Calorie: looking for conversion from heat energy to chemical energy

[GoofyMD]

Hi all, Looking for a bit of help finding articles to show how Calories related to bodily processes.  I know a Calorie (kcal) is the amount of energy needed to raise the temperature of 1 kilogram of water 1 degree Celsius.  I understand the Atwater  system uses the average values of 4 Kcal/g for protein, 4 Kcal/g for carbohydrate, and 9 Kcal/g for fat. But how does the burning of food translate into ATP bonds that we use for energy in vivo? 

 just curious

thanks

[BabcockHall]

The short answer is that oxidation of food creates a proton motive force which is coupled to the synthesis of ATP from ADP and phosphate.  It takes several chapters worth of lectures in biochemistry to provide a detailed explanation.  I would be hesitant to bring heat directly into this discussion.

Edited October 20, 2020 by BabcockHall

[GoofyMD]

Thanks,

I understand glycolysis and remember with fondness memorizing the Kreb's cycle and energy production.  But what I am asking is although we can measure the energy released of  burning food in Joules and we can measure the energy stored in Phosphate bonds in the same units.  It seems to me burning a piece of bread to measure the heat released should not equate to how a human metabolizes that same piece of bread.  no?

[John Cuthber]

A piece of bread is complicated; can we go with a spoonful of sugar?

If you ingest that sugar then  a small fraction of it will be used by the bacteria in your gut.
Most of it will end up in your bloodstream where via complicated pathways of hydrolysis , cleavage and so on,. it will get converted to carbon dioxide and water.
 

But the conservation of energy tells us that , if the starting point is the same- a spoonful of sugar- and the end points are the same- carbon dioxide and water-, then the energy released must be the same.

How much of it is "used" by the body and how much is "wasted" as heat is a separate question.

You can look up the reactions and find out how many molecules of ATP you can produce  by metabolising each molecule of sugar.

And you can also look up the stored energy in a molecule of ATP.

And thus you can work out how much of the energy gets "used" in the body- assuming it  uses the ATP efficiently.
And if you want, you can compare that to how much is released by burning the sugar.

It won't be the same, but, when people talk about energy use in the body, that inefficiency, or wasted heat is included in the calculations.

 

 

[GoofyMD]

1 hour ago, John Cuthber said:

John,

Exactly, thank you.    Now has anyone looked to see how comparable burning sugar  or even 4 Kcal/g for carbohydrates is to the breaking down of chemical bonds?  

I was just thinking about how inaccurate the food calorie system seems to be, especially since so many people judge what to eat or how to loose weight based on calories. 

yes, conservation of energy makes sense. But, If heat energy if being measures in a calorimeter to raise 1 cl of water 1 degree, are we missing the energy expense in light and sound?  And  are metabolized calories entirely different?  What if measured by chemical potential energy, we could eat five times as much licorice for half the calories as we now think?
 

 

[John Cuthber]

55 minutes ago, GoofyMD said:

we missing the energy expense in light and sound? 

They measure the energy content of food by burning it in a bit of kit called a bomb  calorimeter.
There will be some light and sound produced, but those are absorbed in the apparatus and converted to heat.

The point is that all energy can be interconverted, and it's (usually) easiest to measure it as heat.
 

One Calorie (Kcal) is also the energy needed to move something 4.2 metres against a force of 1000 Newtons.

It's the same thing.
If you had a steam engine, you could use the same amount of coal to raise the temperature of the water, or to make the engine lift something.

[CharonY]

3 hours ago, GoofyMD said:

seems to me burning a piece of bread to measure the heat released should not equate to how a human metabolizes that same piece of bread.  no?

A big challenge is that as a whole the metabolization process is very complex and can vary depending on the person, how much the person eats, in what combination with other things the person eats something, the metabolic state of the individual and so on. I.e. calorimetric measures give a rough impression of how much energy can be drawn out, but what precisely happens metabolically is actually quite broad. As with almost all nutritional sciences it is not a complete mechanism-driven biochemical model, but it is more about rough tendencies. At this point we are not even able to perfectly predict metabolic processes in a single cell (we can for major pathways but only if we keep things very, very stable). 

For complex systems such as plants or animals we therefore need to rely on these imperfect approximations, until we manage to get a better understanding.

[GoofyMD]

Thank you all.

It is as I thought, more rough estimates.  I just wanted to make sure I was not missing something.

 

appreciate it.