Mitochondria: what are they, characteristics and functions?

A summary of the characteristics of this type of cell organelles.

Mitochondria are small organelles found in our cells and in the cells of found in our cells and in the cells of virtually all eukaryotic organisms.

Their function is very important for the life of the organism, since they are the producers of a kind of fuel so that metabolic processes can be carried out inside the cell.

Next we will see more in depth what these organelles are, what are their parts, their functions and what hypothesis has been put forward to explain how they originated.

What are mitochondria?

Mitochondria are organelles organelles present in the eukaryotic cell interior that have a very important function for lifeThey are responsible for providing energy to the cell, allowing it to carry out various metabolic processes. Their shape is circular and stretched, having several layers and ridges in its interior, where proteins are coupled that allow carrying out several processes in order to give this energy, in the form of ATP (adenosine triphosphate).

These organelles can be present in a variable number in the cellular medium, and their quantity is directly related to the energy needs of the cell. This is why, depending on the tissue that makes up the cell, more or fewer mitochondria can be expected. For example, in the liver, where there is a high enzymatic activity, liver cells usually present several of these organelles.

Morphology

The mitochondrion is, as one would expect, a very small structure, ranging in size from 0.5 to 1 μm (micrometers) in diameter and up to 8 μm in length, having a hemispherical and stretched shape, like a fat sausage.

The number of mitochondria inside the cell is directly related to the energy requirements of the cell. The more energy required, the more mitochondria the cell needs. The set of mitochondria is called the cell chondriome.

The mitochondria are surrounded by two membranes with different functions in terms of enzymatic activity, separated into three spaces: cytosol (or three spaces: cytosol (or cytoplasmic matrix), intermembrane space and mitochondrial matrix..

1. Outer membrane

It is an outer lipid bilayer, permeable to ions, metabolites and many polypeptides. It contains pore-forming proteins, called porins, which constitute a voltage-dependent anion channel.. These channels allow the passage of large molecules up to 5,000 daltons and a diameter of approximately 20 Å (ångström).

The outer membrane performs rather few enzymatic or transport functions. It contains between 60% and 70% of proteins.

2. Inner membrane

The inner membrane is composed of about 80% proteins, and unlike its outer membrane counterpart, it lacks pores and is highly selective. It contains many enzyme complexes and transmembrane transport systems, which are involved in transmembrane transport.which are involved in the translocation of molecules, i.e., moving them from one site to another.

3. Mitochondrial cristae

In most eukaryotic organisms, mitochondrial cristae are in the form of flattened, perpendicular septa. The number of mitochondrial ridges is thought to be a reflection of cellular activity. The ridges The mitochondrial surface area is significantly increased so that proteins useful for different processes inside the mitochondria can be coupled. that take place inside the mitochondrion.

They are connected to the inner membrane at specific points, where the transport of metabolites between the different compartments of the mitochondrion will be facilitated. In this part of the mitochondrion, functions related to oxidative metabolism, such as the respiratory chain or oxidative phosphorylation, are carried out. Here we can highlight the following biochemical compounds:

• The electron transport chain, composed of four fixed enzyme complexes and two mobile electron transporters.
• An enzyme complex, the hydrogen ion channel and ATP synthase, which catalyzes ATP synthesis (oxidative phosphorylation).
• Transporter proteins, which allow the passage of ions and molecules through them, among the most notable are fatty acids, pyruvic acid, ADP, ATP, O2 and water; they can be highlighted:

4. Intermembrane space

Between both membranes, there is a space containing a liquid similar to the cytoplasm, with a high concentration of protons, due to the pumping of these subatomic particles by the enzymatic complexes of the respiratory chain.

Within this intramembranous medium, several enzymes are located several enzymes, which are involved in the transfer of the high-energy bond of ATPsuch as adenylate kinase or creatine kinase. In addition, carnitine, a substance involved in the transport of fatty acids from the cytoplasm to the mitochondrial interior, where they will be oxidized, can be found.

5. Mitochondrial matrix

The mitochondrial matrix also called the mitosol, contains fewer molecules than the cytosol.The mitochondrial cell, although it can also contain ions, metabolites to be oxidized, circular DNA similar to that of bacteria and some ribosomes (mitorribosomes), which carry out the synthesis of some mitochondrial proteins and contain, in fact, mitochondrial RNA.

It has the same organelles that free-living prokaryotic organisms have, which differ from our cells in that they lack a nucleus.

Several metabolic pathways fundamental to life, such as the Krebs cycle and beta-oxidation of fatty acids, occur in this matrix.

