Ribosome: Types, Structure, and Function

Every protein in every living thing, from a bacterium to a human, is built in the same kind of tiny machine: the ribosome. A single bacterial cell contains thousands of them, all reading genetic messages and stitching amino acids into proteins. Because ribosomes are essential to life, and because the bacterial version is slightly different from the human version, they are also one of the most important targets in medicine. Many common antibiotics work by jamming the bacterial ribosome while leaving ours untouched.

This article explains what ribosomes are, the two types (the smaller 70S found in bacteria and the larger 80S found in our own cells), how they are built, what they do, and why that small difference between the two types is so useful in fighting infection.

What is a ribosome?

A ribosome is the cell structure that makes proteins. It reads the instructions carried by messenger RNA (mRNA) and joins amino acids together, one by one, in the right order to build a protein. Because of this job, the ribosome is often called the "protein factory" of the cell.

Ribosomes are found in every living cell, which is why they are called a universal cell organelle. Each ribosome is built from two parts of unequal size, a large subunit and a small subunit, that come together to do the work. Both subunits are made of ribosomal RNA (rRNA) and proteins joined together, which is why a ribosome is described as a ribonucleoprotein.

The two subunits are not always joined. They come together on an mRNA to build a protein, then separate again afterward and are reused.

Ribosomes are small, dense, rounded, and granular particles of the ribonucleoprotein. The word ribosome is derived from the Greek word “ribo” which means ribonucleic acid, and “soma” which means body. In 1953 Robinson and Brown noted the ribosome in the plant cell while they were studying the bean roots using the electron microscope. Likewise, in 1955 Palade observed the ribosome in the animal cell. In 1958, Palade gave the term ribosome. Ribosomes are also called Palade’s granules/ particles.

The mass of the ribosome is expressed in the Svedberg unit (S). In the prokaryotic cell, it is found freely floating in the cytoplasm. In the eukaryotic cell, they are found in the cytoplasm, on the endoplasmic reticulum, on the nuclear membrane, and inside the cell organelles of mitochondria and chloroplast.

Location of Ribosomes

• A large number of ribosomes are present in yeast cells, reticulocytes or lymphocytes, meristematic plant tissues, embryonic nerve cells, and cancerous cells. These ribosomes are present freely in the cytoplasmic matrix.
• In the actively protein-synthesizing cell, ribosomes are attached to the endoplasmic reticulum as the rough endoplasmic reticulum. Examples: pancreatic cells, plasma cells, hepatic parenchymal cells, Nissls bodies, osteoblasts, serous cells, or the submaxillary gland, chief cells of the glandular stomach, thyroid cells, and mammary gland cells.
• A large number of free ribosomes are present in the cell synthesizing specific proteins. Example: erythroblasts, developing muscle cells, skin, and hair. These specific proteins are associated with intracellular utilization and storage.

Number of ribosomes: In E. coli, ribosome covers 25 percent of the total mass of bacteria. It contains 10,000 ribosomes. In the mammalian cultured cell, 10 million ribosomes are present per cell.

Structure of Ribosomes

A ribosome is in the subspherical structure and consists of two unequal parts. One is a larger unit, and the next is a smaller unit. The larger subunit is dome-shaped, whereas the smaller subunit is in the oblate-ellipsoid cap shape. In the larger subunit, protuberance, stalk, and ridge are present.  In the larger subunits, two sites, the peptidyl (P)  and the aminoacyl (A) site, are present. A third site, the exit (E) site, is where the used tRNA leaves. So the three sites, A, P, and E, mark the path a tRNA takes through the ribosome: it enters at A, moves to P, and leaves at E. The smaller subunit consists of the head, cleft, and platform. The smaller unit fits over the larger subunit like the cap. The cleft provides space for the mRNA in between these two subunits.

Figure: Ribosome

These two subunits remain attached to each other due to the higher concentration of the Mg++ ions. When there is a decrease in the concentration of the Mg++, then these larger and smaller subunits get separated. In the bacterial cell, these two subunits remain freely in the cytoplasm, and only during the protein synthesis do they unite. A single ribosome working on an mRNA is called a monosome. During active protein synthesis, many ribosomes attach to the same mRNA at once, forming a polyribosome or polysome.

Types of ribosomes

Ribosomes are sorted into two types by their size, measured as a sedimentation coefficient in Svedberg units (S). The Svedberg unit describes how fast a particle settles when spun at very high speed: a bigger, denser particle settles faster and has a higher S value. The two types are the 70S ribosome and the 80S ribosome.

One important warning about S values: they do not add up by simple arithmetic. A 70S ribosome is made of a 50S and a 30S subunit, which sounds like it should be 80S, not 70S. The reason is that the Svedberg unit measures rate of settling, which depends on shape and density as well as size, so the values cannot simply be added. This catches out many students, so keep it in mind.

70S Ribosome

70S ribosome is smaller in size with a molecular weight of 2.7×106 daltons. In prokaryotic cells, the 70S ribosome is found in bacteria (including cyanobacteria). The eukaryotic cell 70S ribosome is found in the chloroplasts and mitochondria. 70S ribosomes consist of two subunits: 50S (larger subunit) and 30 S (smaller subunit)

80S Ribosome

It is larger in size, with a molecular weight of about 4×10⁶ daltons. 80S ribosome is absent in the prokaryotic cell. In the eukaryotic cell, 80S ribosome is found either in free form in the cytoplasm or bound with the endoplasmic reticulum and nuclear membrane. 80S ribosome consists of two subunits: 60 S (larger subunit) and 40 S (smaller subunit).

