Endoplasmic reticulum (cell organelle): characteristics, structure and functions

Cells are the elemental units of life. There is not a single living being that is not made up of at least one cell. And it is that these cells, the simplest level of biological organization, are capable of functioning as individual organisms (in unicellular beings) or of organizing themselves among billions of them to give rise to multicellular beings.

Be that as it may, cells, which have an average size of about 10 micrometers (one thousandth of a millimeter), organic structures surrounded by a plasma membrane that protects an internal material where, thanks to the joint work of the different cellular organelles, they have place the functions of relationship, nutrition and reproduction.

Mitochondria, Golgi apparatus, vacuoles, cytoskeleton, centrioles, ribosomes, lysosomes ... There are many different cellular organelles synthesized according to what is encoded in the genetic material of the cell and that are specialized in a specific cellular process.

And in today's article we will talk about an organelle present in all eukaryotic cells (not in bacteria and archaea) that is involved in the synthesis of both proteins and lipids: the endoplasmic reticulum. If you want to know everything about its structure, characteristics and functions, you have come to the right place. Let us begin.


    What is the endoplasmic reticulum?

    The endoplasmic or endoplasmic reticulum is a cellular organelle present in the cytoplasm of all eukaryotic cells and that is specialized in the synthesis of proteins and lipids. It consists of a complex system of membranes arranged in the cytoplasm in the form of interconnected tubules, cisterns and flattened sacs.

    The membranes of the endoplasmic reticulum show continuation with the nuclear membrane and can extend to the vicinity of the plasma membrane (the one that separates the cell interior from the external environment), so that, especially in animal cells, it can represent more than half of all cell membranes.

    In any case, the entire endoplasmic reticulum membrane, with its cisternae, flattened sacs, and tubules, defines a single internal space known as the endoplasmic reticulum lumen, which can represent 10% of the volume of the cytoplasm, which has high concentrations of calcium ions, which is an oxidizing environment and inside which the physiological functions of this organelle take place, which we will comment on later.

    In this sense, the endoplasmic reticulum can be understood as a membranous network present in all eukaryotic cells and which is considered the largest cell organelle. In its internal environment, the lumen, the endoplasmic reticulum fulfills its functions.

    But what are these functions? Basically, the biosynthesis of proteins (practically all the proteins that are secreted outside the cell first pass through the endoplasmic reticulum) and of lipids, as well as intracellular transport and the metabolism of steroids. But let's dive deeper into this amazing organelle.

    What is the morphology of the endoplasmic reticulum?

    As we have already commented, the morphology of the endoplasmic reticulum consists of a system of membranes that extend from the nuclear membrane and within which, the lumen, the physiological functions of the organelle have a reaction.

    Its structure, then, is based on a continuous system of membranes (which are lipid bilayers, such as nuclear) that adopt the architecture of interconnected sacs, cisterns and tubules. These sacs are usually flattened and stacked, giving rise to curved regions that, depending on the metabolic needs of the cell, are restructured.

    Similarly, if the cell needs more lipid synthesis, we may see fewer flat sac shapes (more linked to protein synthesis) and more tubules. But, we repeat, all these morphologies are dynamic and evolve depending on the needs of the cell.

    But what is clear is that the endoplasmic reticulum is always divided into two domains or regions that have a different morphology and that, therefore, perform different functions: the smooth endoplasmic reticulum and the rough endoplasmic reticulum. Let's see the properties of each of them.

    1. Smooth endoplasmic reticulum

    The smooth endoplasmic reticulum is the domain of the endoplasmic reticulum that contains ribosomes in the membrane. It has a more complex and varied morphology than rough and, unlike this, its main function is lipid biosynthesis.

    Ribosomes are organelles inside which the genetic material is translated into proteins. So it is evident that, as they are not attached to the membrane, protein biosynthesis does not occur in the endoplasmic reticulum. And the proteins present in it come, as we will now see, from the rough.

    The smooth endoplasmic reticulum is more irregular in architecture and represents the smallest part of the organelle, consisting of a disordered network of tubules in whose interior (the lumen) different metabolic reactions take place, being the synthesis of structural lipids (those that are part of cell membranes and those that serve for the production of hormones), cell detoxification (that is why liver cells have a large amount of this domain) and calcium homeostasis are the most important.

