Normal metabolism

I will describe how normal, food metabolism works in our bodies, based on my understanding of the current and well-established scientific understanding.  Metabolism is the usual way our bodies derive energy from food.

When we eat food, our body breaks it up into various food-fuels which are absorbed into the bloodstream.  The most common food-fuel in healthy people's blood is glucose, which is formed from the breaking up of the carbohydrates we eat.  Other food fuels include fatty acids, coming from the fats we eat, and amino acids, from the proteins we eat.  Each of these is a molecule - a collection of atoms joined by chemical bonds which contain energy. 

In our blood, these molecules are broken down further in a chemical reaction, releasing their energy, which our bodies then use to fuel themselves.  There are also by-products of using up the food fuels, including carbon dioxide (CO2) which is useless to the body, and water (H2O) which can be re-used by the body.  I'll explain this in more detail, using the diagrams below, taking glucose as the example.

Glucose is a molecule which has carbon, oxygen, and hydrogen atoms.  Our bodies at any time should have millions of glucose molecules.  Here is a diagram of one:

~ As you can see, the grey dots represent carbon atoms, the blue dots oxygen atoms, and the aqua/cyan are all hydrogen atoms (I didn't label them all).  The black lines represent chemical bonds, and also show the arrangement in which the atoms are joined to make this one glucose molecule.

For those who didn't take chemistry class, chemical bonds contain energy.  When any chemical reaction happens, the bonds are broken, releasing energy.  New chemicals bonds are then made between some of the atoms, just in a different arrangement.  The number of atoms stay the same - just the position of bonds changes. 

Our bodies take in oxygen when we breathe.  Oxygen gas comes into our lungs and is absorbed into our blood.  Oxygen gas is made up of two oxygen atoms joined by a chemical bond, hence it's symbol O2.

~ Here is a diagram of what starts to happen when a glucose molecule meets 6 oxygen molecules.

~ So, you can see the glucose molecule is still the middle, now with 6 O2's which were absorbed into the blood from the lungs.  The stars represent energy being released when the bonds are broken.  This energy is then available to be used by the body to stay alive and function.

But when the bonds are broken, this means the glucose molecule is destroyed.  The number of atoms remain the same, but they form new molecules - the following diagram shows what is created:

So you can see, when our bodies metabolise glucose, a food-fuel, then our bodies create carbon dioxide (CO2) (a carbon atom with two oxygen atoms), water (H2O) (an oxygen atom with two hydrogen atoms), and in the process gain a bit of energy which we then use to stay alive and do stuff.  Of course, our bodies are doing this continually, with countless glucose molecules. 

What happens to the H2O (water) and CO2 (carbon dioxide)?  We can re-use the water created, just like we can use the water we drink.  The CO2, however, is useless to us - it travels with the blood back to our lungs, where it passes into our breath, and is breathed out.

(For those wondering, glucose's bonds break, releasing energy, but the making of new bonds to form H2O and CO2 takes energy - just far less than the energy released by the glucose, hence our bodies get a net increase in energy.)