Chemical Composition of Ribosomes

Figure: Composition of ribosomal subunits

• A ribosome is composed of RNA and proteins. In the 70S ribosome, the RNA content is higher than the protein content. E.g., in the ribosome of E. coli, rRNA is 63%, and protein is 37 %. In the 80S ribosome the balance is closer to equal, with a slightly higher share of protein than in the 70S ribosome.
• 70S ribosome consists of three types of rRNA, i.e., 23S rRNA, 16S rRNA, and 5S rRNA. In the larger 50S ribosomal subunit, 23S and 5S rRNA are present. In the smaller 30S ribosomal subunit, 16S rRNA is present.
• 80S ribosome consists of four types of rRNA, i.e., 28S rRNA, 18S rRNA, 5S rRNA, and 5.8S rRNA. In the larger 60S ribosomal subunit, 28S, 5S, and 5.8S rRNA are present. In the smaller 40S ribosomal subunit, 18S rRNA is present.
• The important metallic ions in the ribosome are Mg++, Ca++, and Mn++.

Functions of Ribosomes

• Ribosomes are the sites of protein synthesis. So ribosomes are called the protein factories or workbench of proteins.
• Free ribosomes synthesize the structural and enzymatic proteins for intracellular use.
• The ribosomes, bound with the endoplasmic reticulum, produce secretory, lysosomal, and membrane proteins.
• The ribosome provides enzymes (peptidyl transferase) and initiation factors for the condensation of the amino acids to form polypeptides.
• Ribosomes contain the rRNA for attachments of mRNA and tRNA.

Difference between the 70S and 80S Ribosomes

Why the ribosome difference matters: antibiotics

The difference between the bacterial 70S ribosome and the human 80S ribosome is not just a detail for exams. It is the reason a large group of antibiotics works.

Bacteria must make proteins to survive and multiply, and they do it with their 70S ribosomes. Several antibiotics attack these 70S ribosomes and stop the bacterium from making proteins, which kills it or halts its growth. Because human cells use the larger 80S ribosome instead, these drugs harm the bacterium far more than the patient. This difference is what makes the drugs safe enough to use.

Antibiotics that target the bacterial ribosome include the aminoglycosides (such as gentamicin and streptomycin), the tetracyclines, the macrolides (such as erythromycin and azithromycin), chloramphenicol, and others. Different drugs bind different parts of the ribosome: some jam the small (30S) subunit, others block the large (50S) subunit, but all of them exploit the same fact, that the bacterial 70S ribosome is different from our own.

There is one important exception worth knowing. Human mitochondria contain their own 70S-like ribosomes, a clue to their bacterial origin. This is part of why a few ribosome-targeting antibiotics can, at high doses, cause side effects: they can affect the mitochondrial ribosomes in our cells.

How to Remember

70S in prokaryotes, 80S in eukaryotes. The smaller number goes with the smaller, simpler cell. Prokaryote and 70 (fewer); Eukaryote and 80 (more). Bacteria get the 70S; our cells get the 80S.

The subunits, and why they don't add up. 70S = 50S + 30S. 80S = 60S + 40S. Notice neither adds up arithmetically (50+30 is not 70). That is the giveaway that S-values measure settling speed, not weight. "The numbers don't add, because S is a speed, not a size."

rRNA count: 3 for 70S, 4 for 80S. The prokaryotic 70S has three rRNAs (23S, 16S, 5S). The eukaryotic 80S has four (28S, 18S, 5.8S, 5S). "The bigger ribosome has one more rRNA."

Why antibiotics are safe. Bacterial ribosomes are 70S; ours are 80S. A drug built to jam the 70S ribosome hits the bacterium and spares us. "Different ribosome, safe antibiotic."

A, P, E in order. A tRNA enters at the A site (arrival), moves to the P site (peptide), and exits at the E site (exit). "A, P, E: arrive, peptide, exit."

Key exam facts in one table

Where Students Get Confused

The subunit numbers do not add up (50S + 30S = 70S). Svedberg units measure how fast a particle settles when spun, which depends on shape and density, not just weight. So they cannot be added. This is why a 50S and a 30S subunit make a 70S ribosome, not 80S.

70S is prokaryotic, 80S is eukaryotic, but mitochondria are the exception. Our cells use 80S ribosomes, but the mitochondria inside them contain 70S-type ribosomes. This is a clue that mitochondria came from ancient bacteria, and it is why some antibiotics can occasionally affect them.

A ribosome is not made only of protein. It is roughly half rRNA and half protein (rRNA is the larger part in bacteria). The rRNA is not just structural; it carries out the peptide-bond chemistry, so the ribosome is a ribozyme.

Free versus bound ribosomes make different proteins. Free ribosomes in the cytoplasm make proteins used inside the cell. Ribosomes bound to the endoplasmic reticulum make proteins for secretion or for membranes. Same ribosome, different destination depending on where it sits.

The subunits are separate when not in use. The large and small subunits are not permanently locked together. They join on an mRNA to build a protein, then come apart and are reused.

References

1. Alberts B, Heald R, Johnson A, et al. Molecular Biology of the Cell. 7th ed. W.W. Norton; 2022.
2. Madigan MT, Bender KS, Buckley DH, Sattley WM, Stahl DA. Brock Biology of Microorganisms. 16th ed. Pearson; 2021.
3. Nelson DL, Cox MM. Lehninger Principles of Biochemistry. 8th ed. W.H. Freeman; 2021.