Energy in the organism

The main carrier of energy in our organism is ATP. Its full name is adenosine triphosphate, but we do not need to remember this name. This is a molecule with a "kernel", to which a chain of three phosphate groups is attached. The kernel is composed of a molecule of adenine and a molecule of ribose. This is the molecule of ribose, to which the chain of phosphate groups is attached. When one phosphate group gets detached, a certain amount of energy is released, which is used for life processes, and the ATP molecule ceases to be ATP and becomes ADP – adenosine diphosphate. 

It is possible to separate one more phosphate group with a release of some more energy, even though less than at separation of the first group, and after that, ADP will turn into AMF – adenosine monophosphate. But it is not possible to separate the last phosphate group. Some more energy may still be squeezed out of AMF, but this will be done only in a critical situations, in life or death matters. And, after that, what will be left of the AMF molecule will have to be thrown into the wastebin. 

ATP can approximately be compared with a charged battery (like that of a mobile phone). This is a tiny battery, and it discharges not gradually, but in two steps, with a release of some fixed portions of energy. Though, there are very many such batteries (Just remember that huge Avogadro number showing the number of molecules in one mole of a substance). So, it is always possible to take a required number of  ATP molecules to obtain a required amount of energy. Or not always. If all ATP molecules are completely discharged to zero, processes of life stop, which means death. 

But while life continues, these molecular batteries are constantly recharged. To ADP, one phosphate group is attached, and to AMP, two phosphate groups. For this, energy is required, like when a usual rechargeable battery is charged. After this, ATP molecules charged with energy can be used again as a source of energy for life processes. 

In a battery of a typical gadget, there are losses of energy both when it is charged and when it is discharged. It is the same with the molecular ATP batteries, but these losses are much smaller than in alternative variants. This is why the terrestrial life chose ATP as the main energy carrier at very early stages of evolution..

For how long will the charge in the ATP batteries last, if they are not constantly recharged? Not for long at all. If, at some moment, all ATP molecules are charged, and then their recharging suddenly stops, all their charge will be spent in less than two minutes, and death will come because all life processes will stop. And this is if there is no physical activity at all. If this activity is very intense, there will be not enough for more than 20 seconds.  

So, ATP molecules need to be constantly recharged. More precisely, AMF and ADF molecules need to be charged to obtain charged ATP molecules. To charge the battery of a gadget, we connect it to the electric grid, and the energy from the grid is "pumped" into the battery. This energy comes into the grid from electric power plants. And power plants take it from somewhere, because energy does not appear and disappear, according to the law of conservation of energy. There are several ways of obtaining energy for production of electric power. One of them is burning coal, or natural gas, or petroleum products, that is, fuel. At this, fuel is oxidized by oxygen from the air, and combustion products are released – carbon dioxide when coal is burned; carbon dioxide and water, when natural gas or petroleum products are burned. There are some other combustion products too, though, in smaller quantities, because not only carbon and  hydrogen are present in the fuel, and not all fuel burns up completely.

Perhaps, burning of carbon fuels is not the best way of obtaining electric power, but we do something similar for charging our ATP molecules. No, nothing is burning inside us, but a very similar process of oxidation of molecules that contain a lot of atoms of carbon and hydrogen (almost like methane, the natural gas, or petroleum products). And the main products of oxidation are the same – carbon dioxide and water (and some other substances in smaller quantities). And then, charged ATP molecules transport energy to the places of its use. 

We have two ways of charging ATP molecules. 

The first is oxidative phosphorylation. This is a highly efficient way. Inside our cells, there ae special formations (organelles) named mitochondria. Sometimes, mitochondria are called power plants of cells, because they give energy for all processes in cells, but, like real power plants that provide energy not directly, but through the electric grid, mitochondria supply energy through ATP molecules. Mitochondria "burn" fuel in oxygen that we breathe in and charge ATP. In fact, it is a very complicated and multiple-stage process.  

The second way is very ancient and inefficient. It has been used since the times when there was no oxygen in the atmosphere of Earth. This is fermentation, and it does not need oxygen. This method is still preserved as a backup for situations when there is not enough oxygen. This happens, for instance, when physical activity increases abruptly – discharge of ATP molecules speeds up sharply, and they need to be urgently recharged, but supply of oxygen cannot be increased immediately to match this need, because it takes time to speed up breathing, and then time for the additional oxygen to be delivered (by blood) to the places where it is needed. In fermentation, instead of carbon dioxide and water, other products are formed, mostly lactic acid and even some ethanol (like, for instance, in making sauerkraut or brewing beer – these processes are also examples of fermentation).

A small amount of ATP, mostly in muscles, can be charged without oxygen and without fermentation, at the expense of energy accumulated in phosphocreatine. This mechanism is intended for having a possibility to abruptly increase physical activity, for instance, in case of danger. But when this storage of energy is not enough (and it lasts only 5 to 8 seconds of increased physical activity), the organisms adds fermentation until supply of oxygen increases sufficiently for oxidative phosphorylation. And this phosphocreatine first must be made out of creatine with the energy from ATP. 

