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The ribosomes are few organelles globular without membrane, constituted by proteins (ribonucleoproteins) associated ribonucleic acids ribosomal (rRNA) from the nucleolus.

All cells, prokaryotic or eukaryotic, have ribosomes, although eukaryotic ribosomes are larger than prokaryotic ones.

In the eukaryotic cell can be found:

Structure of ribosomes

The ribosome is made up of two subunits, each with a different sedimentation coefficient.

The two ribosomal subunits, are formed in the nucleolus where two components are joined, in addition to the water: the rRNA and ribosomal proteins. RNA is synthesized in the nucleus while proteins are synthesized in the cytoplasm and are subsequently transported to the nucleolus. The two ribosomal subunits exit the cytoplasm through the nuclear pores and it is in the cytoplasm that they assemble to form the ribosome.

Ribosomes are spherical organelles, with a sedimentation rate of 80 S (in eukaryotic cells) and which are made up of two subunits:

  • The minor subunit, which sediments at values ​​of 40 S.
  • The largest subunit, with a sedimentation rate of 60 S.

The two subunits are separated in the cytoplasm, and only come together when they have to synthesize proteins.

Remember that the prokaryotic ribosome is 70 S, with 50 S being the largest subunit and 30 S the smallest (depending on the surface).

Ribosome function

The ribosomes involved the protein biosynthesis. First, the mRNA binds to the minor subunit of the ribosome and then to the major subunit, beginning the translation of the mRNA. When protein synthesis is complete, the two subunits separate. The mRNA molecules are read by several ribosomes at the same time, which is why they form chains that are called polyribosomes or polysomes.

They are abundant in cells that have to synthesize many proteins, such as pancreatic cells, which synthesize many digestive enzymes and have a very high number of ribosomes.

Ribosomes and pancreatic cells

The cells of the pancreas produce large amounts of proteins, such as insulin and various digestive enzymes, so they will need to contain many ribosomes in order to synthesize these proteins.

Questions that have come out in University entrance exams (Selectividad, EBAU, EvAU)

Canary Islands, July 2020, question 7

Researchers at the National Cancer Research Center develop a new method that allows them to demonstrate how tumor cells produce ribosomes to grow faster (NCYT, 01/09/20209).

a.- What is the chemical composition of ribosomes?
b.- What is its function?
c.- Where can we locate ribosomes in an animal cell?
d.- Where can we locate ribosomes in a plant cell?

Andalusia, September 2018, option A, question 4. (2 points)

An inhibitor of 70S ribosome activity is introduced into a culture of eukaryotic animal and plant cells.

a) Can animal cells synthesize proteins? [0.2]

b) What about plant cells? [0.2]

c) Can animal cells carry out cellular respiration? [0.2]

d) What about plant cells? [0.2]

e) Can plant cells carry out photosynthesis? [0.2] Give reasons for all answers.

Murcia, June 2017, option A, question 2.

Regarding ribosomes:

a) What structure do they have, both in prokaryotes and eukaryotes? (0.5 pts).

b) What chemical composition do they have? (0.3 pts).

c) what function do they perform? (0.2 pts).

d) where can they be located, both in prokaryotic and eukaryotic cells? (0.5 p).

Canary Islands, July 2021, question 9

9. The 2009 Nobel Prize in Chemistry was awarded to V. Ramakrishnan, TA Steitz and AE Yonath for their studies of the structure and function of the ribosome. (Source:

a. What are ribosomes made of?
b. What molecule is binding ribosomes in polyribosomes?
c. List two organelles of the cell that contain ribosomes.
d. Most antibiotics interfere with the function of bacterial ribosomes. State the metabolic function that is affected by these agents by damaging ribosomes.

Navarra, July 2021, question 7

a) What organelle or supramolecular complex is in both eukaryotic and prokaryotic cells ?

b) What is the difference between this organelle between the two cell types?

c) Indicates which components are part of it;

d) What function does it perform in the cell?

Fundamental ideas about ribosomes

The ribosomes: 


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