Carbon Cycle: Two Simple, But Tricky Science Questions

OK, this article is a bit more lighthearted, as a little break from the series of articles about teaching languages. We’ll explore two easy science questions that have very simple answers. The answers are no secret, they were known for ages, and they should be easy to answer for anyone who has completed high school. However, many well-educated people, when asked those questions out of blue, don’t know how to answer them, or give wrong answers.

The questions are:

  1. Where exactly does the fat go, when you lose weight?
  2. What do plants grow from (from which stuff)?

These questions are often misunderstood and they are both related to carbon cycle.

So, let’s start with the first one.

As an answer to the first question, most people would say “You burn/melt the fat”, “You use it for energy”, “You use it instead of food” “Calories in, calories out”… or some variation of that. But, let’s see what it exactly means to “burn” the fat, and where it goes when it’s “burned”. The answer to this question comes from the science of chemistry. First we need to see, what the chemical structure of body fat is. It’s composed of triglycerides, and they are esters of fatty acids. As an example, we’ll take tristearin (C57 H110 O6).

Fat burning is actually the process of oxidation. The reaction, which is extremely simplified here (the actual reactions in body involve many steps, enzymes, ATP, etc…),  releases energy which body can use to function. It uses the oxygen from air and fat (here it’s tristearin), and the products are carbon dioxide and water.

2 C57H110O6 + 163 O2 → 114 CO2 + 110 H2O

So the answer to the first question, “Where the fat actually goes”  – is simple… it turns into carbon dioxide and water. You exhale the carbon dioxide, and you pee, sweat and also exhale water (as water vapor). So it goes mostly in the air… and down the toilet!

The more important question is where the carbon component of fat goes, and the answer is simple… only in the air (as carbon dioxide).

Now, the second question.

Imagine the situation. You put a seed in a flowerpot filled with earth, you water it regularly, and in a few months, you have a large plant, grown in the flowerpot. If you put the whole thing on a scale, before and after growing you’ll see it weighs much more than it did when you started growing it, and the only thing you’ve been putting in the flowerpot is the water.  And the amount of earth in the flowerpot hasn’t decreased. You think, the plant must have grown from earth… it has nutrients… and water… yeah right… And then you remember you learned about photosynthesis, so the sunlight is also involved, it gives the plant energy… so it can grow, and it needs carbon dioxide so that it can breathe… the things become a bit clearer when you remember your high school lessons. But most people do not arrive to the very essence of the whole thing, and the essence is: what exactly do plants grow from, (from which stuff), and how they increase in mass? To answer that, you need to have a very basic understanding of photosynthesis:

6 CO2 + 6 H2O + light→ C6H12O6 + 6 O2

(this is also a summary formula, the actual process involves chlorophyll, and many more steps)

So as you see, the plant uses carbon dioxide (from air) and water, and with the help of solar energy, it turns them into glucose, and oxygen. The glucose is later turned into all the other carbohydrates (cellulose and starch) that make the bulk of plant dry matter. So the answer to the question is: plants grow from air and water. And it’s air that contains carbon component of plants, and if the plant is dehydrated and carbonized, most of the dry matter that remains from plant is carbon, and it has come exclusively from air (carbon dioxide in air). Of course plants also need nutrients from Earth, such as minerals, nitrogen, etc. to grow, but the largest amount of plant mass comes from water and carbon dioxide.

So, when someone asks you what plants grow from, the simplest answer that would really explain the essence would be “from (carbon dioxide in) the air, and from water”. When explained like this it’s simple… but it’s still a bit fascinating when you look at the forest, and realize that all those huge amounts of organic matter have actually come from air.

And now you can also understand how forests can help fight the global warming: when trees grow, they trap the carbon dioxide from the atmosphere and turn it into solid plant material, and lower the amount of the carbon dioxide (which is a greenhouse gas that contributes to the global warming) in the atmosphere.

When I first realized this, I started looking at plants and fat loss a bit differently. It has all of a sudden become more understandable and demystified.

Now you see that those two questions are related to each other. They are both about carbon cycle and they show how carbon, the main constituent of organic matter, circulates in nature, mostly through the atmosphere… So your burned fat, in the form of carbon dioxide, goes in the atmosphere, and then plants can use that very same carbon dioxide to grow. So when you are losing the weight, you are, among the other things, feeding the plants. (Maybe that will motivate some of you 🙂   )

That’s the basic idea of carbon cycle.

The whole process can be explained with a diagram:

carbon cycle


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