Plant Physiology - Musienko, M. M. 2001

Chemical and molecular composition, structure, and functions of the plant cell
Ribosomes

These are small spherical particles with a diameter of 0.2 µm, composed primarily of high-molecular-weight RNA (up to 60%) and protein. Ribosomal RNA (rRNA) is synthesized in the nucleolus. Together with Proteins, they form a complex three-dimensional Structure through self-assembly. Each Cell contains tens of thousands of them. They can be attached to the ER or lie freely in the Cytoplasm. They are also found in The Nucleus, Plastids, and Mitochondria. Therefore, Two Types of ribosomes are distinguished: cytoplasmic ribosomes, with a sedimentation coefficient of 80S (the sedimentation constant, which characterizes The rate of

particle sedimentation during ultracentrifugation, is expressed in Svedberg units — S) and organelle ribosomes with a 70S coefficient. Ribosomes quite often form complexes — polyribosomes, which are formed step-by-step from individual subunits. First, subunit-specific proteins are assembled with the participation of 28S and 18S rRNA. Moreover, the smaller ribosomal subunit interacts with initiator tRNA in the presence of ATP, GTP, and protein initiation factors. This complex joins with mRNA in the presence of magnesium ions, and finally, the large ribosomal subunit attaches to the mRNA.

Ribosomes are responsible for Protein Synthesis in The Cell. The structure of protein molecules synthesized on ribosomes is determined by specific messenger or information RNA (mRNA) molecules, which transfer information from the nuclear genetic material to the ribosomes. Mitochondria and METABOLISM/14.html">Chloroplasts possess their own genetic material. During Protein Synthesis on Ribosomes, the Amino Acids that make up the polypeptide chain are attached sequentially one after another. The ribosome acts as the site where amino acid molecules can take up strictly defined positions relative to one another. Thus, protein synthesis involves mRNA, which carries the Genetic information, transport tRNA, which delivers the required amino acids to the ribosome, and the growing polypeptide chain. Factors responsible for the initiation, elongation, and termination of the polypeptide chain are also required. During protein synthesis, in their active state, most ribosomes attach to long thread-like mRNA molecules, forming polyribosomes in the presence of magnesium ions. This allows the simultaneous synthesis of several dozen molecules of the same protein.