Structural support, catalysis of chemical reactions, regulation of substances passage across the cell membrane, protection against disease, and main components of hair, nails, bones, and tissues- these are all functions performed by proteins. Protein synthesis, essential for cell activity, mainly occurs in tiny cellular structures called ribosomes. Ribosomes’ function is so vital that they are found in all kinds of organisms, from prokaryotic bacteria and archaea to eukaryotes. In fact, it's often said life is just ribosomes making other ribosomes! In the following article, we'll look at ribosomes' definition, structure, and function.

Ribosomes Ribosomes

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Table of contents

    Ribosome definition

    Cell biologist George Emil Palade first observed the ribosomes inside a cell using an electron microscope in the 1950s. He described them as “small particulate components of the cytoplasm”. A few years later, the term ribosome was proposed during a symposium and was later widely accepted by the scientific community. The word comes from “ribo” = ribonucleic acid (RNA), and the Latin word “soma” = body, meaning a body of ribonucleic acid. This name refers to the composition of ribosomes, which are composed of ribosomal RNA and proteins.

    A ribosome is a cellular structure not bounded by a membrane, composed of ribosomal RNA and proteins, and whose function is to synthesize proteins.

    The ribosome’s function in protein synthesis is so critical for all cellular activities that two Nobel prizes have been awarded to research teams that study the ribosome.

    The Nobel prize in Physiology or Medicine was awarded in 1974 to Albert Claude, Christian de Duve, and George E. Palade “for their discoveries concerning the structural and functional organization of the cell”. The recognition of Palade’s work included the discovery and description of ribosome structure and function. In 2009 the Nobel prize in chemistry was awarded for the description of the ribosome structure in detail and its function at the atomic level to Venkatraman Ramakrishnan, Thomas Steitz, and Ada Yonath. The press release stated, “The Nobel Prize in Chemistry for 2009 awards studies of one of life’s core processes: the ribosome’s translation of DNA information into life. Ribosomes produce proteins, which in turn control the chemistry in all living organisms. As ribosomes are crucial to life, they are also a major target for new antibiotics”.

    Ribosome structure

    Ribosomes comprise two subunits (Fig. 1), one large and one small, with both subunits made up of ribosomal RNA (rRNA) and proteins. These rRNA molecules are synthesized by the nucleolus inside the nucleus and combined with proteins. The assembled subunits exit the nucleus to the cytoplasm. Under a microscope, ribosomes look like small dots that can be found free in the cytoplasm, as well as bound to the continuous membrane of the outer nuclear envelope and the endoplasmic reticulum (Fig. 2).

    Ribosome diagram

    The following diagram represents a ribosome with its two subunits while translating a messenger RNA molecule (this process is explained in the next section).

    Ribosome function

    How do ribosomes know how to synthesize a specific protein? Remember that the nucleus previously transcribed the information from genes into messenger RNA molecules -mRNA- (the first step in gene expression). These molecules ended up exiting the nucleus and are now in the cytoplasm, where we also find the ribosomes. In a ribosome, the large subunit is located on top of the small one, and in the space in between the two, the mRNA sequence passes through to be decoded.

    The ribosome small subunit “reads” the mRNA sequence, and the large subunit synthesizes the corresponding polypeptide chain by linking amino acids. This corresponds to the second step in gene expression, the translation from mRNA to protein. The amino acids needed for polypeptide synthesis are brought from the cytosol to the ribosome by another type of RNA molecule, appropriately called transfer RNA (tRNA).

    Ribosomes that are free in the cytosol or bound to a membrane have the same structure and can interchange their location. Proteins produced by free ribosomes are usually used within the cytosol (like enzymes for sugar breakdown) or are destined for mitochondria and chloroplasts membranes or imported to the nucleus. Bound ribosomes generally synthesize proteins that will be incorporated into a membrane (of the endomembrane system) or that will exit the cell as secretory proteins.

    The endomembrane system is a dynamic composite of organelles and membranes that compartmentalize the interior of a eukaryotic cell and work together to perform cellular processes. It includes the outer nuclear envelope, the endoplasmic reticulum, the Golgi apparatus, the plasma membrane, vacuoles, and vesicles.

    Cells that continuously produce a lot of proteins can have millions of ribosomes and a prominent nucleolus. A cell can also change the number of ribosomes to achieve its metabolic functions if needed. The pancreas secretes large amounts of digestive enzymes, thus pancreatic cells have abundant ribosomes. Red blood cells also are rich in ribosomes when immature, as they need to synthesize hemoglobin (the protein that binds to oxygen).

