They are found in almost every cell in our body. Without it, we couldn’t produce energy. We couldn’t breathe, nor would our hearts beat. We couldn’t move a muscle, nothing would work. This refers to a tiny organelle in our cells with the peculiar name mitochondrion.
Scientists have suspected their existence for a long time. But they could only be seen after the invention of the electron microscope around the middle of the last century. Mitochondria are as tiny as bacteria. But what an inner life they have! They are rightly called the powerhouses of our cells because by breaking down food they produce most of the energy that our body uses for all of its functions.
Occurrence and Structure
Mitochondria are found in all cells in our body except red blood cells. Around 1000 mitochondria per cell are typical. This large number already shows us how tiny they must be, when so many of them fit into one cell. They occupy up to 25% of the cell volume. Cells that use a lot of energy have the most mitochondria. These include the muscle, nerve and sensory cells.
Mitochondria are enclosed by two membranes. The outer membrane is smooth and contains many tunnel-like channels through which small molecules can be channeled that our body is constantly metabolizing from our food. The inner membrane, on the other hand, is impermeable to almost all molecules. Only the smallest such as water, carbon dioxide and oxygen can pass. But it has many transport systems in order to selectively channel various metabolic products from our diet into the interior. In order to be able to do all this filtering it is strongly folded to create a large surface.
Inside of the mitochondria, the so-called matrix, many metabolic activities take place. We will come back to that later. The mitochondrion itself has its own DNA like the cell nucleus. So it can divide itself. Its lifespan is not very long, only around 10 to 20 days. It is constantly reproduced by transverse division.
Task of the Mitochondria
The main task is to absorb important metabolic products from our food and convert them into energy. Other residual substances must be converted into a form that can be easily excreted from the body. This includes, for example, the urea cycle, which partially is happening here as well.
Let’s look at the breakdown of fat from our food. We eat fat because, among other things, we want to use it to produce energy. Our body is a real miracle and a huge, elaborate chemistry lab. It takes many complicated steps to go from one tablespoon of oil to energy. We want to summarize the whole process in a somewhat simplified way. Our pancreas provides enzymes for fat digestion. The bile breaks up a large drop of fat into many small droplets so that the enzymes can operate more easily. They split the fat molecules into fatty acids and glycerine, that can be absorbed through the intestinal wall. After they have passed this barrier, they are reassembled back into real fat molecules. Since fat is insoluble in water and cannot simply swim in the lymph or blood, it becomes bound to lipoproteins. With this taxi, the fat finally gets into the cell, where it is to be converted into energy.
This transformation happens in our mitochondria. To do this, the fat molecule must first be broken down into glycerine and fatty acid. However, the fatty acid is too large to get through the membrane into the interior of the mitochondrion. That is why a transport system is needed. Carnitine serves as a taxi, which every athlete knows very well, believing that he needs a lot of it so that he can burn more fat in order to get more energy. If the fatty acids are in the mitochondria, they are together with oxygen converted into energy, water and carbon dioxide via a complicated process called beta oxidation.
The ATP (adenosine triphosphate), as this chemical energy is called, is created inside the inner mitochondrial membrane. The body needs this energy to move its muscles, to produce vital organic molecules and to handle transport processes in the cell and from one cell to another. In an adult human, the amount of ATP that is built up and broken down in his body every day is roughly equivalent to his body weight. What an achievement!
The glycerine from the fat is introduced into the citric acid cycle. This is another extremely complicated cycle that also takes place inside the mitochondria. They are an important hub in the metabolic process. They break down components from our food and at the same time rebuild new substances such as amino acids, the smallest components of protein. The breakdown products of carbohydrates and protein metabolism also enter the citric acid cycle. Components can also be diverted from this process in order to generate energy. We see already the important role of mitochondria in our body´s metabolism.
Fats provide by far the largest amount of energy per weight, followed by carbohydrates. Energy production from protein is not very efficient and is only used when there are not enough fats and carbohydrates available.
Conversion of Nutrients
Carbohydrates are important sources of energy. Some cells like the red blood cells, the nerve and brain cells absolutely depend on glucose. Glucose is therefore stored in the liver in the form of glycogen. This substance can be split up again into glucose between the meals, when no new glucose is available. If more carbohydrates than necessary are consumed and all glycogen stores are filled, carbohydrates are converted into fat. This is stored in the fat cells. Caution: this can lead to obesity!
When fasting, after a while the brain and nerve cells even get used to obtaining energy from fatty acids. But the red blood cells are always dependent on glucose for energy. If almost no carbohydrates are ingested with food, as is the case with some restrictive diets, the body has to produce glucose from amino acids. This path is energetically very demanding.
A Varied Diet
We have seen that some nutrients can be converted into one another. With regard to their function as building blocks for bones, organs, teeth and much more, they cannot be exchanged. It is therefore important that we eat a balanced and varied combination of foods and avoid unilateral diets. The best diet consists of lots of fruits and vegetables, grains, nuts, and seeds. From all this variety of foods our body is optimally supplied, and we can produce enough energy for all its vital functions.
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Esther Neumann studied Nutrition at the University of Vienna. Since then she served as an author for the health magazine “Leben und Gesundheit” and conducted health lectures in various locations of Austria.