Fusion and fission

Mitochondria have the ability to divide and fuse with relative ease, and these are two actions that occur constantly in cells. This involves the mitochondrial DNAs of each of these organelle units mixing and dividing..

In eukaryotic cells there are no individual mitochondria, but a network connected to a variable number of mitochondrial DNAs. One of the possible functions of this phenomenon is to share products synthesized by different parts of the network, to correct local defects or simply to share their DNA.

If two cells with different mitochondria fuse, the mitochondrial network that will emerge from the union will be homogeneous after only 8 hours. Because mitochondria are all the time joining and dividing, it is difficult to establish the total number of these organelles in a cell of a given tissue, although it can be assumed that those tissues that are most worked or require the most energy will possess many mitochondria resulting from fission.

Mitochondrial division is mediated by proteins, very similar to dynamins, which are involved in mitochondrial division.which are involved in the generation of vesicles. The point at which these organelles begin to divide depends very much on their interaction with the endoplasmic reticulum. The membranes of the reticulum surround the mitochondrion, constricting it and eventually splitting it in two.

Functions

The main function that mitochondria have is the production of ATP, which is known as the fuel for cellular processes. However, they also carry out part of the metabolism, they also carry out part of the metabolism of fatty acids through beta-oxidation, as well as acting as a store for calcium and calcium channeling..

In addition, research in recent years has linked this organelle to apoptosis, i.e. cell death, as well as Cancer and the aging of the organism, and the onset of degenerative diseases such as Parkinson's disease or diabetes.

One of the benefits of mitochondria for genetic studies is their DNA, which comes directly from the mitochondria. their DNA, which comes directly from the maternal line.. Genealogy and anthropology researchers use this DNA to establish family trees. This DNA is not subject to genetic recombination due to sexual reproduction.

ATP synthesis

It is in the mitochondria that most of the ATP for non-photosynthetic eukaryotic cells is produced.

They metabolize acetyl-coenzyme Athrough an enzymatic citric acid cycle, producing carbon dioxide (CO2) and NADH. NADH yields electrons to an electron transport chain in the mitochondrial inner membrane. These electrons travel to an oxygen molecule (O2), producing a water molecule (H2O).

This electron transport is coupled to the transport of protons, coming from the matrix and reaching the intermembrane space. It is the proton gradient that allows ATP to be synthesized thanks to the action of a substance called ATP synthase, binding a phosphate to ADP, and using oxygen as the final electron acceptor (oxidative phosphorylation).

The electron transport chain is known as the respiratory chain.contains 40 proteins.

2. Lipid metabolism

A good amount of lipids present in cells are due to mitochondrial activity. In the mitochondria, lysophosphatidic acid is produced, from which lysophosphatidic acid is synthesized.from which triacylglycerols are synthesized.

Phosphatidic acid and phosphatidylglycerol, which is necessary for the production of cardiolipin and phosphatidyl ethanolamine, are also synthesized.

The origin of mitochondria: cells within cells?

In 1980 Lynn Margulis, one of the most important women in science, recovered an old theory about the origin of this organelle, reformulating it as an endosymbiotic theory. According to her version, more updated and based on scientific evidence, about 1.5 billion years ago, a prokaryotic cell, i.e., without a nucleus, was able to obtain energy from organic nutrients using molecular oxygen as an oxidant..

During the process, it fused with another prokaryotic cell, or with what may have been the first eukaryotic cells, and was phagocytosed without being digested. This phenomenon is based on reality, since bacteria have been seen phagocytizing others but without ending their life. The absorbed cell established a symbiotic relationship with its host, supplying it with energy in the form of ATPand the host provided a stable, nutrient-rich environment. This great mutual benefit was consolidated, eventually becoming part of it, and this would be the origin of the mitochondrion.

This hypothesis is quite logical if we take into account the morphological similarities between bacteria, free-living prokaryotic organisms, and mitochondria. For example, both have an elongated shape, have similar layers and, most importantly, their DNA is circular. In addition, the mitochondrial DNA is very different from that of the cell nucleus, giving the impression that they are two different organisms.

Bibliographical references:

• Friedman, J. R., Nunnari, J.. (2014). Mitochondrial form and functions. Nature. 505: 335-343.
• Kiefel, B. R., Gilson, P. R., Beech P. L. (2006). Cell biology of mitochondrial dynamics. International review of cytology. 254: 151-213.
• MacAskill, A. F., Kittler, J. T. (2010). Control of mitochondrial transport and localization in neurons. Trends in cell biology. 20: 102-112