    Interestingly, we can find ribosomes in other parts of a eukaryotic cell, besides the cytoplasm and the rough endoplasmic reticulum. Mitochondria and chloroplasts (organelles that transform energy for cellular use) have their own DNA and ribosomes. Both organelles most likely evolved from ancestral bacteria that were engulfed by the ancestors of eukaryotes through a process called endosymbiosis. Therefore, as previous free-living bacteria, mitochondria and chloroplasts had their own bacterial DNA and ribosomes.

    What would be an analogy for ribosomes?

    Ribosomes are often referred to as the “cell factories” due to their protein-building function. Because there are so many (up to millions!) ribosomes inside a cell, you can think of them as the workers, or machines, that actually do the assembly job in the factory. They get copies or blueprints (mRNA) of the assembly instructions (DNA) from their boss (nucleus). They do not make the protein components (amino acids) themselves, these are in the cytosol. Therefore, ribosomes only link the amino acids in a polypeptide chain according to the blueprint.

    Why are ribosomes important?

    Protein synthesis is essential for cell activity, they function as diverse vital molecules, including enzymes, hormones, antibodies, pigments, structural components, and surface receptors. This essential function is evidenced by the fact that all cells, prokaryotic and eukaryotic, have ribosomes. Although bacterial, archaeal, and eukaryotic ribosomes differ in subunits size (prokaryotic ribosomes are smaller than eukaryotic ones) and specific rRNA sequences, they all are composed of similar rRNA sequences, have the same basic structure with two subunits where the small one decodes mRNA, and the large one joins amino acids together. Thus, it seems that ribosomes evolved early in the history of life, which also reflects the common ancestry of all organisms.

    The importance of protein synthesis for cell activity is exploited by many antibiotics (substances that are active against bacteria) that target bacterial ribosomes. Aminoglycosides are one type of these antibiotics, like streptomycin, and bind to the ribosomal small subunit preventing the accurate read of mRNA molecules. The proteins synthesized are non-functional, which leads to bacterial death. As our ribosomes (eukaryotic ribosomes) have enough structural differences from prokaryotic ones, they are not affected by these antibiotics.
    But what about mitochondrial ribosomes? Remember that they evolved from an ancestral bacterium, therefore their ribosomes are more similar to prokaryotic than to eukaryotic ones. Changes in mitochondrial ribosomes after the endosymbiotic event may prevent them from being affected as much as bacterial ones (the double membrane could serve as protection). However, recent research suggests that most of these antibiotics’ side effects (kidney injury, hearing loss) are associated with mitochondrial ribosome dysfunction.

    Ribosomes - Key takeaways

    • All cells, prokaryotic and eukaryotic, have ribosomes for protein synthesis.
    • Ribosomes synthesize proteins through the translation of the information encoded in mRNA sequences into a polypeptide chain.
    • Ribosomal subunits are assembled in the nucleolus from ribosomal RNA (transcribed by the nucleolus) and proteins (synthesized in the cytoplasm).
    • Ribosomes can be free in the cytosol or bound to a membrane have the same structure and can interchange their location.
    • Proteins produced by free ribosomes are usually used within the cytosol, destined to mitochondria and chloroplasts membranes, or imported to the nucleus.
    Frequently Asked Questions about Ribosomes

    What are 3 facts about ribosomes?

    Three facts about ribosomes are: they are not delimited by a bilayered membrane, their function is to synthesize proteins, they can be free in the cytosol or bound to the rough endoplasmic reticulum membrane.

    What are ribosomes?

    Ribosomes are cellular structures not bounded by a bilayered membrane and whose function is to synthesize proteins.

    What is the function of ribosomes?

    The function of ribosomes is to synthesize proteins through the translation of mRNA molecules.

    Why are ribosomes important?

    Ribosomes are important because they synthesize proteins, which are essential for cell activity. Proteins function as diverse vital molecules including enzymes, hormones, antibodies, pigments, structural components, and surface receptors.

    Where are ribosomes made?

    Ribosomal subunits are made in the nucleolus inside the cell nucleus.

    Test your knowledge with multiple choice flashcards

    Which organelle assembles the ribosomal subunits?

    All the eukaryotic ribosomes are found free in the cytosol

    Ribosome synthesis is performed by____through the translation of_____molecules.


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