General characteristics and sedimentation rate
Ribosomes stand out among other cellular structures for their tiny size and colossal numbers. These non-membranous organelles are typically classified by their sedimentation coefficient—an indicator that reflects the rate of particle sedimentation during centrifugation (measured in Svedberg units, S).
Depending on the cell type, there are two main classes:
- Eukaryotic ribosomes are larger and heavier, with a sedimentation coefficient of 80S.
- Prokaryotic ribosomes are smaller in size and have a sedimentation rate of 70S.
Morphology and chemical composition
Each functional ribosome consists of two unequal parts: the large and small subunits. The formation of these subunits in eukaryotes takes place in a specialized intranuclear region—the nucleolus.
The foundation of the organelle is formed by three main chemical components: ribosomal RNA (rRNA), various protein molecules, and magnesium ions. The presence of magnesium ions is critically important, as they maintain the stable spatial structure of the organelle.
The composition of eukaryotic ribosomal subunits is strictly determined:
- The small subunit includes protein molecules complexed with a single rRNA molecule.
- The large subunit also contains proteins, but they are bound to three rRNA molecules.
Intracellular topography and polysomes
Inside the cell, ribosomes are distributed unevenly and can occupy several localizations. A significant portion of them is freely located in the hyaloplasm (cytoplasm). Another pool is firmly bound to the membranes of the endoplasmic reticulum (ER), forming its rough type. In addition, intrinsic ribosomes are found inside semi-autonomous organelles—mitochondria and plastids.
During intensive protein biosynthesis, multiple ribosomes can simultaneously bind to a single messenger RNA (mRNA) molecule. Such functional complexes are called polysomes. The formation of polysomes allows the cell to dramatically accelerate the production of identical protein molecules from a single template.
Functional role
The sole yet vital function of ribosomes is protein synthesis (translation). However, the fate of the created molecules directly depends on where the organelle was located at the time of operation:
- Proteins synthesized on ribosomes attached to the ER membrane are translocated directly into the cisternae of the endoplasmic reticulum for further modification and transport.
- Proteins assembled on free ribosomes do not leave the hyaloplasm and are used for the cell's own needs (a classic example is the accumulation of hemoglobin in the cytoplasm of erythrocytes).
Briefly: answers to topical questions
What are the sedimentation coefficients for the large and small subunits of the 80S ribosome separately?
The sedimentation coefficients for the subunits of the eukaryotic 80S ribosome are 60S for the large subunit and 40S for the small subunit. The S (Svedberg) value characterizes the rate of particle sedimentation during ultracentrifugation and is proportional to their molecular weight.
- The large subunit is 60S, containing 5S, 5.8S, and 28S rRNA, as well as about 50 types of proteins.
- The small subunit is 40S, including 18S rRNA and about 33 types of ribosomal proteins.
What specific molecular mechanisms ensure the attachment of the ribosome to the ER membrane?
The attachment of the ribosome to the endoplasmic reticulum membrane is ensured by the interaction of the signal peptide, the signal recognition particle, and ER receptors.
- SRP (signal recognition particle) is a carrier protein that binds to the signal peptide of the synthesized protein and blocks translation.
- The ribosome-SRP complex binds via protein-protein interaction to the SRP receptor on the ER membrane.
- The translocon (Sec61) is a protein channel to which the ribosome is handed off to continue translation.
The disassembly of the complex occurs due to GTP hydrolysis, after which the SRP detaches, and protein synthesis continues through the channel into the ER lumen.
To avoid confusing the composition of eukaryotic subunits: the small subunit has little RNA (only 1 molecule), while the large subunit has a lot of RNA (as many as 3 molecules).
Class and exam questions
Where are ribosomal subunits formed?
The assembly of the large and small ribosomal subunits takes place in the nucleolus.
What is the role of magnesium ions in the ribosomal structure?
Magnesium ions are necessary to stabilize and maintain the correct spatial structure of the rRNA-protein complex.
What is a polysome?
This is a working complex formed when multiple ribosomes bind to a single mRNA molecule during translation.
Where do proteins from free ribosomes go?
Proteins synthesized on ribosomes lying freely in the cytoplasm remain in the hyaloplasm (e.g., hemoglobin).
What else to study on this topic
- Translation mechanisms and the role of mRNA
- Differences between 70S ribosomes in mitochondria and plastids versus cytoplasmic 80S ribosomes
- The process of subunit formation in the nucleolus
- Entry and transport of proteins within ER cisternae
- Organization of functional complexes—polysomes
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