1. What Do Your Cells Do With the Oxygen You Breathe In?
When you breathe in, oxygen enters your blood from your lungs and is carried to cells throughout your body. Inside these cells are tiny parts called mitochondria. This is where the body makes most of the energy source (ATP) used to power its functions, as part of the process of extracting energy from food [1].
Inside the mitochondria, oxygen ends up becoming water. This happens because "electric particles" (electrons) taken from food are passed along through several stages, and oxygen is the one that receives them at the very end [1]. It is important to note that oxygen itself does not become energy. The source of power is food, while oxygen’s job is to receive the electrons [2].
You can think of this like a bucket brigade. In a bucket brigade, the last person empties the bucket so everyone in the line can pass the next one. However, in your body, what is actually being passed along are electrons, not water. The water in the analogy and the water made from oxygen are different things.
2. What Happens to Your Body's Energy Production If Oxygen Runs Out?
Your body carries energy extracted from food in a small molecule called ATP and uses it when needed. In the mitochondria, ATP is made from ADP and phosphate in connection with the step-by-step passing of electrons [1].
So, what happens if the final receiver, oxygen, is gone? The passing of electrons stops, and ATP production via the electron transport chain stops. Glucose can still make ATP through a different path called "glycolysis" without oxygen, but this only yields 2 ATP molecules per glucose molecule [4].
In the pathway that uses oxygen, it is estimated that one glucose molecule can produce a maximum of about 30 ATP molecules (specifically 29.85) [5]. Dividing 29.85 by 2 shows this is about 15 times more energy. However, about 30 is the theoretical maximum; actual cells produce fewer. The numbers 36 to 38 found in older textbooks are now considered outdated [5].
Textbooks explain that about 90% of the ATP made when glucose is broken down with oxygen is produced by this system that uses the flow of electrons [3]. This is an approximate value and varies depending on conditions.
3. Why Does the Small Brain Use So Much of Your Body's Oxygen?
The brain weighs only about 2% of your body weight, yet it consumes about 20% of the oxygen used by the whole body when you are at rest [6]. Dividing 20 by 2 shows that, per unit of weight, the brain uses about 10 times more oxygen than the body average.
This value is for adults who are quiet. In children, the brain makes up a larger proportion of the body, and the rate of oxygen use is said to be higher. Numbers like "25%" seen in general articles use different counting methods.
The brain’s nerve cells work constantly, even when you are not thinking. Therefore, it is thought that the flow of oxygen used to make ATP is always necessary. It can also be said that the brain is especially vulnerable when oxygen levels fall.
4. Are There Really Animals That Live Without Using Oxygen?
Most animals live using oxygen. However, in 2020, a team from Tel Aviv University reported that a parasite living in salmon muscle (Henneguya salminicola) is the first multicellular animal reported not to use oxygen for respiration [8]. The report describes it as an animal with fewer than 10 cells and had almost lost its mitochondrial genes. The report states that it does not breathe with oxygen.
So, how does it get energy? This point is still not clear [8].
Another example involves sulfur. In 2017, Tohoku University and others reported that mammalian cells also have "sulfur respiration" separate from oxygen-using respiration. Oxygen respiration produces water, but sulfur respiration is thought to produce toxic hydrogen sulfide [7]. However, this does not mean sulfur can replace oxygen. The group considers sulfur respiration essential for growth.
5. How Can You Explore Energy and Oxygen in Everyday Life?
What you can do today: Look for the "Energy (kcal)" numbers on the back of snack or bread packages. This number shows how much energy the food contains. How it changes into ATP inside the body is not clear from the label alone.
If you want to read more: The JST Science Portal article on "sulfur respiration" [7] is available in Japanese. The explanation of the electron transport chain on Kanroo! [2] is written so you can understand the flow of reactions through diagrams.