Ribosomes

by Marianne Bird

Ribosomes are made up of specific proteins and ribosomal RNA (rRNA). Prokaryotic and Eukaryotic ribosomes are similar to one another but differ in their number of proteins and in the rRNA/protein ratios.  The rRNA and proteins arrange themselves into two groups, differing in sizes, known as the large and small subunits of the ribosome. These two differing subunits fit together, slightly longer in axis than they are in diameter. The protein subunit is thought to provide structure for the rRNA subunit to add amino acids, translating mRNA into proteins. Ribosomes are located inside the cellular membrane, they can be floating in the cytoplasm or collected along the rough endoplasmic reticulum near the nucleus.

The main function of the Ribosomes is protein synthesis. They translate genetic code into amino acids that then build protein chains. Messenger RNA from the cell nucleus moves along the ribosome while ribosomal RNA adds amino acids to the polypeptide chain through out protein syntheses. The proteins synthesized by the ribosomes rely on the rough endoplasmic reticulum for transportation. The function of the ribosomes is important for all cells because proteins synthesis is essential for cellular repair and for instructing chemical processes. They are also the catalyst for peptidyl hydrolysis and transfer.

The ribosomes that are found floating in the cytoplasm are called Free Ribosomes and the ones connected to the endoplasmic reticulum are considered membrane-bound. They are the same but produce proteins for different things. When a free ribosome starts protein synthesis, it can become membrane-bound to the endoplasmic reticulum; there, the protein made is used in the plasma membrane. The proteins that are produced by the free ribosomes, are released in the cytosol and used within the cell. Free ribosomes can move about anywhere in the cytoplasm, except for into the nucleus or other organelles. Ribosomes are not considered organelles because they are entirely particulate and do not have a phospholipid membrane.

See the large and small sub-unit illustrated below in green and yellow, producing proteins first as a free ribosome and then attaching to the endoplasmic reticulum.

"A ribosome translating a protein that is secreted into the endoplasmic reticulum."

Works cited:
https://www.cellsalive.com/cells/cell_model_js.htm
https://en.wikipedia.org/wiki/Ribosome#Structure