Mitochondria: what are they, characteristics and functions

Mitochondria are small organelles found in our cells and in those of practically 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.

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

• Related article: "The most important cell parts and organelles: an overview"

What are Mitochondria

Mitochondria are a organelles present in the eukaryotic cell interior that have a very important function for life, since they are responsible for providing energy to the cell, allowing it to carry out various metabolic processes. Its shape is circular and stretched, having several layers and ridges inside, where they fit together. proteins that allow various processes to be carried out in order to give this energy, in the form of ATP (adenosine triphosphate).

These organelles can appear in a variable number in the cell environment, and their quantity is directly related to the energy needs of the cell. That is why, depending on the tissue that forms the cell, more or less mitochondria can be expected. For example, in the liver, where there is high enzyme activity, liver cells often have several of these organelles.

Morphology

The mitochondrion is, as you might 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 stretched, hemispherical shape, like a fat sausage.

The amount of mitochondria inside the cell is directly related to its energy needs. The more energy that is required, the more mitochondria the cell will need. The set of mitochondria is called the cellular chondriome.

Mitochondria are surrounded by two membranes with different functions in terms of enzymatic activity, separated in 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. Contains pore-forming proteins, called porins, that make up a voltage-gated anion channel. These channels allow the passage of large molecules of up to 5,000 daltons and an approximate diameter of 20 Å (ångström)

Rather, the outer membrane performs few enzymatic or transport functions. Contains between 60% and 70% protein.

2. Inner membrane

The inner membrane is made up of about 80% proteins, and unlike its counterpart, the outer one, it lacks pores and is highly selective. Contains many enzyme complexes and transmembrane transport systems, which are involved in the translocation of molecules, that is, moving them from one place to another.

3. Mitochondrial ridges

In most eukaryotic organisms, mitochondrial ridges appear as flattened, perpendicular septa. The number of ridges in the mitochondria is believed to be a reflection of their cellular activity. Ridges represent a significant increase in surface area so that proteins useful for different processes can be coupled that take place inside the mitochondria.

They are connected to the inner membrane at specific points, in which the transport of metabolites between the different compartments of the mitochondria will be facilitated. In this part of the mitochondria, functions related to oxidative metabolism are carried out, such as the respiratory chain or oxidative phosphorylation. 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, that catalyzes the synthesis of ATP (oxidative phosphorylation).
• Transporter proteins, which allow the passage of ions and molecules through it, among the most notable we have fatty acids, pyruvic acid, ADP, ATP, O2 and water; can be highlighted:

4. Intermembrane space

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

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

5. Mitochondrial matrix

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

It has the same organelles as free-living prokaryotic organisms, which differ from our cells by lacking a nucleus.

In this matrix there are several fundamental metabolic pathways for life, such as the Krebs cycle and the beta-oxidation of fatty acids.

Fusion and fission

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

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

If two cells that have different mitochondria fuse, the network of mitochondria that will emerge from the union will be homogeneous after only 8 hours. Because mitochondria are constantly joining and dividing, it is difficult to establish the total number of these organelles in a cell of a certain tissue, although it can be assumed that those tissues that work the most or require the most energy will have many mitochondria as a result of fissions.

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

• You may be interested: "Major cell types of the human body"

Features

The main function of mitochondria is the production of ATP, which is known as the fuel for cellular processes. Nevertheless, they also carry out part of the metabolism of fatty acids through beta-oxidation, in addition to acting as a store for calcium.

In addition, in research in recent years, this organelle has been related to apoptosis, this is cell death, in addition to cancer and aging of the body, and the appearance of degenerative diseases such as Parkinson's or diabetes.

One of the benefits for genetic testing offered by mitochondria is their DNA, which comes directly from the maternal line. Researchers in genealogy and anthropology use this DNA to establish family trees. This DNA is not subject to genetic recombination due to sexual reproduction.

1. ATP synthesis

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

They metabolize acetyl-coenzyme A, by means of an enzymatic cycle of citric acid, and producing carbon dioxide (CO2) and NADH. NADH gives up electrons to an electron transport chain in the inner mitochondrial membrane. These electrons travel until they reach an oxygen molecule (O2), producing a water molecule (H2O).

This transport of electrons is coupled to that 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, attaching a phosphate to ADP, and using oxygen as the final electron acceptor (phosphorylation oxidative).

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 thanks to mitochondrial activity. Lysophosphatidic acid is produced in the mitochondria, from which triacylglycerols are synthesized.

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

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 its version, more updated and based on scientific evidence, about 1,500 million years ago, a prokaryotic cell, that is, 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, being phagocytosed without being digested. This phenomenon is based on reality, since bacteria have been seen engulfing others but without ending their lives. The absorbed cell established a symbiotic relationship with its host, providing it with energy in the form of ATP., and the host provided a stable and nutrient-rich environment. This great mutual benefit was consolidated, eventually becoming part of it, and this would be the origin of the mitochondria.

This hypothesis is quite logical if the morphological similarities between bacteria, free-living prokaryotic organisms, and mitochondria are taken into account. For example, both are elongated in 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 it is two different organisms.

Bibliographic 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