We have cells, for which, the second way (fermentation) is the main and the only. These are erythrocytes, the red blood cells. Their function is to deliver oxygen to all other cells. So that they deliver oxygen rather than consume it for their own purposes, they do not have mitochondria (and neither the nucleus), and they charge their ATP molecules via fermentation only. 

And sometimes, mutant cells appear in us. In them, mitochondria are broken and do not work. Therefore, these cells use fermentation for obtaining ATP. These are most kinds of cancer cells (or maybe all kinds). 

For oxidative phosphorylation (charging of ATP molecules), mitochondria can use any of the following "fuels": 

● glucose;
● fatty acids;
● ketone bodies (usually called just ketones).

For "burning" this fuel, oxygen is needed.

There is a peculiarity of brain cells. The brain is protected by a special brain barrier, which fatty acids cannot pass through. While molecules of glucose and ketone bodies pass through this barrier without obstacles. Therefore, mitochondria of brain cells use only glucose and ketone bodies as fuels, but not fatty acids. 

The substance for charging ATP molecules through fermentation is glucose. Fermentation does not require oxygen. Fermentation occurs without participation of mitochondria and is a backup mechanism for charging ATP molecules. For the red blood cells (erythrocytes), fermentation is the only source of energy for charging ATP molecules.

Of course, from food. But we almost do not eat glucose in pure form. And ketone bodies are almost absent in our food. And we rarely get fatty acids in pure form; we consume fats, which include fatty acids. Normally, we do not eat amino acids; we eat proteins. Our food passes a complicated way of transformations before it becomes "fuel" for mitochondria or material for fermentation. 

We get glucose from carbohydrates (often called carbs). Each molecule of starch (the main nutrient of grains, potatoes and plants in general) is split by enzymes into many molecules of glucose. Each molecule of sugar is split into a molecule of glucose and a molecule of fructose. Besides, our liver can produce glucose out of fats (around 10% of fats can be converted into glucose) and out of some amino acids. 

We get fatty acids from fats. Fats are combined molecules composed of a molecule of glycerol, to which three molecules of fatty acids are attached. Because of this, fats are also named triglycerides. By splitting a molecule of fat into its components, we obtain three molecules of fatty acids. There are different fatty acids, and not all of them are used as fuel. Some of them have other uses. Also, when we have an excess supply of glucose, we can convert it into fats, and then, when glucose is in a short supply, we use these fats for making glucose and fatty acids. 

Ketone bodies are produced by our liver out of fatty acids. This occurs under special conditions. It is believed that ketone bodies are the best fuel for mitochondria. They can also be consumed by the brain, and it is believed that the brain works much better on a mix of glucose (around 30%) and ketone bodies (around 70%), than on pure glucose. Fatty acids cannot get into the brain as fuels, while ketone bodies do.

We get amino acids from proteins. There are rather many different amino acids, around 500. But living organisms of our planet use only 20 of them to build their proteins. And even to these 20, there are special requirements. Two molecules of the same amino acid may be the same in their atomic composition, but mirrored in their spatial structure, and only one of this pair is suitable for making proteins.

Alien proteins can cause havoc in the organism. Therefore, the organism does not permit their penetration, and when it happens, activates the immune system to eliminate the intrusion. Proteins that we eat are disassembled into amino acids in the stomach and intestines, and then these amino acids are absorbed by our organism. They are used for making our own proteins. How many different proteins exist in our body is not known for sure. Their number is estimated from several hundred thousand to about two million. Mostly, proteins act either as structural elements or as nano-machines performing certain work, or combine these functions.  

Proteins are usually a nutrient in a short supply. We constantly renew our proteins, therefore we need to ensure a regular supply of new amino acids by consuming proteins with food. However, various amino acids are present in food not in the proportions necessary for our own proteins. For this reason, a surplus of certain of the amino acids occurs. Some of these amino acids, our liver can convert into glucose, and thus they become an additional source of energy.

ATP is the source of energy for most processes in our organism. Molecules of ATP can be charged and discharged by attachment and detachment of phosphate groups. When they are discharged, they release energy for processes of life.  All ATP molecules in our organism contain only a small amount of energy, for less than two minutes of life, therefore, discharged ATP molecules must be urgently recharged. 

ATP molecules are charged in one of two ways.
1) In mitochondria, by oxidizing "fuel" – a highly efficient way.
2) By fermentation – an ancient and low-efficient way, but it does not require oxygen. 

And there is one more way of charging ATP molecules without oxygen or fermentation – from phosphocreatine. This way is almost exclusively used in muscles for an urgent increase of physical activity in the case of danger, and it is limited to only several seconds. 

The "fuel" for charging ATP molecules:
1) With oxygen, in mitochondria: glucose, fatty acids and ketone bodies.
2) Without oxygen, by fermentation: glucose.

We take this "fuel" from food, but not immediately. We get glucose from carbohydrates, and can make it out of fats and some amino acids. We get fatty acids from fats. Ketones are produced by our liver from fatty acids. And we can convert excess glucose into fats.