Combustion, Or What Happens When We’re Generating Energy.

We combust fuels externally, whether in cars or in power plants. But what happens internally?

Here’s another basic chemistry lesson that we can go through today. What happens during combustion?

From the perspective of industrial sized power plants and machines, we’re looking at the oxidation of hydrocarbons. Gasoline in cars, for instance, is the most commonly used hydrocarbon for powering cars, and the most common chemical representative for gasoline is octane, or C8H18 (hence flashy sports cars are regularly termed “high octane”.)

This octane (or any hydrocarbon, really) has to react completely with oxygen to form carbon dioxide and water as the end products. The flow of oxygen-containing air into a car’s engine allows it to combust the octane that is being fed into it, such that the chemical energy that is released from the combustion can be converted into mechanical/movement energy for the car to move. In the same way, the chemical energy derived from the combustion of fossil fuels at a power plant can also be used to drive turbines to generate electrical energy.

---

But what happens with insufficient oxygen?

If the oxygen intake is lower than necessary, some of the carbon would not be completely converted into carbon dioxide. There would be other by products such as carbon monoxide, which is toxic to human life.

There would also be carbonised residues left within the pipes, which leave an icky, gummy mess that we wouldn’t really want to clean out ourselves. Even worse, though, some of this uncombusted material can be emitted into the atmosphere, which we can sometimes see as black soot particles emanating from the exhaust pipe of big trucks.

When there is black soot/smoke, we can tell immediately that the oxygen intake into the truck is insufficient, and it’s neither good for the truck’s energy efficiency nor the environment.

---

In the human body…

Our cells also make use of a low temperature combustion reaction to extract biochemical energy out of the carbohydrates (glucose) or the fats (ketones) that we consume, as I examine in The Science Of Ketosis. We are feeding in molecules that contain carbon, hydrogen and water.

Even without the cells, we can burn the carbohydrates or the fats chemically. Just heat a tablespoon of sugar over an open flame. It will react with atmospheric oxygen, releasing some carbon dioxide and water, before finally leaving a blackened, sooty, carbonaceous mess that is difficult to clean.

But our cells do it differently. The glucose in the diet is converted into pyruvate, and the pyruvate is converted into acetyl-CoA. This acetyl-CoA is then sent into the tricarboxylic acid (TCA) cycle. One molecule of glucose produces 2 molecules of pyruvate.

Now, there are 2 ways of metabolising the pyruvate in the body: aerobically and anaerobically. This is a biochemical combustion of the pyruvate.

When there is sufficient oxygen in the blood to metabolise all the pyruvate aerobically, the pyruvate is completely converted into carbon dioxide and water, similar to the combustion of octane in car engines. This is known as aerobic metabolism.

When there is insufficient oxygen in the blood, the pyruvate gets converted into lactate. This is known as anaerobic metabolism.

When we are operating at a sedentary pace, our fuel burn rate is lower, and we can take normal breaths.

However, if we are engaged in a vigorous activity, our fuel burn is higher, and we will need to take deeper and more frequent breaths to increase our oxygen intake, which promotes the aerobic metabolism of the pyruvate.

If we are unable to sustain our breathing pace, we would have to start relying on anaerobic metabolism to continue producing ATP for energy use. However, anaerobic metabolism is much more energy inefficient than aerobic metabolism, producing only 2 molecules of ATP per molecule of glucose (as compared to 34 molecules of ATP per molecule of glucose in aerobic metabolism).

Hence, the challenge is to be able to sustain the ATP production rates required for the vigorous activity. ATP production is a sequential process — in Where Does The Body Get Its Energy And Stamina From?, I outline the various nutrients that support the electron transport chain. We do see that ATP synthesis is an oxidative phosphorylation process — meaning that oxygen is involved. And definitely, the majority of the ATP that our bodies produce are derived from aerobic metabolism, which again underscores the importance of a good oxygen intake via our lungs.

In this article, I examine the feed source — the acetyl-CoA that is necessary to feed the TCA cycle, which then provides the NADH that is necessary for the electron transport chain to function.

Hence, for the sports people who take their craft seriously, the VO2 max rate is an important parameter for them to maintain. It is a parameter that indicates how much oxygen they can consume while working at peak activity rates. A lower VO2 max rate indicates that one has a lower peak activity, and that could mean the difference between getting an Olympic medal or going home empty, especially for the professional athletes.

Interestingly, it is the same phenomenon that is expressed in different ways. Insufficient oxygen to a vehicle engine? Soot and smoke appear, and that highlights the energy inefficiency in fuel combustion. Insufficient oxygen to the body? Our bodies would also exhibit energy inefficiency in fuel combustion, just that we’d be exhibiting it in a biochemical sense!

Dr J's recommendations for a sustained energy boost to the cells can be found right here!

---

This article was originally published in Medium.