Mitochondria: what they are, characteristics and functions

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

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

Below, we will take a closer look at what these organelles are, what are their parts, functions, and what hypotheses have been raised to explain their origin.

    What are Mitochondria

    Mitochondria are those organelles present inside the eukaryotic cell that have a very important function for life, Since they are responsible for providing energy to the cell, which allows it to perform various metabolic processes. Its shape is circular and stretched, with several layers and ridges inside, where proteins are coupled 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 varying numbers in the cellular environment, 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, we can expect more or less mitochondria. For example, in the liver, where enzyme activity is high, liver cells usually have more than one of these organelles.

    morphology

    Mitochondria is, as one would expect, a very small structure, ranging from 0.5 to 1 microns (micrometers) in diameter and up to 8 microns in length, having a hemispherical and stretched shape, like a thick sausage .

    The amount of mitochondria inside the cell is directly related to the energy needs of the latter.. The more energy it takes, the more mitochondria the cell will need. The set of mitochondria is called a cell chondrioma.

    The 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 external lipid bilayer, permeable to ions, metabolites and many polypeptides. It contains proteins that form pores, called porins, which form a voltage-gated anion channel.. These channels allow the passage of large molecules up to 5000 daltons and with an approximate diameter of 20 Å (ångström)

    The outer membrane performs relatively few enzymatic or transport functions. It contains between 60% and 70% protein.

    2. Internal membrane

    The inner membrane is made up of about 80% protein and, unlike its counterpart, the outer membrane has no pores, being very selective. It contains many enzyme complexes and transmembrane transport systems, Which are involved in the translocation of molecules, that is, the movement from one place to another.

    3. Mitochondrial ridges

    In most eukaryotic organisms, mitochondrial ridges appear as flattened septa that are perpendicular to each other. It is believed that the number of peaks in the mitochondria is a reflection of its cellular activity. the crests they involve a significant increase in surface area so that useful proteins can be coupled for different processes that take place inside the mitochondria.

    They connect to the inner membrane at specific points, where 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 performed, such as the respiratory chain or oxidative phosphorylation. here we can highlight the following biochemical compounds:

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

    4. Intermembrane space

    Between the two 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 enzyme complexes of the respiratory chain.

    In this intramembranous medium, they are located various enzymes, involved in the transfer of the high energy binding of ATP, Such as myokinase or creatine kinase. Additionally, carnitine can be found, a substance involved in the transport of fatty acids from the cytoplasm to the interior of the mitochondria, where they will be oxidized.

    5. Mitochondrial matrix

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

    It has the same organelles as free prokaryotic organisms, which differ from our cells in having no nucleus.

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

    Fusion and fission

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

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

    If two cells that have different mitochondria merge, the network of mitochondria that emerge from the junction will be homogeneous after only 8 hours. Because mitochondria are constantly coming together 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 the tissues that work the most or require the most energy will have many fission mitochondria.

    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 depends greatly on their interaction with the endoplasmic reticulum. The membranes of the reticulum surround the mitochondria, constrict them and eventually split them into 2.

      functions

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

      In addition, research in recent years has linked this organelle to apoptosis, which is cell death, in addition to cancer and the aging of the body, and the onset of degenerative diseases such as Parkinson’s disease or diabetes.

      One of the benefits of genetic study offered by mitochondria is its 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 ATP is produced for non-photosynthetic eukaryotic cells.

      Metabolize acetyl-coenzyme A, By an enzymatic cycle of citric acid, and producing carbon dioxide (CO2) and of NADH. NADH donates 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 electron transport is coupled with that of protons, originating from the matrix and reaching the intermembrane space. It is the proton gradient that allows ATP to be synthesized by the action of a substance, called ATP synthase, by 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 is due to mitochondrial activity. In the mitochondria, lysophosphatidic acid is produced, From which triacylglycerols are synthesized.

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

      The origin of mitochondria: cells within cells?

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

      In the process, it fused with another prokaryotic cell, or what could have been the first eukaryotic cells, being phagocytosed without being digested. This phenomenon is based on reality, as bacteria have been seen to swallow others without ending their lives. The absorbed cell has established a symbiotic relationship with its host, providing energy in the form of ATP., And the guest offered him a stable and nutrient-rich environment. This great mutual benefit was consolidated, eventually becoming a part, and it would be the origin of the mitochondria.

      This hypothesis makes perfect sense when considering the morphological similarities between bacteria, free 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, JR, Nunnari, J .. (2014). Mitochondrial form and functions. Nature. 505: 335-343.
      • Kiefel, BR, Gilson, PR, Beech PL (2006). Cell biology of mitochondrial dynamics. International journal of cytology. 254: 151-213.
      • MacAskill, AF, Kittler, JT (2010). Control of transport and mitochondrial localization in neurons. Trends in cell biology. 20: 102-112

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