    2. Rough endoplasmic reticulum

    The rough endoplasmic reticulum is the domain of the endoplasmic reticulum that contains ribosomes in the membrane. It is the region closest to the nuclear membrane and receives this name because ribosomes take on the appearance of granules attached to this reticulum.

    Riboforins are proteins that make possible the binding of ribosomes to the reticulum membrane. These ribosomes, as we have said, are responsible for the synthesis of proteins, which, after being synthesized in the membrane, "fall" to the lumen of the reticulum.

    It consists of a network of tubules less disordered than the smooth one and, as we have said, it has a high density of ribosomes on its surface. Tubules tend to adopt a more or less straight architecture (remember that in the smooth there were more curves) and it is also common to see flattened cisterns or sacks.

    What functions does the endoplasmic reticulum have?

    After understanding exactly what the endoplasmic reticulum is, analyzing its morphology and presenting its division into rough and smooth, it is time to talk about its cellular functions. To facilitate understanding, we will see the functions in general and, already within each one of them, if necessary, we will indicate whether it belongs to the smooth or rough domain. Let's go there.

    1. Protein biosynthesis

    The rough endoplasmic reticulum, through ribosomes anchored to its membrane, is specialized in protein synthesis. All the proteins secreted or that will form part of the internal cellular environment culminate their synthesis in the endoplasmic reticulum.

    2. Lipid biosynthesis

    In the membranes of the smooth endoplasmic reticulum, the synthesis of most of the lipids that will be necessary takes place. for the renewal of cell membranes (lipid bilayers), as well as for the production of hormones.

    3. Cell detoxification

    The smooth endoplasmic reticulum also intervenes in cellular detoxification processes, by metabolizing toxic substances from both the exterior (such as carcinogens) and the interior of the cell (metabolic waste substances). The reticulum converts these substances into water-soluble compounds that, after all its process, will be eliminated from the body through urine. Hence, hepatocytes (liver cells) have high amounts of smooth endoplasmic reticulum.

    4. Protein transport

    The endoplasmic reticulum plays an essential role in the transport and traffic of proteins that must be secreted to the outside (or to other organelles, such as the Golgi apparatus) of the cell is concerned.

    5. Calcium storage

    The smooth endoplasmic reticulum is the intracellular reservoir of calcium par excellence. It is capable of, through calcium pumps, "sequestering" the molecules of this mineral to store it and expel it from the cell when necessary.

    6. Product accumulation

    As with calcium, the endoplasmic reticulum in general has the important function of serving as a warehouse for all types of cellular products and metabolic substances. The lumen of the reticulum serves for storage of products.

    7. Dephosphorylation of glucose-6-phosphate

    When glycogen (the form in which glucose is stored) is broken down, glucose-6-phosphate is formed, which is unable to leave the cell because it cannot cross the plasma membrane. And here glucose-6-phosphatase comes into play, an enzyme that acts on the endoplasmic reticulum and that stimulates dephosphorylation (removing, by hydrolysis, a phosphate group) of glucose-6-phosphate. In this way, we obtain glucose, which can already pass into the blood.

    8. Protein glycosylation

    Protein glycosylation takes place in the rough endoplasmic reticulum, a process of adding a carbohydrate to a protein. More concretely, asparagine amino acids receive a complex of 14 sugars in their radical. Subsequently, these proteins that have incorporated a carbohydrate radical and have become glycoproteins are sent to the Golgi apparatus for further processing.

    9. Protein quality control

    Essential control of protein quality also takes place in the rough endoplasmic reticulum. Chaperones are important proteins in the folding and maturation of synthesized proteins, but also in the detection of errors. Defective proteins are detected and eliminated inside the cell.

    10. Formation of disulfide bridges

    The lumen of the endoplasmic reticulum is an oxidizing environment, which makes possible the formation of, thanks to disulfide isomerase, disulfide bridges, a covalent bond between the sulfhydryl groups of cysteine. This part is essential since it makes possible a correct structure of the proteins.

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