2O and CO2 incredibly efficiently. (67% of usable energy from this reaction is captured and stored in ATP, the remainder is given off as heat)

This process is significantly more efficient than any 'burning' we do in industry for example, but is exactly the same process as what is happening in a coal power plant, just on a much more efficient scale.

Even if we take proteins -> amino acids -> back to proteins, we are using energy in the form of ATP (the cellular battery) that was the product of the burning in the mitochondrial power plants, in order to have allowed the amino acids to be ready to be formed into new protein.

So we literally DO burn ALL of our calories (You could argue that glycolysis isn't burning, but this process is very inefficient and is responsible for <<<0.1% of ATP production) This is undeniable scientific fact.

[funkwyrm]

Cool explanation. I didn't conceptually get that the mitochondrial energy process is the same as a coal power plant. But if everything gets burned, how do I build new cell walls and stuff? Where does the actual matter come from?

Some of the matter must be used to create matter directly, rather than as simple an energy source. Or if all the matter in my body is grown/created entirely from energy and cell division (can't be true of essential vitamins, minerals, amino acids, and fatty acids, by definition) isn't growing and repairing my body going to be a huge factor in energy consumption?

[robbiep]

Thankyou. It is quite amazing and fully understanding it carries a sense of deep beauty and appreciation for the complexities and interdependencies by which our body sustains life.

re your question: You have 2 overarching (and this is simplifying slightly) thermodynamic gradients operating in the body: Catabolism and Anabolism.

Catabolic processes break down polymers (of which everything is composed). For example: in the Carbohydrate family, we have Starches or Glycogen; a polymer of Glucose monomers. In the Fats family, Triacylglycerides (TAGs) and Fatty acids are 3 and 2 polymer complexes of the standard 'fat' chain. Proteins are polymers of the 20 biological amino acid monomers, which are combined in an infinite variety of combinations and lengths to create the different structures of our body.

We break Fats and Carbs down to their monomers to ingest them into the body (enzymes etc in the Gut). Malabsorption here will result in excess nutrients in our GIT which bacteria can then feast on, causing medical symptoms of maldigestion. Bacteria also operate in a complex synergy with our GIT to keep everything 'regular' however that's far beyond the scope of this discussion, and pushing the limits of the understanding of science even.

Further catabolic processes go on to take fats and carbs down to the burning stage in the mitochondria, which results in the production of ATP, the body's fuel source (you can think of carbs and fats as the crude unrefined elements and then ATP is the refined, useful fuel which every source in the cell which requires energy can utilise)

Production of ATP is in itself a Anabolic process by definition, as it is a more energetic molecule than it's precursors. Note that the production of ATP is coupled to a catabolic process, the burning of carbon in the Krebs Cycle inside the mitochondria.

ATP is the energy source which is then utilised by different proteins in the cell to provide the energy to catalyse the formation of monomers into polymers - this is the process that creates structural proteins, the elongation and duplication of DNA, elements of the Phospholipid Bilayer (the cell membrane - which has 2 fatty acids/fats as the tail) and all the other cellular components.

Note that Anabolism is an energetically expensive process, as you are building molecules which have more energy than the precursors, and so you require a fuel source for this. this is really what we mean when biologists say that the whole system is driven by thermodynamics.

---

TL;DR - We break everything down into it's monomer subunits to get it into our bloodstream. Special transports exist for Amino Acids, Vitamins, Nucleotides, Simple Carbs, Fats etc. Once inside, our protein cellular machinery stores some energy (in Glycogen or Fat), Burns some energy (Energy consumption is coupled to energy expenditure at the mitochondrial level - rate of production of ATP is limited by rate of consumption; O2 is the limiting factor when consumption is high), and uses amino acids and fats, and carbons from Carbohydrates as different structural and functional components in our cell.

So yes, you are right, we don't burn everything to H2O and CO2. But essentially everything that produces the useful energy in our body does go through this process.

I hope this sort-of answers your question even though I got a bit side-tracked

[Mz]

Lay(wo)man's understanding: You burn the oxygen, one molecule at a time. But there is more in your food than oxygen.

[robbiep]

I think you mean carbon instead of oxygen, and it's 2 at a time, not one at a time.

Your analogy is roughly correct but in the base, reductionist biochemistry of which the body functions, you have fats (chains of carbons with hydrogens on them - directly burned this way), carbs (basic form C6H12O6) which are also directly burnt this way with water coming off, and protein, which has occasional nitrogen atoms and some sulphur atoms. Proteins can also carry metals - so bits of iron or copper or other things we need to make some proteins function correctly are 'carried' into the intestinal system this way. The only other thing we really need is lots and lots of Phosphate, which we take in largely through consumption of DNA in things we eat

If proteins are to be burnt in mitochondria the nitrogens are converted to urea and excreted.

Vitamins are just arrangements of carbon atoms with the occasional nitrogen and oxygen and hydrogen around. Some have a single copper or zinc atom there too.

We really don't use any more than this, our body literally does just run on carbon. To complete your statement, although there is more in food than just carbon, there really isn't much that we are interested in (biochemically speaking) than the carbon, although the body has uses for some of the myriad forms that are created in other plants and animals that we can't synthesise ourselves.

[Mz]

You know, I feel really crappy today. I am not finding anything that readily matches my recollection of the process, though glycolysis and atp come up in my search and that partially matches my memory. Maybe you could link me to something that fairly clearly shows what you mean? My understanding is "burn" = "consume oxygen".

Thanks.

[robbiep]

burning is oxidation, and oxidation requires O2 (although it doesn't always require O2 -oxidation chemically speaking is stripping Electrons and Hydrogen atoms out of a molecule. Which liberates energy.

So you are right. My take on your comment was that you had confused Carbon and oxygen... Since Carbon is the fuel and Oxygen is the oxidising agent , and the convention is basically to talk about the fuel as the one that is being consumed.

I don't have any videos or anything but if you are interested I have attached a link to my old biochemistry notes which, especially if you have a rusty background in anything resembling science, should hopefully jump you along a bit

http://sdrv.ms/SHJQHs

[Mz]

Thanks. I will try to look for something some other time to clarify my understanding. Some time when I feel less crappy.