What are cell organelles or organelles?

The term organelle is used in cell biology to refer to cellular subunits or components found in the cystoplasm and are specialized in a certain function. For example, the nucleus, which stores genetic material, the mitochondria, which produce chemical energy in the form of ATP, and the Endoplasmic Reticulum, where proteins are synthesized.

They are also known as organelles, organelles, organoids or simply cellular elements. Of these, only the organelle is listed in the RAE Dictionary, although the organelle is widely used.

General characteristics

Organelles are intracellular structures that are delimited from the rest of the cytoplasm and that have specialized for specific functions. This specialization is believed to have had a very important evolutionary role. For example, organelles have made it possible to isolate very complex chemical reactions from the rest of cellular reactions. One of the main advantages of this isolation is being able to control the conditions inside the organelle, for example the pH, to optimize the performance of the reaction without affecting the rest of the cell.

The structure and morphology of the organelles is highly variable. Some are so large that they can be seen under a light microscope. For example, the mitochondria or the Golgi apparatus can be observed under an optical microscope, although to see details of their surface and structure it is necessary to use an electron microscope.

Some organelles are scattered throughout the cytoplasm while others tend to be found in specific areas. For example, the nucleus typically appears near the cell center and the Golgi apparatus and endoplasmic reticulum appear together and near the nucleus.

The types and number of organelles is not constant among the different cells of an organism but varies according to the specific function of the cell. For example, muscle cells tend to have a greater number of mitochondria, the organelles responsible for energy production.

There are also organelles typical of taxonomic phyla. For example, all eukaryotic photosynthesizing organisms (plants and algae) have chloroplasts, a type of organelle specialized in oxygenic photosynthesis and not found in animals, fungi, or bacteria.

Possible endosymbiotic origin

Some organelles contain their own genetic material, for example mitochondria and chloroplasts, and are believed to arise from a phenomenon of symbiogenesis. According to this theory, an independent single-celled microorganism established an endosymbiotic relationship with the cells of its host billions of years ago. Little by little, it abandoned its autonomy, delegating certain functions to its host until they were acquired and integrated as part of the cell itself.

In the case of chloroplasts, it is believed that they come from cyanobacteria that established an endosymbiotic relationship with some protist organism and gave rise to algae and all the plants we know today.

There are organelles that also appear to have been acquired by endosymbiosis but do not currently contain genetic material. For example flagella. In any case, the theory of symbiogenesis could have been key in the evolution of eukaryotic cellswhich would be reflected in the great number and variety of organelles that they present and how scarce they are in prokaryotic cells.

In fact, until recently it was thought that prokaryotic cells did not even have internal compartments, but numerous microcompartments have already been described in prokaryotic cells, sometimes encapsulated by a protein structure such as carboxysomes, by a membranous structure such as magnetosomes, and even by a lipid membrane like the nucleoid of bacteria of the genus Planctomycetes.

However, for some biologists, the term organelle should only be used to refer to specialized structures of endosymbiotic origin and that have a lipid membrane. But despite this, the term organelle is very common and widely used to name cellular elements in general. Some authors differentiate between organelles with a membrane and organelles without a membrane, which they also call biomolecular complexes (ribosomes, nucleosomes, carboxysomes, etc.).

Important functions and organelles

As has been said, one of the fundamental characteristics of the organelles is their specialization in specific functions, the vast majority of them key to cell life. From energy production to protein synthesis through nutrient fixation, the variety of organelle functions is enormous. The most outstanding can be summarized in the following:

DNA protection

The nucleus it is the organelle that differentiates cells into two main basic types, prokaryotic cells, which do not have a nucleus, and eukaryotic cells, which have a nucleus where the genetic material is surrounded by a double lipid membrane. The function of the nucleus is, in addition to the protection of the DNA, to control the activity of the rest of the cell through the transcription of the information contained in that DNA.

The membrane of the nucleus controls all the molecules that enter and leave the interior space and has specific receptors for certain substances. Both characteristics are key in the communication of the nucleus with the rest of the cell through chemical signals that allow it to modulate its activity.

protein synthesis

Some organelles with related functions that require continuous and intense communication, They tend to be close and even connected. This is the case of endoplasmic reticulumconnected to the nucleus, and the Golgi apparatusconnected to the endoplasmic reticulum.

In the endoplasmic reticulum, the synthesis of new proteins takes place. This organelle has numerous ribosomes that translate information from mRNA coming from the core. From here, some proteins pass into the Golgi apparatus where they are modified by glycosylation (add carbohydrates), phosphorylation (add phosphate) or proteolyzed to form the final protein or active conformation.

The Golgi apparatus also packages and distributes proteins to their final destination by creating transport vesicles.

For example, proinsulin is synthesized in the endoplasmic reticulum of pancreatic cells and modified to the active form of insulin in the Golgi apparatus. Insulin is packaged in secretory granules that will be responsible for secreting insulin into the extracellular space.

Energy production

All eukaryotic cells have specialized organelles for energy production. mitochondriapresent in animals, fungi and plants, and chloroplasts, present in plants and algae. Mitochondria produce ATP from organic molecules and it is the main pathway for obtaining energy in animals and fungi. In plants it is a secondary energy source, the main one being oxygenic photosynthesis carried out in chloroplasts.

There are no mitochondria or chloroplasts in bacteria, but some bacteria have other types of cells. micro compartments specialized in energy production. For example, in carboxysomes CO is fixedtwo atmosphere and is transformed into organic molecules that can later be used for energy. Carboxysomes have been found in cyanobacteria, chemolithotrophic bacteria, and some photosynthesizing bacteria.

Others

There are many other organelles, most of them in the midst of the dialectical debate as to whether or not they should be called by this term. Without getting into that debate, some common examples of other organelles are:

  • ribosomes
  • lysosomes
  • cilia
  • flagella
  • acrosomes
  • vesicles
  • melanosomes
  • acrosome
  • magnetosomes
  • thylakoids
  • myofibrils
  • centriole
  • And a long etcetera
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