Energy in the cell. Use and storage

Hello! I wanted to devote this article to the cell nucleus and DNA. But before that you need to touch on how the cell stores and uses energy (thanks to spidgorny ). We will deal with energy issues almost everywhere. Let's understand them in advance.

What can you get energy from? Yes, of all! Plants use light energy. Some bacteria too. That is, organic substances are synthesized from inorganic at the expense of light energy. + There are chemotrophs. They synthesize organic substances from inorganic substances due to the oxidation energy of ammonia, hydrogen sulfide, and other substances. And there we are with you. We are heterotrophs. Who are they? These are those who can not synthesize organic matter from inorganic. That is, chemosynthesis and photosynthesis, it is not for us. We take ready organic (eat). We disassemble it into pieces and either use it as a building material or destroy it for energy.
What exactly can we disassemble into energy? Proteins (first assaying them for amino acids), fats, carbohydrates, and ethyl alcohol (but this is optional). That is, all these substances can be used as sources of energy. But for its storage, we use fats and carbohydrates . I love carbohydrates! In our body, the main storage carbohydrate is glycogen.


It consists of glucose residues. That is, it is a long, branched chain consisting of identical links (glucose). If necessary, we split off energy one by one from the end of the chain and oxidize it to obtain energy. This method of obtaining energy is characteristic of all cells of the body, but especially a lot of glycogen in the cells of the liver and muscle tissue.

Now let's talk about fat. It is stored in special connective tissue cells. Name them - adipocytes. In fact, these are cells with a huge fat drop inside.
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If necessary, the body removes fat from these cells, partially splits and transports. At the place of delivery, the final splitting occurs with the release and conversion of energy.

Quite a popular question: "Why can not you store all the energy in the form of fat, or glycogen?"
These sources of energy have a different purpose. From glycogen energy can be obtained quite quickly. Its splitting begins almost immediately after the start of muscular work, reaching a peak by 1-2 minutes. Fat splitting proceeds by several orders of magnitude slower. That is, if you sleep, or slowly go somewhere - you have a constant expenditure of energy, and it can be provided by splitting fats. But as soon as you decide to accelerate (the servers have fallen, they have run up), a lot of energy will be required and you will not be able to quickly split it. Here we need glycogen.

There is one more important difference. Glycogen binds a lot of water. Approximately 3 g of water per 1 g of glycogen. That is, for 1 kg of glycogen it is already 3 kg of water. Not optimal ... With fat easier. Lipid molecules (fats = lipids), in which energy is stored is not charged, unlike water molecules and glycogen. Such molecules are called hydrophobic (literally, those who are afraid of water). Water molecules are polarized. It looks like this.


In essence, positively charged hydrogen atoms interact with negatively charged oxygen atoms. It turns out a stable and energetically favorable state.
Now imagine lipid molecules. They are not charged and cannot interact normally with polarized water molecules. Therefore, a mixture of lipids with water is energetically unfavorable. Lipid molecules are not able to adsorb water, as glycogen does. They “pile up” into so-called lipid drops, they are surrounded by a phospholipid membrane (one side is charged and facing the water from the outside, the other is not charged and looks at the lipids of the drop). As a result, we have a stable system that effectively stores lipids and nothing superfluous.

Okay, we figured out the forms in which energy is stored. And what happens to her next? Here we split the glucose molecule from glycogen. Turned it into energy. What does it mean?
We make a small digression.

About 1,000,000,000 reactions occur in a cell every second. When the reaction proceeds, one substance is transformed into another. What happens to his internal energy? It may decrease, increase or not change. If it decreases -> energy is released. If it increases -> you need to take energy from the outside. The body usually combines such reactions. That is, the energy released during the course of one reaction goes to the second.

So in the body there are special compounds, macroergs, which are able to accumulate and transfer energy during the reaction. In their composition there is one, or several chemical bonds, in which this energy accumulates. Now you can return to the glucose. The energy released during its decay is stored in the bonds of these macroergs.

Let us consider an example.

The most common macroergus (energy currency) of the cell is ATP (Adenosine triphosphate).

It looks like this.


It consists of a nitrogenous base adenine (one of 4 used to encode information in DNA), sugar, ribose, and three phosphoric acid residues (therefore, Adenosine TRIPOSPHATE). It is in the bonds between the residues of phosphoric acid that energy is accumulated. When one phosphoric acid residue is removed, ADP (Adenosine Diphosphate) is formed. ADP can release energy by tearing off another residue and turning it into AMP (AdenosineMONO phosphate). But the effectiveness of the split of the second residue is much lower. Therefore, usually, the body tends to get ATP again from ADP. It happens like this. With the breakdown of glucose, the energy released is spent on the formation of a bond between the two phosphoric acid residues and the formation of ATP. The process is multistage and while we omit it.


The resulting ATP is a universal source of energy. It is used everywhere, ranging from protein synthesis (for the connection of amino acids need energy), ending with muscular work. Muscle contraction motor proteins use the energy stored in ATP to change their conformation. Conformational change is the reorientation of one part of a large molecule relative to another. It looks like this.


That is, the chemical energy of the bond goes into mechanical energy. Here are real examples of proteins that use ATP for work.

Meet this myosin . Motor protein. It moves large intracellular formations and is involved in muscle contraction. Please note that he has two legs. Using the energy stored in 1 ATP molecule, he performs one conformational change, in fact one step. The most obvious example of the transition of the chemical energy of ATP to mechanical.


The second example is the Na / K pump. At the first stage, it binds three Na molecules and one ATP. Using the energy of ATP, he changes the conformation, throwing Na out of the cell. Then it binds two potassium molecules and, returning to the original conformation, transfers potassium to the cell. The piece is extremely important, it allows you to maintain the level of intracellular Na in the normal state.



But seriously, then:


Pause. Why do we need ATP? Why can't we use the energy stored in glucose directly? Tritely, if you oxidize glucose to CO2 at a time, an extreme amount of energy is instantly released. And most of it will dissipate in the form of heat. Therefore, the reaction is divided into stages. Each one releases a little energy, it is stored, and the reaction continues until the substance is completely oxidized.

Summing up. Energy is stored in fats and carbohydrates. From carbohydrates, it can be extracted faster, but more can be stored in fats. For carrying out reactions, the cell uses high-energy compounds in which the energy of the breakdown of fats, carbohydrates, etc. is stored ... ATP - the main such compound in the cell. In essence, take and use. However, not the only one. But more about that later.

PS I tried to simplify the material as much as possible, so some inaccuracies appeared. I ask zealous biologists to forgive me.